Antimicrobial Activity of 1,3,4-Thiadiazole Derivatives

Abstract

The 1,3,4-thiadiazole core has attracted significant attention due to its unique electronic structure, physicochemical properties, and wide-ranging pharmacological potential. This heterocyclic scaffold exhibits a broad spectrum of biological activities, often attributed to its capacity to modulate enzyme function, interact with receptors, and disrupt key biochemical pathways in both pathogens and host cells. Additionally, 1,3,4-thiadiazoles typically display favorable pharmacokinetic properties, including high metabolic stability and appropriate lipophilicity, which enhance their drug-likeness and bioavailability. This review presents an overview of antibacterial and antifungal compounds bearing the 1,3,4-thiadiazole scaffold that have been reported over the past five years. This publication details the chemical structures of novel 1,3,4-thiadiazole derivatives and reports the results of antibacterial and antifungal activity assays conducted against a range of microbial strains. Furthermore, it provides conclusions regarding the structural features that influence the observed biological activity of the synthesized compounds. Antimicrobial activity assessments conducted against ten Gram-negative and nine Gram-positive bacterial strains revealed that 79 newly synthesized 1,3,4-thiadiazole derivatives exhibited either superior inhibitory efficacy relative to standard reference antibiotics or achieved a high level of bacterial growth suppression, defined as 90–100% inhibition. In antifungal assays, the compounds were evaluated against 25 fungal species representing 15 genera. Among the tested derivatives, 75 compounds demonstrated antifungal potency exceeding that of reference antifungal agents or produced growth inhibition within the 90–100% range. The information provided herein may serve as a valuable resource for medicinal and agricultural chemists engaged in the development of novel drug candidates and plant protection agents.

1. Introduction

An important class of five-membered heterocyclic compounds is represented by thiadiazoles, which contain one sulfur atom and two nitrogen atoms in their structure [1,2,3]. This system gives rise to four possible isomers (Figure 1), among which the 1,3,4-thiadiazole isomer is the most widely studied and utilized due to its extensive applications in industry [4,5,6,7,8,9,10,11], medicine [12,13,14,15,16,17], and agriculture [18,19,20,21,22,23,24].

The chemistry of heterocyclic compounds employs various methods for the construction of the 1,3,4-thiadiazole ring [1,2,14]. The most widely used approach is undoubtedly cyclodehydration, typically preceded by a sulfurization step, of diacylhydrazine derivatives (Scheme 1a) or monothiodiacylhydrazines, which are most often formed as intermediates in the reaction of thiohydrazides with carboxylic acid derivatives (Scheme 1b). Another frequently applied method for the synthesis of 2,5-disubstituted 1,3,4-thiadiazoles involves the oxidative cyclization of thioacylhydrazones (Scheme 1c). Additional strategies include the sulfurization of hydrazides followed by cyclocondensation, most commonly with ortho-esters (Scheme 1d), cyclodehydrosulfurization of dithioacylhydrazine derivatives (Scheme 1e), or ring rearrangement reactions, most often involving 1,3,4-oxadiazoles (Scheme 1f). All sulfurization-based methods are typically carried out in the presence of P₄S₁₀ or Lawesson’s reagent.

The emergence of heterocyclic compounds as key pharmacophores in modern medicinal chemistry has fueled extensive research into novel scaffolds with potent and selective biological activities. The favorable pharmacokinetic properties of 1,3,4-thiadiazole derivatives are attributed to their unique chemical structure and mesoionic nature. These compounds exhibit enhanced lipophilicity and membrane permeability, facilitating effective interactions with biological targets [25,26]. The ring structure offers a balance between hydrophilicity and lipophilicity, which supports membrane permeability and bioavailability. Furthermore, studies on drug–1,3,4-thiadiazole conjugates have assessed their compliance with Lipinski’s Rule of Five, a set of guidelines predicting good oral bioavailability. These compounds demonstrated appropriate molecular weight, hydrogen bond donor and acceptor counts, and lipophilicity (log P values), all of which suggest favorable pharmacokinetic properties [26]. In addition, the electron-rich nature of thiadiazoles and their ability to form hydrogen bonds or coordinate with metal ions allow them to function as enzyme inhibitors or receptor ligands [27,28,29]. These attributes make 1,3,4-thiadiazole derivatives promising candidates for drug development, offering an optimal balance between efficacy and safety (Figure 2).

When evaluating antimicrobial activity, several standard methods and parameters are commonly used across microbiology, pharmacology, and materials science. These tests help determine the effectiveness of a compound, extract, or material against specific bacterial or fungal strains. Testing antifungal activity shares many similarities with antibacterial testing; however, fungi—especially yeasts and filamentous species—exhibit different growth characteristics, and methods are therefore adapted accordingly. The most commonly used parameters for assessing antimicrobial efficacy include the following:

• MIC (minimum inhibitory concentration): The lowest concentration that prevents visible bacterial or fungal growth.
• MBC (minimum bactericidal concentration): The lowest concentration that kills 99.9% of bacterial cells.
• MFC (minimum fungicidal concentration): The lowest concentration that kills 99.9% of fungal cells (confirmed by subculturing).
• Zone of inhibition: The diameter (in mm) of the clear, growth-free area around a disk or well.
• Inhibition rate: Typically used in colorimetric assays; represents the reduction in microbial growth relative to an untreated control (%).

Over the years, there has been a notable surge in the synthesis and pharmacological evaluation of 1,3,4-thiadiazole derivatives (Figure 3). This growing interest is driven by their demonstrated activity across a wide range of biological domains, including antibacterial, antifungal, antiviral, anticancer, anti-inflammatory, and central nervous system (CNS) disorders [30,31,32,33,34,35]. Recent studies have also highlighted their potential as enzyme inhibitors, receptor modulators, and multi-target agents, often exhibiting favorable pharmacokinetic and safety profiles [36,37,38,39,40,41,42].

The objective of this review is to provide a systematic analysis of studies published between 2020 and 2025, highlighting the most notable advancements in the biological evaluation of 1,3,4-thiadiazole-based compounds. Particular attention is given to compounds exhibiting antibacterial and antifungal properties for antimicrobial applications. By highlighting the most recent advances, this article seeks to provide medicinal chemists and pharmaceutical scientists with an up-to-date and focused perspective on the potential of the 1,3,4-thiadiazole scaffold in drug discovery and plant protection products.

2. 1,3,4-Thiadiazole Derivatives with Biological Activity

2.1. Antibacterial Activity

2.1.1. Disubstituted 1,3,4-Thiadiazole Derivatives

Among the published articles, the most frequently studied derivatives are 1,3,4-thiadiazole derivatives substituted at positions 2 and 5 of the heterocyclic ring.

Ammara et al. synthesized oxazolidinone derivatives, two of which contained a 1,3,4-thiadiazole ring (1, 2, Figure 4) [43]. The obtained compounds incorporated an oxazolidinone core in the (S)-configuration at the C-5 position, which, according to the authors, is essential for antibacterial activity, as well as a fluorine atom substituted at the phenylene linker, a modification that typically enhances potency by 2–8 fold. Compound 1 exhibited low activity against Enterococcus faecalis (31971), Enterococcus faecalis (31972), and Enterococcus faecium, with MIC values of 64 μg/mL, 32 μg/mL, and 32 μg/mL, respectively. In contrast, compound 2 showed moderate activity against Enterococcus faecalis (31971), Enterococcus faecalis (31972), and Enterococcus faecium, with MIC values of 2 μg/mL, 2 μg/mL, and 1 μg/mL, respectively. It showed no activity against the oxazolidinone-resistant strain Enterococcus faecalis (31903), with MIC > 64 μg/mL.

Xiong et al. synthesized a series of dihydropyrrolidone derivatives containing the 1,3,4-thiadiazole moiety (3a–t, Figure 5) and evaluated their activity against Staphylococcus epidermidis, Staphylococcus aureus, Enterococcus faecalis, and Enterococcus faecium [44]. The compounds and their corresponding MIC values are presented in

Table 1. Antibacterial activity of compounds 3a–n.

Entry	Compound	R2	R3	MIC (μM)
				S. epidermidis	S. aureus	E. faecalis	E. faecium
1	3a	H	2-Cl	>100	>100	>100	>100
2	3b	4-OH	2-Cl	50	100	>100	>100
3	3c	2-OH	2-Cl	6.25	12.5	12.5	12.5
4	3d	3-OH	2-Cl	>100	>100	>100	>100
5	3e	3-Br-4-OH	2-Cl	>100	>100	>100	>100
6	3f	4-OCH2COOCH3	2-Cl	>100	>100	>100	>100
7	3g	4-OCH2COOCH3	2-Cl	>100	>100	>100	>100
8	3h	2-OH-5-OCH3	H	>100	>100	>100	>100
9	3i	2-OH	4-Cl	25	25	50	50
10	3j	2-OH	3-Cl	25	50	25	25
11	3k	2-OH-5-OCH3	2-Cl	50	50	100	100
12	3l	2,4-diOH	H	>100	>100	>100	>100
13	3m	3,4-diOH	H	>100	>100	>100	>100
14	3n	2,3-diOH	H	50	50	25	25
15	3o	2-OH	H	>100	>100	>100	>100
16	3p	2-OH	2-Cl	>100	>100	>100	>100
17	3q	2-OH	H	>100	>100	>100	>100
18	3r	2-OH	H	>100	>100	>100	>100
19	3s	2-OH	H	>100	>100	>100	>100
20	3t	2-OH	H	>100	>100	>100	>100
21	Daptomycin	-	-	2	2	2	2

. Derivatives 3c, 3i, and 3j, characterized by the presence of a hydroxyl group at the ortho position of the benzene ring and a chlorine atom at the R³ site, as well as 3n, containing two hydroxyl groups at the R² site, exhibited the highest activity against Gram-positive bacteria. Moreover, all compounds demonstrated low cytotoxicity and hemolytic activity.

Gumus et al. synthesized a series of 1,3,4-thiadiazole derivatives from thiosemicarbazide-substituted coumarins, yielding compounds 4a–i and 5a–i (Figure 6) [45]. Selected compounds were screened for antibacterial activity against Helicobacter pylori. The tested compounds did not exhibit antibacterial activity (MIC > 128 μg/mL).

Mao et al. obtained phenylthiazole derivatives containing a 1,3,4-thiadiazole thione moiety (6a–p, Figure 7) [46]. The synthesized compounds were tested for antibacterial activity against Ralstonia solanacearum and Xanthomonas oryzae pv. oryzae. The activity was evaluated at concentrations of 100 μg/mL and 200 μg/mL, with the data presented in

Table 2. Antibacterial activity of compounds 6a–p.

Entry	Compound	R	Inhibition Rate (%)
			R. solanacearum		Xoo
			200 μg/mL	100 μg/mL	200 μg/mL	100 μg/mL
1	6a	H	100	42.45	<10	17.24
2	6b	2-F	100	92.00	98.92	42.24
3	6c	2-Cl	100	87.04	34.30	<10
4	6d	2-Br	82.09	74.37	47.08	13.48
5	6e	2-CH3	78.63	45.96	32.33	<10
6	6f	2-OCH3	47.74	31.78	18.70	<10
7	6g	2-OCF3	80.08	48.26	28.65	<10
8	6h	3-F	94.87	93.81	76.17	54.66
9	6i	3-CH3	95.40	94.00	80.07	14.95
10	6j	3-OCH3	70.57	67.56	45.34	31.37
11	6k	3-OCF3	100	100	100	72.63
12	6l	4-F	53.19	33.59	28.59	12.09
13	6m	4-CH3	74.07	71.37	49.37	41.59
14	6n	4-OCH3	64.10	53.41	48.40	44.61
15	6o	4-OCF3	43.47	35.11	<10	<10
16	6p	4-NH2	71.88	60.60	53.00	41.26

. The highest activity against Ralstonia solanacearum was observed for compounds 6b (R = 2-F), 6h (R = 3-F), 6i (R = 3-CH₃), and 6k (R = 3-OCF₃), which showed inhibition rates of 92.00%, 93.81%, 94.00%, and 100%, respectively, at a concentration of 100 μg/mL. Moreover, compound 6k exhibited a high inhibition rate of 72.63% against Xanthomonas oryzae pv. oryzae at the same concentration. The authors concluded that the antibacterial activity of the investigated phenylthiazole derivatives is strongly enhanced by electron-withdrawing substituents at the meta-position of the benzene ring, whereas for the ortho- and para-analogues the inhibition rate is much lower.

Hafidh et al. synthesized hybrid silica gels incorporating 1,3,4-thiadiazole rings in their structure (7a, 7b, Figure 8) [47]. The compounds were evaluated for antibacterial activity against Escherichia coli, Salmonella typhimurium, Staphylococcus aureus, and Enterococcus faecium. The results of the biological screening are presented in

Table 3. Antibacterial activity of compounds 7a,b.

Entry	Compound	MIC
		E. coli	S. typhimurium	S. aureus	E. feacium
1	7a a	0.12	0.12	0.03	0.12
2	7b a	0.5	0.25	0.06	0.25
3	Gentamicin b	7.81	7.81	3.91	7.81

^a (MIC mg/mL); ^b (MIC µg/mL).

.

Mehta’s group synthesized a series of 1,3,4-thiadiazole derivatives containing a benzo[d]imidazole scaffold (8a–o, Figure 9) [48]. The resulting compounds were evaluated for antibacterial activity against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Streptococcus pyogenes, with the results presented in

Table 4. Antibacterial activity of compounds 8a–o.

Entry	Compound	R	MIC (μg/mL)
			E. coli	P. aeruginosa	S. aureus	S. pyogenes
1	8a	H	100	12.5	125	125
2	8b	2-Br	250	250	100	250
3	8c	4-Br	100	62.5	125	125
4	8d	2-Cl	50	125	250	100
5	8e	4-Cl	100	50	12.5	100
6	8f	2-F	125	250	250	250
7	8g	3-F	125	250	100	100
8	8h	4-F	100	62.5	100	125
9	8i	2-OH	100	100	62.5	100
10	8j	4-OH	50	12.5	100	100
11	8k	4-CH3	100	125	250	100
12	8l	4-OCH3	250	250	62.5	100
13	8m	2-NO2	100	100	250	250
14	8n	3-NO2	125	250	125	250
15	8o	4-NO2	250	12.5	100	62.5
16	Chloramphenicol	-	50	50	50	50
17	Ciprofloxacin	-	25	25	50	50
18	Norfloxacin	-	10	10	10	10

. The highest MIC values were observed for compounds 8j (R = 4-OH) and unsubstituted 8a (R = H) against Pseudomonas aeruginosa, both with values of 12.5 μg/mL. In contrast, compound 8e (R = 4-Cl) exhibited the lowest MIC value, also 12.5 μg/mL, against Staphylococcus aureus. The studies demonstrated that electron-donating substituents, such as hydroxyl and methoxy groups at the ortho and para positions of the benzylidene fragment, enhanced the biological activity against Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus. Electron-withdrawing groups, such as bromo, chloro, and fluoro at the para and ortho positions, also showed significant activity against the same bacterial strains.

The group of Danilova evaluated a series of bisamino-1,3,4-thiadiazoles linked via alkyl and alkenyl spacers (9a–d, 10, Figure 10) against Staphylococcus aureus, Citrobacter amalonaticus, and Escherichia coli [49]. Among the tested compounds, 9a (n = 1) exhibited activity against Staphylococcus aureus at a concentration of 3.36 × 10⁻⁴ M, with a zone of inhibition measuring 1.6 mm. In the case of Citrobacter amalonaticus, only compound 10 was active at a concentration of 3.01 × 10⁻⁴ M, also with a zone of inhibition of 1.6 mm. None of the tested compounds showed activity against Escherichia coli.

Zahoor et al. obtained Schiff base derivatives containing a 1,3,4-thiadiazole moiety (11a–l, Figure 11) [50]. The compounds were tested for antibacterial activity against Escherichia coli. The results are presented in

Table 5. Antibacterial activity of compounds 11a–l.

Compound	11a	11b	11c	11d	11e	11f	11g	11h	11i	11j	11k	11l	Streptomycin
Inhibition rate (%)	42.3	32.4	40.1	38.2	27.1	31	25.5	18.1	36.5	19.7	14.2	18.4	44

. Four of the investigated compounds—11a, 11c, 11d, and 11i—exhibited bacterial inhibition rates of 42.3%, 40.1%, 38.2%, and 36.5%, respectively, in comparison to Streptomycin (44%). The authors concluded that the presence of electron-withdrawing substituents (CF₃, NO₂, Cl, F) within the benzylidene fragment at favorable positions—mainly para and, less frequently, ortho—ensures strong inhibition. In contrast, weaker inhibition was observed for derivatives bearing bulky groups (CH₃, Br, naphthyl), particularly at the meta-positions, due to steric hindrance.

Li et al. obtained a series of 2,5-disubstituted 1,3,4-thiadiazole compounds (12a–r, Figure 12) [51]. The compounds were tested for antibacterial activity against Xanthomonas oryzae pv. oryzae. The results of the screening, performed at a concentration of 100 μg/mL, are presented in

Table 6. Antibacterial activity of compounds 12a–r.

Entry	Compound	R1	R2	Inhibition Rate (%)
1	12a	CH3	CH3	72
2	12b	CH3	CH2CH3	72
3	12c	CH3	CH2CH2CH3	79
4	12d	CH3	CH2C6H5	78
5	12e	CH3	CH2C6H4-4-F	63
6	12f	CH3	CH2C6H4-4-Cl	67
7	12g	CH3	CH2C6H4-3-Cl	61
8	12h	CH3	CH2C6H4-2-Cl	52
9	12i	CH3	CH2C6H3-2,4-diCl	76
10	12j	CF3	CH3	72
11	12k	CF3	CH2CH3	62
12	12l	CF3	CH2CH2CH3	75
13	12m	CF3	CH2C6H5	62
14	12n	CF3	CH2C6H4-4-F	68
15	12o	CF3	CH2C6H4-4-Cl	71
16	12p	CF3	CH2C6H4-3-Cl	83
17	12q	CF3	CH2C6H4-2-Cl	67
18	12r	CF3	CH2C6H3-2,4-diCl	76

. Bioassay results demonstrated that all tested compounds exhibited superior antibacterial activity against Xanthomonas oryzae pv. oryzae, with inhibition rates ranging from 52% to 79%, compared to the Thiodiazole copper standard (16%) (J, Figure 2). Enhanced in vitro antibacterial activity against Xanthomonas oryzae pv. oryzae was observed for compound 12p, which carries a trifluoromethyl group in the pyrimidine ring and a 3-chlorobenzyl unit.

Panwar and his coworkers obtained derivatives of [2-phenyl-1-(p-tolyl)pyrido[3,2-f]quinazolin-4(1H)-yl]-1,3,4-thiadiazolyl]-4-piperazine (13a–i, Figure 13) [52]. The compounds were tested for antibacterial activity against Staphylococcus aureus, Escherichia coli, and Proteus vulgaris at a concentration of 250 μg/mL. The results of the analysis are presented in

Table 7. Antibacterial activity of compounds 13a–i.

Entry	Compound	R	Zone of Inhibition (mm)
			S. aureus	E. coli	P. vulgaris
1	13a	H	6	0	6
2	13b	2-Cl	14	16	14
3	13c	3-Cl	12	16	12
4	13d	4-Cl	14	18	14
5	13e	3-NO2	10	8	8
6	13f	4-NO2	8	10	12
7	13g	3-OCH3-4-OH	10	12	10
8	13h	4-CH3	10	12	12
9	13i	2-OH	12	14	14
10	Ampicillin trihydrate	-	16	20	20

. It was observed that chloro-substituted thiadiazoles 13b–d exhibited stronger antimicrobial activity compared to other derivatives bearing nitro, methoxy, hydroxy, or methyl groups. Among the isomeric chloro-derivatives, the 2-chlorophenyl substitution was particularly favorable for antimicrobial potency. In summary, the synthesized compounds exhibited moderate antibacterial potential, although lower than that of the standard, Ampicillin trihydrate.

Blaja et al. obtained tetranorlabdane compounds bearing 1,3,4-thiadiazole units (14a–c, Figure 14) [53]. The compounds were tested against two bacterial strains: the Gram-positive Bacillus polymyxa and the Gram-negative Pseudomonas aeruginosa (

Table 8. Antibacterial activity of compounds 14a–c.

Entry	Compound	MIC (μg/mL)
		B. polymyxa	P. aeruginosa
1	14a	2.5	2.5
2	14b	>256	>256
3	14c	>256	>256
4	Kanamycin	4	4

). The results indicate that compound 14a, containing free amino group adjacent to 1,3,4-thiadiazole ring, possesses significant antibacterial activity, with an MIC value of 2.5 μg/mL.

Ibrahim et al. synthesized a modified chitosan–thiadiazole conjugate (15, Figure 15) [54]. The antibacterial properties of this derivative were investigated against various pathogens, including Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, and Staphylococcus aureus. Zones of inhibition for the individual bacterial strains, measured at a concentration of 50 μg/mL, are presented in

Table 9. Antibacterial activity of compound 15.

Entry	Compound	Zone of Inhibition (mm)
		E. coli	P. aeruginosa	B. subtilis	S. aureus
1	15	11	11.5	11	8

. The synthesized compound exhibited moderate antibacterial potential.

Dinh Thanh et al. obtained a series of thioureas containing a 1,3,4-thiadiazole moiety (16a–i, Figure 16) [55]. The compounds were tested against Gram-positive bacteria (Bacillus subtilis, Clostridium difficile, Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pneumoniae) and Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Salmonella typhimurium). The results of the analysis are presented in

Table 10. Antibacterial activity of compounds 16a–i.

Entry	Compound	MIC (μg/mL)
		BS	CD	SA	SE	SP	EC	KP	PA	ST
1	16a	12.5	200	3.125	3.125	12.5	1.56	50	25	1.56
2	16b	6.25	6.25	25	100	12.5	50	6.25	1.56	12.5
3	16c	1.56	0.78	6.25	50	3.125	200	12.5	50	3.125
4	16d	3.125	25	50	25	1.56	12.5	25	100	25
5	16e	1.56	50	12.5	1.56	3.125	3.125	3.125	12.5	6.25
6	16f	400	400	400	400	400	400	400	400	400
7	16g	0.78	3.125	100	12.5	0.78	6.25	400	6.25	50
8	16h	200	12.5	0.78	0.78	200	0.78	1.56	0.78	100
9	16i	100	1.56	1.56	6.25	50	25	0.78	3.125	0.78
10	Ciprofloxacin	3.125	6.25	3.125	3.125	6.25	1.56	1.56	1.56	1.56
11	Vancomycin	1.56	1.56	1.56	0.78	1.56	-	-	-	-

BS: Bacillus subtilis; CD: Clostridium difficile; SA: Staphylococcus aureus; SE: Staphylococcus epidermidis; SP: Streptococcus pneumoniae; EC: Escherichia coli; KP: Klebsiella pneumonia; PA: Pseudomonas aeruginosa; ST: Salmonella typhimurium.

. Almost all of the thioureas exhibited remarkable antibacterial activity. Among the studied compounds, 16a, unsubstituted at the 5 position of the 1,3,4-thiadiazole ring, 16h, bearing a pentyl substituent at the R site, and 16i, with an isopentyl substituent, were the most effective inhibitors against Staphylococcus aureus, with MIC values ranging from 0.78 to 3.125 μg/mL, in comparison to the standards Ciprofloxacin and Vancomycin. Structure–activity relationship analysis led to the general conclusion that the presence of short alkyl chains, such as methyl or ethyl (16b, 16c), is beneficial and results in good or moderate activity against many bacterial strains. It was also observed that chain elongation at the 5 position of the 1,3,4-thiadiazole ring increased activity, except for the n-butyl group (16f), which was completely inactive. In particular, compounds bearing five-carbon atom substituents (16h, 16i) exhibited very strong activity against several Gram-positive strains (Staphylococcus aureus, Staphylococcus epidermidis, Clostridium difficile) and Gram-negative strains (Klebsiella pneumoniae, Pseudomonas aeruginosa). Chain branching did not strongly affect overall activity but influenced the susceptibility of specific strains.

Zhao et al. obtained a series of pyrrolamide derivatives, one of which contained a 1,3,4-thiadiazole ring (17, Figure 17) [56]. The compound was tested for its inhibitory effect against Staphylococcus aureus (Gyrase IC₅₀ = 0.137 μmol/L) and Escherichia coli (Gyrase IC₅₀ = 6.87 μmol/L). For the investigated thiadiazole 17, the MIC values were 0.125 μg/mL and 16 μg/mL, respectively.

Hangan et al. obtained copper complexes of 1,3,4-thiadiazole derivatives bearing dimethylformamide (DMF) or 1,10-phenanthroline (phen) ligands (18, 19, Figure 18), with the formulas [Cu₄(18)₄(OH)₄(DMF)₂(H₂O)] and [Cu(19)₂(phen)(H₂O)] [57]. The biological activity of the heterocyclic ligands was evaluated against four Gram-positive bacteria (Methicillin-susceptible Staphylococcus aureus (MSSA), Methicillin-resistant Staphylococcus aureus (MRSA), Staphylococcus lentus, and Enterococcus faecium) and two Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) (

Table 11. Antibacterial activity of compounds 18 and 19 and their complexes.

Bacterial Strains	18 Complex MIC µM/L	19 Complex MIC µM/L	18 MIC µM/L	19 MIC µM/L
S. aureus MRSA	2	2	25	12.5
S. aureus MSSA	2	2	12.5	12.5
E. coli	2	2	25	25
E. faecium	n.a	0.2	n.a	12.5
P. aeruginosa	n.a	0.2	n.a	25
S. lentus	n.a	2	n.a	25

n.a—not active.

). Compound 18 exhibited antibacterial activity against Staphylococcus aureus (MRSA), Staphylococcus aureus (MSSA), and Escherichia coli; however, it showed no activity against Enterococcus faecium, Pseudomonas aeruginosa, or Staphylococcus lentus. Coordination of compound 18 with Cu²⁺ ions led to a reduction in MIC to 2 µM/L. Compound 19 showed activity against all tested bacterial strains, and its transformation into a copper complex resulted in a significant reduction in MIC values.

Kumar et al. obtained a series of (4-substituted-phenyl-1,3,4-thiadiazol-2-yl)-4-(4-substituted-phenyl)azetidin-2-one derivatives (20a–g, Figure 19) [58]. The newly synthesized compounds were screened for antibacterial activity against Pseudomonas aeruginosa, Staphylococcus aureus, Klebsiella pneumoniae, Enterococcus faecalis, and Escherichia coli (

Table 12. Antibacterial activity of compounds 20a–g.

Entry	Compound	R1	R2	MBC (µM)
				SA	EF	PA	EC	KP
1	20a	Br	4-OH	14.30	28.60	14.30	14.30	14.30
2	20b	Br	4-NH2	14.33	28.67	14.33	43.10	14.33
3	20c	OCH3	3,5-diCl	14.17	28.34	28.34	28.34	28.34
4	20d	OCH3	4-NO2	14.99	14.99	29.98	29.98	14.99
5	20e	OCH3	4-Br	13.86	13.86	13.86	13.86	13.86
6	20f	Cl	4-NO2	14.85	14.85	29.69	14.85	14.85
7	20g	Br	4-Cl	6.87	6.87	6.87	13.74	13.74

SA: Staphylococcus aureus; EF: Enterococcus faecalis; PA: Pseudomonas aeruginosa; EC: Escherichia coli; KP: Klebsiella pneumoniae.

). Results of the minimum bactericidal concentration (MBC) test indicated that the compounds exhibited moderate antibacterial potential. The authors demonstrated that the type and position of substituents on the phenyl rings of 4-substituted phenyl-1,3,4-thiadiazol-2-yl)-4-(4-substituted-phenyl)azetidin-2-one derivatives significantly influenced their antimicrobial activities. Electron-withdrawing groups (EWGs), such as chloro or nitro at the para position, enhanced antimicrobial activity. The best results were obtained for derivative 20g, containing bromine at the R¹ site and chlorine at the para position of the benzene ring (R²), which demonstrated notable activity against both Gram-positive and Gram-negative bacteria.

Liu’s group obtained a series of gallic acid amide derivatives containing a 1,3,4-thiadiazole core (21a–i, Figure 20) [59]. The compounds were tested for biological activity against Vibrio harveyi (

Table 13. Antibacterial activity of compounds 21a–i.

Compound	21a	21b	21c	21d	21e	21f	21g	21h	21i	Streptomycin Sulfate
MIC (mg/mL)	0.0625	0.0313	0.0625	-	0.0625	0.0625	0.0625	-	-	44

). Among the described compounds, derivative 21b, containing a 4-fluorophenyl group adjacent to the 1,3,4-thiadiazole ring, exhibited the most promising activity, with an MIC value of 0.0313 mg/mL.

Gurunani et al. obtained Sparfloxacin derivatives containing a 1,3,4-thiadiazole ring (22a–j, Figure 21) [60]. The compounds were tested for activity against Gram-negative and Gram-positive bacteria (Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, and Bacillus subtilis), as well as Mycobacterium tuberculosis (

Table 14. Antibacterial activity of compounds 22a–j.

Entry	Compound	R	MIC [μg/mL]
			S. aureus	B. subtilis	E. coli	P. aeruginosa	M. tuberculosis
1	22a	H	15.67	16.67	31.00	65.33	Resist
2	22b	Cl	1.62	1.33	8.67	8.67	3.12
3	22c	Br	2.00	2.00	3.33	5.00	6.25
4	22d	F	4.67	7.33	7.67	7.33	3.12
5	22e	NO2	1.67	1.67	4.67	2.67	Resist
6	22f	CH3	8.00	16.33	17.33	32.67	6.25
7	22g	OCH3	8.67	8.33	16.67	16.67	1.60
8	22h	NH2	5.33	4.00	16.00	8.00	1.60
9	22i	OH	7.33	9.00	15.33	16.33	Resist
10	22j	C6H5	1.33	2.00	4.00	9.00	0.8

). Almost all compounds synthesized by the authors exhibited moderate to good antibacterial activity. The highest MIC values against Gram-negative bacteria were observed for derivatives 22b, 22e, and 22j, bearing chlorine, nitro, and phenyl groups, respectively.

Wujec and coworkers obtained a series of 1,3,4-thiadiazole derivatives (23a–s, Figure 22) [61]. Almost all of the synthesized compounds exhibited no or only negligible antibacterial activity against Gram-positive and Gram-negative bacteria, except for compound 23p, which contains a 4-bromophenyl substituent. This compound showed activity against Staphylococcus epidermidis with an MIC value of 31.25 µg/mL and Micrococcus luteus with an MIC value of 15.63 µg/mL.

Alqahtani et al. obtained a series of compounds bearing a benzothiazolotriazole scaffold connected to a 1,3,4-thiadiazole ring (24a–c, Figure 23) [62]. The synthesized compounds were evaluated for their antibacterial activity against a panel of bacteria, including Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Proteus mirabilis, and Acinetobacter baumannii. Among the obtained compounds, only derivative 24b (R = Br) showed moderate activity against Staphylococcus aureus, with an MIC value of 128 µg/mL.

Pham’s group synthesized a series of 5-substituted-2-amino-1,3,4-thiadiazole derivatives (25a–l, Figure 24) [63]. The compounds were tested against Escherichia coli, Pseudomonas aeruginosa, Streptococcus faecalis, Methicillin-resistant strains of Staphylococcus aureus, and Methicillin-susceptible strains of Staphylococcus aureus. The compounds exhibited weak activity against the tested strains, with MIC values ranging from 126 to 1024 µg/mL.

Baddi et al. obtained a derivative of 1,3,4-thiadiazole (26, Figure 25), occurring in the form of two L/D isomers [64]. Both isomers were tested against Gram-positive (Bacillus subtilis and Staphylococcus aureus) and Gram-negative (Pseudomonas aeruginosa and Escherichia coli) bacteria. The hydrogel derived from the D-26 isomer exhibited greater antibacterial activity than the L-26 hydrogel, with zones of inhibition measuring 35 mm, 27.5 mm, and 24 mm for Bacillus subtilis, Staphylococcus aureus, and Pseudomonas aeruginosa, respectively.

Muğlu et al. synthesized a range of 1,3,4-thiadiazole derivatives substituted with a thiophene ring (27a–g, Figure 26) [65]. The obtained compounds were tested against Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus, Bacillus cereus, Bacillus subtilis (6051), Bacillus subtilis (6633)). Among the tested compounds, only derivatives 27a (R = CH₃) and 27f (R = 3-FC₆H₄) exhibited antibacterial activity, compound 27a against both Gram-positive and Gram-negative bacteria and compound 27f against Gram-positive bacteria only. Zones of inhibition at a concentration of 256 μg/mL for compound 27a against Bacillus subtilis (6633), Bacillus subtilis (6051), Staphylococcus aureus, Bacillus cereus, and Escherichia coli were 14 mm, 13 mm, 14 mm, 15 mm, and 22 mm, respectively. For compound 27f, the inhibition zones were 16 mm, 14 mm, 15 mm, and 15 mm, respectively.

Sunitha et al. obtained a series of azo-imine thiadiazoles (28a–e, Figure 27) [66]. The compounds were tested against Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli and Klebsiella pneumoniae), with Streptomycin as the standard drug (

Table 15. Antibacterial activity of compounds 28a–e.

Entry	Compound	Zone of Inhibition (mm)
		S. aureus	B. subtilis	K. pneumonia	E. coli
1	28a	10	9	12	8
2	28b	14	15	12	13
3	28c	10	11	-	8
4	28d	-	11	9	12
5	28e	8	13	-	14
6	Streptomycin	17	16	15	15

). Among the synthesized dyes, compound 28b bearing a hydroxyl group at the para position of the benzene ring, 28d with chlorine at the same position, and 28e containing a nitro group showed good activity against Escherichia coli, with zones of inhibition of 13 mm, 12 mm, and 14 mm, respectively. In addition, compound 28b was also active against Bacillus subtilis.

Yu et al. obtained a series of thiochroman-4-one derivatives incorporating carboxamide and 1,3,4-thiadiazole thioether moieties (29a–o, Figure 28) [67]. The compounds were tested against Xanthomonas oryzae pv. oryzae and Xanthomonas axonopodis pv. citri using Bismerthiazol and Thiodiazole copper (J, Figure 2) as standard drugs (

Table 16. Antibacterial activity of compounds 29a–o.

Entry	Compound	R1	R2	Inhibition Rate (%)
				Xoo		Xac
				200 μg/mL	100 μg/mL	200 μg/mL	100 μg/mL
1	29a	CH3	Cl	100	94	90	78
2	29b	CH2CH3	Cl	92	78	86	70
3	29c	CH2CH2CH3	Cl	81	70	74	61
4	29d	CH2C6H5	Cl	60	44	51	40
5	29e	CH2C6H4-4-F	Cl	64	50	57	45
6	29f	CH3	F	90	75	80	61
7	29g	CH2CH3	F	74	60	68	54
8	29h	CH2CH2CH3	F	68	54	60	48
9	29i	CH2C6H5	F	51	38	47	31
10	29j	CH2C6H4-4-F	F	54	41	51	39
11	29k	CH3	CH3	45	32	40	30
12	29l	CH2CH3	CH3	37	28	32	20
13	29m	CH2CH2CH3	CH3	30	20	25	14
14	29n	CH2C6H5	CH3	24	16	16	8
15	29o	CH2C6H4-4-F	CH3	28	18	21	12
16	Bismerthiazol	-	-	70	52	57	35
17	Thiodiazole copper	-	-	63	45	35	15

). Compounds 29a–g exhibited 74–100% and 60–94% in vitro antibacterial activity against Xanthomonas oryzae pv. oryzae at concentrations of 200 and 100 μg/mL, respectively. Meanwhile, compounds 29a–h demonstrated 60–90% and 48–78% in vitro antibacterial activity against Xanthomonas axonopodis pv. citri at the same concentrations, both exceeding the activity of the Bismerthiazol and Thiodiazole copper standards.

Shu et al. obtained a series of galactoside derivatives containing a 1,3,4-thiadiazole moiety (30a–t, Figure 29) [68]. The compounds were tested against Xanthomonas oryzae pv. oryzae and Xanthomonas axonopodis pv. citri at concentrations of 200 and 100 μg/mL. The inhibition rates ranged from 31.5% to 64.2% and 40.8% to 57.7% against Xanthomonas oryzae pv. Oryzae and from 18.3% to 36.2% and 19.8% to 36.1% against Xanthomonas axonopodis pv. citri, respectively. These values were lower than those observed for the Thiodiazole copper standard (J, Figure 2) (70.1%, 43.6%, 80.2%, and 46.1%).

Prasad et al. obtained a series of quinoline-bridged thiophenes connected to a 1,3,4-thiadiazole ring (31a–e, Figure 30) [69]. The compounds were tested against Escherichia coli and Staphylococcus aureus. The study found that all synthesized compounds (31a–e) exhibited notable antibacterial activity against both Escherichia coli and Staphylococcus aureus, although their activity was weaker than that of the Chloramphenicol standard.

Acar Çevik et al. obtained a series of benzimidazole derivatives containing a 1,3,4-thiadiazole scaffold (32a–k, Figure 31) [70]. The antibacterial activity of all compounds was evaluated by determining their minimum inhibitory concentration (MIC) against Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Enterococcus faecalis, Bacillus subtilis, and Staphylococcus aureus (

Table 17. Antibacterial activity of compounds 32a–k.

Entry	Compound	MIC (μg/mL)
		EC	KP	PA	EF	BS	SA
1	32a	125	125	250	7.81	15.63	125
2	32b	125	125	125	7.81	31.25	125
3	32c	125	125	125	31.25	125	125
4	32d	62.5	62.5	125	3.9	125	125
5	32e	125	62.5	125	7.81	125	125
6	32f	<0.97	31.25	125	7.81	125	31.25
7	32g	7.81	62.5	62.5	7.81	62.5	31.25
8	32h	1.95	15.63	31.25	7.81	31.25	7.81
9	32i	<0.97	125	125	3.9	125	250
10	32j	3.9	125	125	7.81	125	125
11	32k	3.9	125	125	3.9	125	62.5
12	Azithromycin	<0.97	<0.97	<0.97	<0.97	<0.97	<0.97

EC: Escherichia coli; KP: Klebsiella pneumoniae; PA: Pseudomonas aeruginosa; EF: Enterococcus faecalis; BS: Bacillus subtilis; SA: Staphylococcus aureus.

). Compounds 32f and 32i exhibited the greatest antibacterial activity against Escherichia coli, with MIC values below 0.97 µg/mL. In their search for structure–activity relationships, the authors found that antibacterial activity increased in compounds bearing alkylamine substituents at the 5 position of the thiadiazole ring, with longer chain or cyclic substituents (n-butyl, cyclohexyl) conferring higher potency. Among other derivatives exhibiting activity against the Escherichia coli strain, compound 32h, containing an isopropyl group (MIC = 1.95 µg/mL), as well as compounds 32j and 32k, bearing a p-methoxyphenyl substituent and an isobutyl substituent, respectively, should be highlighted (MIC = 3.90 µg/mL). For the Enterococcus faecalis strain, compounds 32d, 32i, and 32k, containing phenyl, n-butyl, and isobutyl groups, respectively, were the most active (MIC = 3.90 µg/mL).

Rdaiaan’s group obtained a series of 5-phenyl-1,3,4-thiadiazole derivatives containing a benzimidazole scaffold (33a–g, Figure 32) [71]. The derivatives were tested for antibacterial activity against Gram-positive (Staphylococcus aureus, Staphylococcus epidermidis) and Gram-negative (Pseudomonas aeruginosa, Escherichia coli) bacteria. Activity data at a concentration of 25 mg/mL are presented in

Table 18. Antibacterial activity of compounds 33a–g.

Entry	Compound	R	Zone of Inhibition (mm)
			S. aureus	S. epidermidis	P. aeruginosa	E. coli
1	33a	H	18	19	11	22
2	33b	2-F	20	19	11	18
3	33c	2-I	16	15	13	-
4	33d	2-Cl	20	19	-	-
5	33e	3-Cl	16	15	-	15
6	33f	4-Cl	20	18	-	17
7	33g	4-NO2	24	23	-	15

. The synthesized compounds exhibited moderate to good antibacterial activity against these bacterial strains.

The same group of authors also obtained another series of benzimidazole derivatives containing a 1,3,4-thiadiazole ring (34a–e, Figure 33) [72]. The compounds were tested for antibacterial activity against Gram-positive (Staphylococcus aureus, Staphylococcus epidermidis) and Gram-negative (Pseudomonas aeruginosa, Escherichia coli) bacteria. Biological activity data, represented by zones of inhibition measured at a concentration of 100 mg/mL, are presented in

Table 19. Antibacterial activity of compounds 34a–e.

Entry	Compound	R	Zone of Inhibition (mm)
			S. aureus	S. epidermidis	P. aeruginosa	E. coli
1	34a	H	14	12	17	15
2	34b	2-Br	13	12	14	15
3	34c	2-Cl	11	13	20	16
4	34d	3-NO2	14	0	11	11
5	34e	3,5-diNO2	21	11	27	18

.

Garg and coworkers obtained derivatives of 1,3,4-thiadiazole attached to 2,3-disubstituted thiazolidinones (35a–j, Figure 34) [73]. The compounds were tested for antibacterial activity against Gram-positive (Staphylococcus aureus, Streptococcus pyogenes) and Gram-negative (Pseudomonas aeruginosa, Escherichia coli) bacteria. The results of the zone of inhibition assay at a concentration of 250 μg/mL are presented in

Table 20. Antibacterial activity of compounds 35a–j.

Entry	Compound	R	Zone of Inhibition (mm)
			S. aureus	S. pyogenes	P. aeruginosa	E. coli
1	35a	H	21	20	22	20
2	35b	N(CH3)2	23	24	26	22
3	35c	(OCH3)2	20	21	22	19
4	35d	Cl	17	17	21	18
5	35e	NO2	14	15	16	15
6	35f	OH	21	23	23	22
7	35g	OCH3	19	21	20	20
8	35h	CH3	20	18	19	19
9	35i	NH2	24	25	26	26
10	35j	F	20	19	19	18
11	Ampicillin	-	25	25	26	24

. Biological screening showed that all tested compounds were active against all strains. Some of the compounds—35a (R = H), 35b (R = N(CH₃)₂), 35f (R = OH), and 35i (R = NH₂)—exhibited activity comparable to the standard drug Ampicillin. The authors concluded that modifying the substitution at the distal phenyl ring attached to the thiazolidinone at the 2 position enhances antimicrobial activity. This effect is likely attributable to increased lipophilicity, which facilitates permeation through microbial lipid membranes. Overall, structural features, such as thiazolidinone–thiadiazole conjugation, appropriate aromatic substitution, and lipophilic character, contribute to superior antimicrobial potency compared with reference drugs.

Weaam prepared three metal complexes containing a 2,5-dihydrazinyl-1,3,4-thiadiazole ligand coordinated to chromium, cobalt, and copper atoms (36, Figure 35) [74]. The compounds were tested for antibacterial activity against Escherichia coli and Staphylococcus aureus. The results obtained from the analysis are presented in

Table 21. Antibacterial activity of compound 36 and its complexes.

Entry	Compound	Zone of Inhibition (mm)
		E. coli	S. aureus
1	36	11	22
2	Cr(36)2Cl2]Cl]	21	26
3	[Co(36)2Cl2]Cl	28	15
4	[Cu(36)Cl2]	32	27
5	Ciprofloxacin	30	25

. The synthesized complexes exhibited biological activity comparable to that of Ciprofloxacin, which was used as the standard.

Gidwani’s group obtained three derivatives of 2-amino-1,3,4-thiadiazole (37–39, Figure 36) [75]. The compounds were tested for antibacterial activity against Gram-positive (Bacillus subtilis) and Gram-negative (Escherichia coli) bacteria. Compounds 37 and 38 exhibited MIC values of 1000 μg/mL against Bacillus subtilis, whereas among the tested compounds only 38 showed activity against Escherichia coli, also with an MIC of 1000 μg/mL. None of the synthesized compounds demonstrated superior activity compared to the standard drug Ciprofloxacin (MIC = 25 μg/mL) against the tested microbial strains.

Nawar et al. synthesized 1,3,4-thiadiazole derivatives containing the Amoxicillin scaffold (40a–g, Figure 37) [76]. The compounds were tested for antibacterial activity against Gram-negative (Proteus mirabilis and Escherichia coli) and Gram-positive (Mycobacterium tuberculosis) bacteria (

Table 22. Antibacterial activity of compounds 40a–g.

Entry	Compound	R	Zone of Inhibition in (mm)
			E. coli	M. tuberculosis	P. mirabilis
1	40a	OH	24	18	19
2	40b	NO2	18	12	17
3	40c	Cl	22	16	15
4	40d	OCH3	17	15	18
5	40e	CH3	12	10	11
6	40f	N(CH3)2	16	14	12
7	40g	OC2H5	21	19	14
8	Amoxicillin	-	20	17	18

) and exhibited considerable zones of inhibition. The best result was observed for derivative 40a, containing a hydroxyl group at the R site, which demonstrated greater activity than Amoxicillin.

Weaam also obtained metal complexes containing the 1,3,4-thiadiazole ligand (41, Figure 38) coordinated with iron, nickel, and copper atoms [77]. The complexes were tested for antibacterial activity against Escherichia coli and Staphylococcus aureus. The study data are presented in

Table 23. Antibacterial activity of compound 41 and its complexes.

Entry	Compound	Zone of Inhibition (mm)
		E. coli	S. aureus
1	41	13	15
2	[Fe(41)2Cl2]Cl	31	29
3	[Ni(41)Cl2]	20	27
4	[Cu(41)Cl2]	33	23
5	Ciprofloxacin	30	25

. The best activity against Escherichia coli was observed for the copper complex, while the iron and nickel complexes were most effective against Staphylococcus aureus. All of the obtained complexes exhibited greater antibacterial activity compared to the free 1,3,4-thiadiazole ligand.

Kaur et al. obtained a range of sulfonamides containing a 1,3,4-thiadiazole moiety (42a–ac, Figure 39) [78]. These compounds were screened for antibacterial activity against Enterococcus faecium. The MIC values for each of the tested derivatives are presented in

Table 24. Antibacterial activity of compounds 42a–ac.

Entry	Compound	MIC (μg/mL)	Entry	Compound	MIC (μg/mL)
1	42a	1	16	42p	0.25
2	42b	1	17	42q	0.06
3	42c	0.25	18	42r	0.06
4	42d	0.5	19	42s	0.06
5	42e	0.125	20	42t	0.007
6	42f	0.015	21	42u	0.06
7	42g	0.015	22	42v	1
8	42h	2	23	42w	1
9	42i	0.5	24	42x	1
10	42j	0.25	25	42y	8
11	42k	0.25	26	42z	2
12	42l	0.25	27	42aa	>64
13	42m	0.25	28	42ab	>64
14	42n	2	29	42ac	>64
15	42o	0.25	30	Acetazolamide	2

. The authors noted a correlation between lipophilicity and activity; the greater the lipophilicity, the stronger the activity against Enterococcus faecium. Biological screening revealed that many of the synthesized compounds exhibited high antibacterial activity compared to the standard drug Acetazolamide. The most potent derivatives against Enterococcus faecium were 42t, containing a 3-cyclohexylpropanoyl group at the R site, 42f, bearing a 3-methylbutanoyl group at the same position, and 42g, with a heptanoyl group, with MIC values of 0.007, 0.015 and 0.015 μg/mL, respectively. In the search for correlations between the structure of the synthesized sulfonamides and their biological activity, the authors found that in addition to lipophilic groups (alkyl and cycloalkyl), chain extension by a methylene linker can also dramatically increase potency. In contrast, the presence of heteroatoms in pendant groups was found to reduce activity.

Kracz et al. obtained derivatives of 1,3,4-thiadiazoles and their zinc complexes (43–47, Figure 40) [79]. The antibacterial properties against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria were evaluated by determining the MIC values (

Table 25. Antibacterial activity of compounds 43–45 and their complexes 46 and 47.

Entry	Compound	MIC (μg/mL)
		S. aureus	P. aeruginosa	E. coli
1	43	500	-	1000
2	44	-	-	-
3	45	-	-	-
4	46	500	-	1000
5	47	500	-	1000

). The tested compounds exhibited low antibacterial efficacy. The study revealed that acylation of the amino and hydroxyl groups leads to a decrease in antibacterial activity.

Abdel-Motaal et al. obtained a benzimidazole-2-yl derivative of 1,3,4-thiadiazole containing a furan-2-yl substituent (48, Figure 41) [80]. The compound was tested for antibacterial activity against Gram-positive bacteria (Staphylococcus aureus), Gram-negative bacteria (Escherichia coli), and bacterial spores (Bacillus pumilus). The study demonstrated high activity of derivative 48 against the tested microorganisms, with zones of inhibition of 18.96 mm for Staphylococcus aureus, 18.20 mm for Bacillus pumilus, and 17.33 mm for Escherichia coli.

Scheme 1 thiadiazole moiety (49a–i, 50a–i, 51a–h, 52a–h, Figure 42) [81]. The compounds were tested against two bacterial strains: Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola. The EC₅₀ values for the tested compounds are presented in

Table 26. Antibacterial activity of compounds 49a–i, 50a–i, 51a–h, and 52a–h.

Entry	Compound	EC50 (μg/mL)		Entry	Compound	EC50 (μg/mL)
		Xoo	Xoc			Xoo	Xoc
1	49a	51.02	51.81	18	50i	31.78	26.54
2	49b	55.9	63.50	19	51a	100.39	107.15
3	49c	54.78	62.12	20	51b	107.8	118.91
4	49d	40.6	42.53	21	51c	61.66	61.20
5	49e	47.6	86.29	22	51d	190.41	163.17
6	49f	95.37	64.08	23	51e	182.31	167.98
7	49g	51.73	72.13	24	51f	175.29	176.67
8	49h	48.63	47.62	25	51g	138.08	140.29
9	494i	44.45	45.80	26	51h	143.93	147.33
10	50a	51.34	60.72	27	52a	16.03	27.69
11	50b	78.94	77.23	28	52b	28.47	36.47
12	50c	38.74	46.97	29	52c	3.14	8.83
13	50d	40.52	48.17	30	52d	51.83	70.02
14	50e	33.73	40.23	31	52e	31.13	44.20
15	50f	88.08	89.66	32	52f	41.31	66.12
16	50g	80.52	65.66	33	52g	44.66	44.59
17	50h	33.25	35.49	34	52h	59.56	61.33

. The strongest inhibitory activity against Xanthomonas oryzae pv. oryzae was exhibited by compounds 50c, 50e, 50h, 50i, 52a, 52b, and 52c, with EC₅₀ values of 38.74, 33.73, 33.25, 31.78, 16.03, 28.47, and 3.14 μg/mL, respectively. Against Xanthomonas oryzae pv. oryzicola, the most active compounds were 50h, 50i, 52a, 52b, and 52c, with EC₅₀ values of 35.49, 26.54, 27.69, 36.47, and 8.83 μg/mL, respectively. Among all tested compounds, 52c, bearing an allyl group at the R¹ site and a 2-chloroethyl group at the R² site, exhibited the strongest inhibitory activity against both strains. The results indicated that in the case of thiol derivatives (49a–i, 50a–i), modifications to the vanillin substituents (R¹) had little impact on antibacterial activity. However, substituting hydrogen at the R² position with an alkyl group significantly reduced activity, indicating that thiol derivatives (50e, 50h, 50i) are generally more potent than their corresponding thioether analogues (51a–h). Replacement of sulfur with a sulfone group enhanced activity for compounds bearing the same vanillin and R¹ and R² groups, suggesting that sulfone derivatives (52a–h) may interact covalently or through hydrogen bonding with protein targets, thereby leading to stronger antibacterial effects.

Ali et al. obtained 1,3,4-thiadiazole derivatives of Resveratrol (53a–d, Figure 43) [82]. The compounds were tested for antibacterial activity against Gram-positive (Staphylococcus aureus, Streptococcus pyogenes) and Gram-negative (Escherichia coli, Klebsiella pneumoniae) bacteria. The zones of inhibition for the compounds, measured at a concentration of 500 μg/mL, are presented in

Table 27. Antibacterial activity of compounds 53a–d.

Entry	Compound	R	Zone of Inhibition (mm)
			S. aureus	S. pyougenes	K. pneumoniae	E. coli
1	53a	H	15	6	8	17
2	53b	C7H15	10	10	10	12
3	53c	C3H7	15	8	10	13
4	53d	C6H5	19	0	0	12

. The best results were obtained for compound 53c, bearing propyl substituents, and 53d, substituted with phenyl groups, which exhibited strong activity against Staphylococcus aureus. The unsubstituted derivative 53a showed notable activity against Escherichia coli.

Mahmoud et al. synthesized a series of benzimidazole derivatives containing a 1,3,4-thiadiazole ring (54a, 54b, 55a, 55b, Figure 44) [83]. The synthesized compounds were tested for antibacterial activity against Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, and Escherichia coli. Compounds 54a and 54b exhibited very good activity against all tested bacteria, with zones of inhibition ranging from 18 to 23 mm at a concentration of 100 mg/mL. Compound 55a (R = OH) demonstrated excellent activity against Escherichia coli (25 mm) and good activity against Staphylococcus aureus (14 mm) and Bacillus subtilis (19 mm), also at a concentration of 100 mg/mL.

Lungu et al. obtained a series of homodrimane sesquiterpene–thiadiazole hybrid compounds (56a–e, Figure 45) [84]. The synthesized derivatives were tested for antibacterial activity against Gram-positive Bacillus subtilis and Gram-negative Pseudomonas aeruginosa bacterial strains. The determined minimum inhibitory concentration (MIC) values revealed that compound 56a, bearing a mercapto substituent at the 2 position of the 1,3,4-thiadiazole ring, and compound 56c, containing an allylamino substituent, exhibited promising non-selective antibacterial activity, with MIC values of 0.094 and 0.5 µg/mL, respectively.

Pardeshi’s group synthesized a series of benzamide–thiadiazole-based derivatives substituted at the benzamide fragment with various aryl groups (57a–l, Figure 46) [85]. The compounds were tested for antibacterial activity against Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Pseudomonas aeruginosa (

Table 28. Antibacterial activity of compounds 57a–l.

Entry	Compound	R	Zone of Inhibition (mm)
			S. aureus	B. subtilis	E. coli	P. aeruginosa
1	57a	H	14	13	15	14
2	57b	4-CH3	12	13	12	15
3	57c	4-OCH3	11	12	11	10
4	57d	3-CH3	15	14	16	14
5	57e	3-OCH3	15	16	15	14
6	57f	4-C2H5	12	11	12	14
7	57g	3-CH3-4-OCH3	14	13	14	12
8	57h	4-Cl-3-CH3	11	11	13	12
9	57i	4-Cl	12	12	11	12
10	57j	4-Br	10	11	12	11
11	57k	4-Cl-2-CH3	13	14	15	13
12	57l	4-Br-2-CH3	15	13	14	12
13	Streptomycin	-	17	18	20	18

). The synthesized derivatives exhibited good antibacterial properties, and the standard drug Streptomycin was used for comparative purposes.

Brahimi and coworkers obtained 1,3,4-thiadiazole derivatives of castor oil extract (58a, 58b, Figure 47) [86]. The synthesized compounds were tested for their in vitro antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus, Enterococcus faecalis, Bacillus cereus) and Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli, Klebsiella planticola, Salmonella, Proteus vulgaris) using nutrient agar medium (

Table 29. Antibacterial activity of compounds 58a,b.

Entry	Compound	Zone of Inhibition in mm and (MIC) in µg/mL
		SA	EF	BC	PA	EC	KP	S	PV
1	58a	10 (6.25)	12 (25)	-	-	8 (25)	10 (25)	10 (6.25)	8 (25)
2	58b	8 (50)	8 (50)	7 (100)	7 (100)	-	10 (25)	10 (25)	-
3	Ampicillin	25	8	32	15	20	32	18	22

SA: Staphylococcus aureus; EF: Enterococcus faecalis; BC: Bacillus cereus; PA: Pseudomonas aeruginosa; EC: Escherichia coli; KP: Klebsiella planticola; S: Salmonella; PV: Proteus vulgarus.

). The compounds exhibited moderate antimicrobial properties, with the best result observed for derivative 58a, containing a mercapto group adjacent to the 1,3,4-thiadiazole ring, which showed good activity against Enterococcus faecalis (12 mm) and outperformed the standard drug Ampicillin.

2.1.2. Bicyclic 1,3,4-Thiadiazole Derivatives

Another notable group of derivatives exhibiting antibacterial activity comprises fused systems in which the 1,3,4-thiadiazole ring is incorporated into a bicyclic structure. Typical examples include fusion with aromatic or heteroaromatic rings, such as benzene, pyrazole, pyridine, or pyrrole.

Parrino’s group obtained a series of 1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one derivatives (59a–v, Figure 48) [87]. The compounds were tested for biological activity against Gram-positive pathogens Staphylococcus aureus and Enterococcus faecalis and Gram-negative pathogens Pseudomonas aeruginosa and Escherichia coli. The best results against Staphylococcus aureus were observed for derivatives 59e (R = F, R¹ = H, R² = C₆H₅, MIC = 50 μg/mL), 59f (R = H, R¹ = CH₃, R² = C₆H₅, MIC = 100 μg/mL), 59k (R = H, R¹ = H, R² = CH₃, MIC = 50 μg/mL), and 59l (R = H, R¹ = CH₃, R² = CH₃, MIC = 50 μg/mL). Regarding activity against Enterococcus faecalis, the most promising compounds were 59j (R = F, R¹ = CH₃, R² = C₆H₅, MIC = 50 μg/mL) and 59k (R = H, R¹ = H, R² = CH₃, MIC = 25 μg/mL).

Mahdavi’s group synthesized [1,2,4]triazolo[3,4-b][1,3,4]thiadiazole derivatives (60a–n, Figure 49) [88]. All compounds were tested against Proteus mirabilis and showed weak activity (MIC = 256 μg/mL) compared to the standard drug Ciprofloxacin (MIC = 0.25 μg/mL).

Wu et al. synthesized a series of quinazolin-4(3H)-one derivatives containing the 1,2,4-triazolo[3,4-b][1,3,4]thiadiazole moiety (61a–ai, Figure 50) [89]. The compounds were tested for biological activity against four bacterial strains: Xanthomonas axonopodis pv. citri, Xanthomonas oryzae pv. oryzicola, Xanthomonas oryzae pv. oryzae, and Pseudomonas syringae pv. actinidiae at a concentration of 100 µg/mL, employing the commercial antibacterial agent BMT as the positive control (

Table 30. Antibacterial activity of compounds 61a–ai.

Entry	Compound	Inhibition Rate (%)
		Xac	Xoc	Xoo	Psa
1	61a	77.3	27.1	30.3	87.5
2	61b	78.3	51.1	39.5	42.0
3	61c	69.1	52.7	37.8	41.5
4	61d	66.0	42.4	53.1	49.9
5	61e	63.9	98.4	87.6	32.1
6	61f	74.9	99.3	100	38.6
7	61g	78.9	93.0	100	47.7
8	61h	72.9	44.0	92.8	34.1
9	61i	76.9	23.3	31.7	27.1
10	61j	78.8	21.9	34.9	55.0
11	61k	69.4	47.2	32.2	24.6
12	61l	71.3	49.1	15.7	43.9
13	61m	80.9	15.1	26.0	52.3
14	61n	83.9	47.1	28.4	27.7
15	61o	74.6	58.6	20.9	32.1
16	61p	72.8	53.3	96.1	34.9
17	61q	79.4	13.8	6.3	57.7
18	61r	59.6	40.3	13.7	35.4
19	61s	68.5	33.2	21.5	28.2
20	61t	69.2	72.8	74.9	28.7
21	61u	66.7	18.6	34.0	33.2
22	61v	48.9	98.6	90.9	28.4
23	61w	53.2	73.1	17.5	51.0
24	61x	71.2	60.7	10.5	47.9
25	61y	70.7	46.3	100	34.2
26	61z	76.9	26.6	18.8	24.2
27	61aa	75.2	43.9	13.7	34.3
28	61ab	70.1	42.9	17.0	38.1
29	61ac	76.3	38.1	6.7	31.1
30	61ad	70.4	99.9	97.6	31.0
31	61ae	67.6	32.5	25.4	25.0
32	61af	21.9	100	98.1	38.4
33	61ag	53.1	99.7	95.1	45.4
34	61ah	48.7	87.2	82.5	37.5
35	61ai	74.9	77.7	93.3	47.0
36	BMT	58.8	56.6	60.0	64.5

).

Kamoutsis et al. obtained a range of fused 1,3,4-thiadiazole arrangements (62a–s, Figure 51) [90]. The obtained compounds were tested for activity against Bacillus cereus, Staphylococcus aureus, Listeria monocytogenes, Pseudomonas aeruginosa, Escherichia coli, and Salmonella typhimurium. The results of antibacterial analyses are presented in

Table 31. Antibacterial activity of compounds 62a–s.

Entry	Compound	MBC (μg/mL)
		BC	SA	LM	PA	EC	ST
1	62a	20	40	20	200	10	20
2	62b	20	20	20	40	10	20
3	62c	20	40	10	20	10	20
4	62d	20	0.20	0.020	0.040	0.020	0.020
5	62e	20	36	40	20	10	40
6	62f	20	20	36	40	10	10
7	62g	20	40	20	20	20	20
8	62h	36	40	40	73	20	20
9	62i	40	40	20	40	10	20
10	62j	20	20	40	60	10	10
11	62k	23	40	20	67	10	20
12	62l	40	37	20	150	10	10
13	62m	20	40	20	200	47	20
14	62n	20	60	80	80	13	20
15	62o	10	40	20	60	10	20
16	62p	20	80	20	80	10	20
17	62q	20	40	20	80	10	20
18	62r	20	40	20	60	10	20
19	62s	10	20	20	40	10	10

BC: Bacillus cereus; SA: Staphylococcus aureus; LM: Listeria monocytogenes; PA: Pseudomonas aeruginosa; EC: Escherichia coli; ST: Salmonella typhimurium.

. The best results were obtained for the 62s derivative, containing a phenyl group at the R site, characterized by MBC = 10–40 μg/mL and MIC = 5–20 μg/mL.

Bhadraiah et al. obtained bicyclic 1,3,4-thiadiazolo[3,2-α]pyrimidine analogues (63a–i, Figure 52) [91]. The compounds were tested for bactericidal activity against Gram-positive (Bacillus cereus, Staphylococcus aureus) and Gram-negative (Escherichia coli, Klebsiella pneumoniae) bacterial strains. Biological tests revealed that derivative 63e, containing two methoxy groups at the R¹ and R² sites, exhibited the best activity among the synthesized compounds, while derivatives 63b and 63h showed moderate properties (

Table 32. Antibacterial activity of compounds 63a–i.

Entry	Compound	R1	R2	MBC (μg/mL)
				B. cereus	S. aureus	E. coli	K. pneumoniae
1	63a	H	H	280	260	250	265
2	63b	H	OCH3	130	140	145	135
3	63c	H	Cl	250	255	260	240
4	63d	OCH3	H	150	165	210	195
5	63e	OCH3	OCH3	130	145	120	135
6	63f	OCH3	Cl	150	140	160	210
7	63g	Cl	H	280	290	275	270
8	63h	Cl	OCH3	185	215	200	210
9	63i	Cl	Cl	240	260	250	250

).

Jin’s group synthesized a series of fused imidazo[2,1-b][1,3,4]thiadiazole derivatives (64a–g, 65a–g, 66a–g, Figure 53) [92]. The obtained compounds were tested for antibacterial activity against Staphylococcus aureus (4220), Staphylococcus aureus (209), Escherichia coli, Pseudomonas aeruginosa, Methicillin-resistant Staphylococcus aureus (3167), and Quinolone-resistant Staphylococcus aureus (3505). The tested compounds exhibited no or low activity against Gram-positive and Gram-negative bacteria. However, selected derivatives demonstrated activity against multi-drug-resistant Gram-positive strains. Specifically, compounds 64c (R = 3-F, MIC₅₀ = 12.79 μg/mL), 64e (R = 2-CH₃, MIC₅₀ = 17.84 μg/mL), 65e (R = 2-CH₃, MIC₅₀ = 15.81 μg/mL), and 66a (R = H, MIC₅₀ = 18.49 μg/mL) showed moderate activity against Methicillin-resistant Staphylococcus aureus. In the case of Quinolone-resistant Staphylococcus aureus, moderate activity was observed for fluorine- or methyl-substituted compounds 64c–f (MIC₅₀ = 9.06–19.09 μg/mL), the unsubstituted compound 65a (MIC₅₀ = 16.69 μg/mL), the fluorine-substituted compound 65c (MIC₅₀ = 17.24 μg/mL), the methyl-substituted compound 65e (MIC₅₀ = 18.02 μg/mL), and fluorine- or methyl-substituted compounds 66c–g (MIC₅₀ = 12.98–17.44 μg/mL).

2.1.3. Multi-Substituted 1,3,4-Thiadiazole Derivatives

Highly substituted 1,3,4-thiadiazole derivatives represent a less commonly explored class of compounds.

Abdel-Aziem and coworkers obtained two coumarin-linked thiadiazole derivatives (67a,b, Figure 54) [93]. The newly synthesized derivatives were initially investigated for their antibacterial activity. Six pathogenic microbes were selected for assessment: Bacillus pumilis and Streptococcus faecalis, representing Gram-positive bacteria, and Escherichia coli and Enterobacter cloacae, representing Gram-negative bacteria. The standard antibacterial drugs Penicillin G (for Gram-positive bacteria) and Ciprofloxacin (for Gram-negative bacteria) were used as references (

Table 33. Antibacterial activity of compounds 67a,b.

Entry	Compound	Zone of Inhibition (mm)
		B. pumilis	S. faecalis	E. coli	E. cloacae
1	67a	19	21	20	17
2	67b	21	16	19	18
3	Penicillin G	25	19	-	-
4	Ciprofloxacin	-	-	30	27

). The obtained compounds exhibited moderate biological activity in comparison with the standard drugs.

Dai et al. obtained 5-sulfonyl-1,3,4-thiadiazole-substituted flavonoids (68a–ah, Figure 55) [94]. The compounds were tested for antibacterial activity against two bacterial strains: Xanthomonas axonopodis pv. citri and Xanthomonas oryzae pv. oryzae. The inhibition rates at a concentration of 50 μg/mL are presented in

Table 34. Antibacterial activity of compounds 68a–ah.

Entry	Compound	R1	R2	R3	Inhibition Rate (%)
					Xac	Xoo
1	68a	H	CH3	CH3	0	93.91
2	68b	6-Br	CH3	CH3	48.28	70.30
3	68c	6-F	CH3	CH3	22.99	88.34
4	68d	6-Cl	CH3	CH3	43.03	91.57
5	68e	6-CH3	CH3	CH3	43.10	82.61
6	68f	7-Br	CH3	CH3	46.51	80.95
7	68g	H	CH2CH3	CH3	27.30	97.01
8	68h	6-Br	CH2CH3	CH3	67.96	83.62
9	68i	6-F	CH2CH3	CH3	46.98	98.99
10	68j	6-Cl	CH2CH3	CH3	59.99	93.42
11	68k	6-CH3	CH2CH3	CH3	63.43	97.62
12	68l	H	CH2CH2CH3	CH3	69.68	65.17
13	68m	6-Br	CH2CH2CH3	CH3	31.39	90.31
14	68n	6-F	CH2CH2CH3	CH3	29.60	98.14
15	68o	6-Cl	CH2CH2CH3	CH3	57.04	62.75
16	68p	6-CH3	CH2CH2CH3	CH3	36.71	61.30
17	68q	7-Br	CH2CH2CH3	CH3	40.52	48.02
18	68r	H	CH3CHCH3	CH3	44.11	97.22
19	68s	6-Br	CH3CHCH3	CH3	48.99	37.73
20	68t	6-F	CH3CHCH3	CH3	18.75	98.06
21	68u	6-Cl	CH3CHCH3	CH3	37.57.	45.04
22	68v	7-Br	CH3CHCH3	CH3	57.26	9.64
23	68w	6-F	allyl	CH3	57.40	99.39
24	68x	6-CH3	allyl	CH3	39.94	1.49
25	68y	H	Bn	CH3	45.19	36.92
26	68z	6-Br	Bn	CH3	34.34	47.66
27	68aa	6-F	Bn	CH3	29.02	46.53
28	68ab	6-Cl	Bn	CH3	66.52	83.21
29	68ac	6-CH3	Bn	CH3	29.09.	38.58
30	68ad	7-Br	Bn	CH3	42.31	54.08
31	68ae	6-F	CH3	CH2CH3	55.82	70.22
32	68af	6-Cl	CH3	CH2CH3	87.43	78.29
33	68ag	H	i-Pr	CH2CH3	38.43	96.81
34	68ah	6-CH3	i-Pr	CH2CH3	46.48	8.80
35	Bismerthiazole	-	-	-	35.85	31.6

. Compounds 68a (R¹ = H, R² = CH₃, R³ = CH₃), 68c (R¹ = 6-F, R² = CH₃, R³ = CH₃), 68g (R¹ = H, R² = CH₂CH₃, R³ = CH₃), 68i (R¹ = 6-F, R² = CH₂CH₃, R³ = CH₃), 68m (R¹ = 6-Br, R² = CH₂CH₂CH₃, R³ = CH₃), and 68n (R¹ = 6-F, R² = CH₂CH₂CH₃, R³ = CH₃) exhibited remarkable antibacterial activity against Xanthomonas oryzae pv. oryzae, with EC₅₀ values below 10 μg/mL, which were superior to that of Bismerthiazol (70.89 μg/mL). Preliminary structure–activity analysis indicated that the introduction of substituents at the 6 position of the flavonoid core significantly affected antibacterial activity. Introducing small groups, such as hydrogen or fluorine, at this position markedly increased potency. Alkyl substitutions on the sulfone group had minimal impact, whereas the presence of a benzyl group reduced antibacterial activity.

Gomha et al. obtained a series of 1,4-dihydropyridine hybrids with 1,3,4-thiadiazole (69a–h, Figure 56) [95]. The compounds were assayed in vitro for their antibacterial activity against Gram-positive bacteria (Staphylococcus aureus, Staphylococcus epidermidis, Bacillus subtilis, Streptococcus pyogenes) and Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, Salmonella typhimurium) at a concentration of 30 μg/mL; Gentamicin and Ampicillin were used as reference drugs (

Table 35. Antibacterial activity of compounds 69a–h.

Entry	Compound	Zone of Inhibition (mm)
		SA	SE	BS	SP	PA	EC	KP	ST
1	69a	19.3	19.4	22.7	-	-	22.3	18.2	20.4
2	69b	18.1	18.7	20.1	-	-	19.3	19.2	18.4
3	69c	19.4	22.7	18.3	-	-	25.2	21.2	20.6
4	69d	19.9	22.0	17.6	-	-	23.1	22.4	23.0
5	69e	23.6	22.4	25.5	-	-	26.9	26.3	27.2
6	69f	22.4	20.9	23.8	-	-	24.3	22.5	26.3
7	69g	20.1	19.2	24.3	-	-	23.7	19.3	21.6
8	69h	22.1	19.8	24.3	-	-	24.2	21.3	23.3
9	Ampicillin	23.7	22.4	32.4	24.5	-	-	-	-
10	Gentamicin	-	-	-	-	22.3	25.4	22.6	23.3

SA: Staphylococcus aureus; SE: Staphylococcus epidermidis; BS: Bacillus subtilis; SP: Streptococcus pyogenes; PA: Pseudomonas aeruginosa; EC: Escherichia coli; KP: Klebsiella pneumoniae; ST: Salmonella typhimurium.

). Derivative 69e, bearing a 2-oxoindolin-3-ylidene moiety, indicated higher inhibitory activity against all of the examined bacteria than the reference standards.

Rashdan’s group synthesized a series of tri-substituted 1,3,4-thiadiazole derivatives (70a–e, Figure 57) [96]. The compounds were evaluated for antimicrobial activity against Escherichia coli, Pseudomonas aeruginosa, Proteus vulgaris, Bacillus subtilis, and Staphylococcus aureus (

Table 36. Antibacterial activity of compounds 70a–e.

Entry	Compound	MIC (μg/mL)
		E. coli	P. aeruginosa	P. vulgaris	B. subtilis	S. aureus
1	70a	20	40	20	10	20
2	70b	-	-	-	40	80
3	70c	40	160	80	40	20
4	70d	160	-	-	80	160
5	70e	-	-	-	-	-
6	Ciprofloxacin	5	7	1.25	2.5	1.25

). Compound 70a, containing phenylamino group at the R² site, demonstrated notable broad-spectrum efficacy across all tested strains, with low effective concentrations ranging from 20 to 40 µg/mL.

The same research group also obtained another series of 1,3,4-thiadiazole derivatives (71a–f, Figure 58) [97]. These compounds were tested against four pathogenic bacteria: Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and Bacillus subtilis (

Table 37. Antibacterial activity of compounds 71a–f.

Entry	Compound	Zone of Inhibition (mm)
		K. pneumoniae	P. aeruginosa	S. aureus	B. subtilis
1	71a	19	20	13	15
2	71b	13	15	10	12
3	71c	16	18	13	15
4	71d	15	17	12	14
5	71e	20	22	15	17
6	71f	22	25	18	20
7	Ciprofloxacin	18	20	15	17

). Compound 71f, containing a nitro group at the R¹ site and a phenylamino group at the R² site, exhibited the strongest antibacterial activity against all evaluated strains.

2.2. Antifungal Activity

2.2.1. Disubstituted 1,3,4-Thiadiazole Derivatives

As previously noted, Mao et al. synthesized a series of derivatives containing a 1,3,4-thiadiazole thione moiety (6a–p, Figure 7) [46]. In addition to antibacterial evaluation, the compounds were assessed for antifungal activity against Sclerotinia sclerotiorum, Rhizoctonia solani, Magnaporthe oryzae, and Colletotrichum gloeosporioides at a concentration of 50 μg/mL (

Table 38. Antifungal activity of compounds 6a–p.

Entry	Compound	R	Inhibition Rate (%)
			S. sclerotiorum	R. solani	M. oryzae	C. gloeosporioides
1	6a	H	32.14	30.65	28.57	28.27
2	6b	2-F	90.48	72.32	55.36	52.98
3	6c	2-Cl	67.85	40.78	15.70	16.12
4	6d	2-Br	33.04	23.21	29.46	26.19
5	6e	2-CH3	17.81	52.85	19.11	33.22
6	6f	2-OCH3	50.00	38.69	31.25	25.89
7	6g	2-OCF3	<10	31.78	34.13	24.34
8	6h	3-F	82.14	57.44	38.99	45.83
9	6i	3-CH3	83.63	52.98	31.85	38.69
10	6j	3-OCH3	69.05	54.76	30.95	42.56
11	6k	3-OCF3	59.52	40.48	41.96	43.15
12	6l	4-F	32.14	39.88	26.49	35.42
13	6m	4-CH3	50.00	27.98	22.02	33.04
14	6n	4-OCH3	55.36	49.40	33.63	33.93
15	6o	4-OCF3	23.51	33.63	48.51	41.37
16	6p	4-NH2	80.06	31.55	22.62	31.85
17	Thifluzamide	-	72.92	98.51	25.89	26.49
18	Carbendazim	-	98.21	100	100	96.13

). Compound 6b, containing a fluoro substituent at the ortho position of the benzene ring, demonstrated excellent efficacy against S. sclerotiorum (EC₅₀ = 0.51 µg/mL), comparable to that of the commercial fungicide Carbendazim (EC₅₀ = 0.57 µg/mL). The authors concluded that derivatives containing electron-withdrawing substituents at the ortho position exhibited enhanced antifungal activity (e.g., 6b, R = 2-F), whereas meta-substituted derivatives showed primarily antibacterial potency (e.g., 6k, R = 3-OCF₃).

Hafidh et al. investigated hybrid silica gels incorporating 1,3,4-thiadiazole rings (7a, 7b, Figure 8) for their antimicrobial potential, including antifungal activity against Candida albicans [47]. The MIC values were 0.25 mg/mL for compound 7a, containing a (5-amino-1,3,4-thiadiazol-2-yl)amino moiety, and 0.5 mg/mL for 7b, containing a (5-mercapto-1,3,4-thiadiazol-2-yl)thio moiety, indicating relatively low antifungal efficacy in comparison with the reference agent Gentamicin (MIC = 7.81 µg/mL).

Baoyu Li et al. synthesized a series of derivatives incorporating a 1,3,4-thiadiazole ring (72a–y, Figure 59) [98]. The compounds were evaluated for antifungal activity against Physalospora piricola, Colletotrichum orbiculare, Cercospora arachidicola, Gibberella zeae, Alternaria solani, Rhizoctonia solani, Fusarium oxysporum, and Bipolaris maydis at a concentration of 50 μg/mL (

Table 39. Antifungal activity of compounds 72a–y.

Entry	Compound	Inhibition Rate (%)
		PP	CO	CA	GZ	AS	RS	FO	BM
1	72a	43	7	45	46	47	45	3	0
2	72b	89	80	70	67	70	74	69	74
3	72c	53	10	45	40	33	21	11	18
4	72d	42	0	85	83	77	18	6	3
5	72e	51	13	60	60	63	68	17	18
6	72f	74	50	55	62	57	39	40	41
7	72g	68	30	10	30	47	17	23	27
8	72h	42	7	70	25	43	13	14	21
9	72i	49	3	20	25	20	11	11	9
10	72j	74	27	45	49	57	45	26	24
11	72k	79	57	30	37	47	27	43	41
12	72l	30	0	25	19	17	16	9	0
13	72m	47	40	40	25	37	12	31	35
14	72n	79	47	45	43	33	16	31	32
15	72o	49	13	30	27	33	16	20	21
16	72p	70	40	45	57	53	62	34	32
17	72q	42	0	50	37	27	16	11	9
18	72r	70	53	70	49	47	19	46	41
19	72s	77	53	45	32	33	12	43	47
20	72t	42	13	35	35	33	12	11	24
21	72u	15	0	55	48	23	12	0	12
22	72v	42	7	55	40	63	33	9	18
23	72w	57	0	55	24	40	10	14	9
24	72x	51	17	45	27	27	10	11	24
25	72y	79	57	55	35	60	31	46	38
26	Boscalid	89	50	90	18	67	87	40	38

PP: Physalospora piricola; CO: Colletotrichum orbiculare; CA: Cercospora arachidicola; GZ: Gibberella zeae; AS: Alternaria solani; RS: Rhizoctonia solani; FO: Fusarium oxysporum; BM: Bipolaris maydis.

). Compound 72b, containing a 2-methylphenyl group in the 1,3,4-thiadiazole-amide fragment, showed broad-spectrum activity with high inhibition rates ranging from 67% to 89% across all tested phytopathogens. Among the tested 1,3,4-thiadiazoles, compound 72d, with a 4-methylphenyl substituent in the 1,3,4-thiadiazole-amide fragment, demonstrated particularly strong activity against Cercospora arachidicola, Gibberella zeae, and Alternaria solani, with inhibition rates of 85%, 83%, and 77%, respectively, all superior or comparable to those of the reference fungicide Boscalid. In the search for correlations between the structure of the synthesized compounds and their biological activity, the authors found that small groups at the ortho position of the terminal benzene ring (e.g., CH₃ or F) enhanced activity. The same trend was observed for bulky (e.g., tert-butyl) and electron-donating groups (e.g., OCH₃) at the para position. In contrast, halogens or electron-withdrawing groups (e.g., NO₂) generally reduced antifungal activity.

A series of 1,3,4-thiadiazole derivatives synthesized by Mehta and coworkers (8a–o, Figure 9 [48], initially tested for antibacterial properties, was also evaluated for antifungal activity against Candida albicans, Aspergillus niger, and Aspergillus clavatus.

Table 40. Antifungal activity of compounds 8a–o.

Entry	Compound	R	MIC (μg/mL)
			C. albicans	A. niger	A. clavatus
1	8a	H	500	500	500
2	8b	2-Br	1000	>1000	>1000
3	8c	4-Br	250	100	500
4	8d	2-Cl	1000	1000	1000
5	8e	4-Cl	100	>1000	100
6	8f	2-F	250	500	500
7	8g	3-F	500	1000	>1000
8	8h	4-F	1000	1000	1000
9	8i	2-OH	250	100	500
10	8j	4-OH	1000	100	1000
11	8k	4-CH3	500	1000	1000
12	8l	4-OCH3	250	>1000	>1000
13	8m	2-NO2	1000	500	500
14	8n	3-NO2	>1000	500	500
15	8o	4-NO2	1000	100	250
16	Nystatin	-	100	100	100
17	Griseofulvin	-	500	100	100

presents the minimum inhibitory concentration (MIC) values of the tested compounds. The strongest activity against Candida albicans was observed for compounds 8c (R = 4-Br), 8e (R = 4-Cl), 8f (R = 2-F), 8i (R = 2-OH), and 8l (R = 4-OCH₃), with MIC values of 250, 100, 250, 250, and 250 μg/mL, respectively. The two previously mentioned derivatives, 8c and 8i, as well as compounds 8j (R = 4-OH) and 8o (R = 4-NO₂), showed activity against Aspergillus niger with MIC values of 100 μg/mL, while compound 8e inhibited Aspergillus clavatus with an MIC of 100 μg/mL.

The previously discussed series of 1,3,4-thiadiazoles (9a–d, 10, Figure 10), synthesized by Danilova’s group [49], was also tested for antifungal activity against Fusarium oxysporum, Alternaria alternata, and Bipolaris sorokiniana. Among the tested compounds, only 5,5’-methylenebis(1,3,4-thiadiazol-2-amine) (9a) exhibited activity, showing inhibition against Alternaria alternata at a concentration of 200 μg/mL.

He et al. synthesized a series of indole derivatives incorporating a 1,3,4-thiadiazole ring (73a–x, Figure 60) [99]. Selected compounds were evaluated for antifungal activity against Botrytis cinerea, Tomato Botrytis cinerea, and Phomopsis sp., with EC₅₀ values presented in

Table 41. Antifungal activity of selected compounds 73a–x.

Entry	Compound	Pathogens	EC50 (μg/mL)
1	73b	Botrytis cinerea	2.7
2	73g	Botrytis cinerea	2.9
3	73o	Botrytis cinerea	5.2
4	Azoxystrobin	Botrytis cinerea	14.5
5	73d	Tomato Botrytis cinerea	3.5
6	73e	Tomato Botrytis cinerea	14.1
7	73v	Tomato Botrytis cinerea	25.9
8	Azoxystrobin	Tomato Botrytis cinerea	26.5
9	73b	Phomopsis sp.	6.4
10	73d	Phomopsis sp.	5.3
11	73o	Phomopsis sp.	7.8
12	Azoxystrobin	Phomopsis sp.	10.4

. Among them, compound 73b, containing a 2-methylphenyl group in the amide fragment, exhibited the highest level of activity against Botrytis cinerea, with an EC₅₀ of 2.7 μg/mL, surpassing the efficacy of the reference fungicide Azoxystrobin (EC₅₀ = 14.5 μg/mL). The authors observed that compounds unsubstituted at the indole fragment (73a–g, R¹ = H) generally exhibited stronger inhibition than those bearing halogen substituents (73h–x; R¹ = Br, Cl, F). The presence of electron-donating R² substituents on the benzene ring of the opposite amide group improved antifungal activity compared with electron-withdrawing substituents. Furthermore, it was noted that the parent indole structure displayed higher activity when the benzamide fragment carried an electron-donating substituent, particularly when R² = CH₃.

Xue and coworkers synthesized a series of chalcone derivatives containing a 1,3,4-thiadiazole moiety (74a–x, Figure 61) [100]. The compounds were tested against Rhizoctonia solani, Phomopsis sp., and Phytophthora capsici. Inhibition rates at a concentration of 100 μg/mL are summarized in

Table 42. Antifungal activity of compounds 74a–x.

Entry	Compound	Inhibition Rate (%)
		R. solani	Phomopsis sp.	P. capsici
1	74a	73.2	83.9	63.1
2	74b	59.4	82.6	39.5
3	74c	76.2	71.3	83.3
4	74d	83.7	91.3	73.0
5	74e	73.6	71.3	74.7
6	74f	78.2	81.3	41.2
7	74g	35.6	15.7	21.9
8	74h	72.8	60.9	82.8
9	74i	61.5	70.0	68.2
10	74j	54.0	59.1	50.6
11	74k	60.3	57.4	59.7
12	74l	67.4	80.0	75.5
13	74m	58.6	56.1	85.0
14	74n	46.4	60.9	84.1
15	74o	59.4	30.4	75.1
16	74p	71.6	82.6	77.7
17	74q	32.2	29.6	30.5
18	74r	53.1	75.2	60.1
19	74s	71.6	67.2	84.5
20	74t	73.2	39.1	85.0
21	74u	67.8	47.8	41.2
22	74v	52.7	39.1	71.7
23	74w	40.2	35.7	55.8
24	74x	73.6	75.7	82.4

. Bioactivity screening revealed that several derivatives exhibited notable antifungal activity with high levels of inhibition. Further evaluation showed that compound 74d, bearing 4-methylphenyl substituents at opposite terminal positions, had an EC₅₀ value of 14.4 μg/mL against Phomopsis sp., significantly outperforming the reference fungicides Azoxystrobin (32.2 μg/mL) and Fluopyram (54.2 μg/mL). In general, it was found that compounds with electron-donating groups at the R² or R¹ positions exhibited stronger activity than those with electron-withdrawing groups. It was also observed that derivatives containing an alkyl linker composed of four methylene groups (n = 4) were more active than their counterparts with three methylene groups (n = 3).

The previously described series of 1,3,4-thiadiazole-containing Schiff bases synthesized by Zahoor and his team (11a–l, Figure 11) [50] was also evaluated for antifungal activity against Alternaria alternata. Inhibition rates are summarized in

Table 43. Antifungal activity of compounds 11a–l.

Compound	11a	11b	11c	11d	11e	11f	11g	11h	11i	11j	11k	11l	Terinafine
Inhibition rate (%)	43.4	3.4	25.8	31.9	4.3	7.3	11.1	14.2	34.3	21.2	9.4	0	50.7

. Compounds 11a (R = 2-NO₂-4-CF₃C₆H₃), 11d (R = 4-Cl-3-OHC₆H₃), and 11i (R = 3,5-diFC₆H₃) showed notable antifungal efficacy, with inhibition levels of 43.4%, 31.9%, and 34.3%, respectively, compared to the reference drug Terinafine (50.7%).

Dai et al. synthesized a series of disubstituted 1,3,4-thiadiazole derivatives (75a–ag, Figure 62) [101]. The compounds were evaluated for antifungal activity against Botrytis cinerea, Alternaria solani, Rhizoctonia solani, Fusarium graminearum, and Colletotrichum orbiculare. In vitro inhibition rates at a concentration of 10 μg/mL are presented in

Table 44. Antifungal activity of compounds 75a–ag.

Entry	Compound	R1	R2	Inhibition Rate (%)
				B. cinerea	A. solani	R. solani	F. graminearum	C. orbiculare
1	75a	H	CH3	62.1	43.4	64.8	32.3	51.1
2	75b	6-Br	CH3	100	87.0	92.4	70.0	87.4
3	75c	6-F	CH3	34.4	43.7	50.4	28.7	43.8
4	75d	6-Cl	CH3	100	81.7	83.0	44.5	75.1
5	75e	6-CH3	CH3	59.3	41.3	53.7	28.7	48.6
6	75f	7-Br	CH3	11.0	32.3	75.4	38.3	13.3
7	75g	H	CH2CH3	100	65.6	85.6	48.7	100
8	75h	6-Br	CH2CH3	100	84.9	91.8	71.8	100
9	75i	6-F	CH2CH3	100	100	100	49.4	100
10	75j	6-Cl	CH2CH3	100	100	100	73.9	100
11	75k	6-CH3	CH2CH3	59.9	50.4	62.6	41.1	61.2
12	75l	H	Pr	64.3	53.9	76.3	36.1	77.4
13	75m	6-Br	Pr	100	71.4	91.2	79.0	93.2
14	75n	6-F	Pr	84.9	63.6	86.8	76.1	98.9
15	75o	6-Cl	Pr	82.7	69.2	100	59.4	95.4
16	75p	6-CH3	Pr	39.8	36.6	38.9	28.3	48.8
17	75q	7-Br	Pr	16.1	23.9	23.9	14.7	16.9
18	75r	H	i-Pr	49.1	79.1	89.1	40.6	100
19	75s	6-Br	i-Pr	100	98.5	100	65.6	100
20	75t	6-F	i-Pr	44.8	84.9	93.5	51.0	100
21	75u	6-Cl	i-Pr	100	66.2	100	75.6	100
22	75v	6-CH3	i-Pr	51.8	46.3	45.8	40.0	68.6
23	75w	7-Br	i-Pr	22.0	36.1	62.1	26.1	27.1
24	75x	H	allyl	85.3	59.0	93.9	35.0	83.3
25	75y	6-Cl	allyl	100	84.0	100	21.4	30.2
26	75z	6-CH3	allyl	24.9	19.0	57.4	36.2	32.8
27	75aa	7-Br	allyl	33.9	41.7	70.7	17.5	20.0
28	75ab	H	Bn	13.9	27.0	23.7	17.3	16.7
29	75ac	6-Br	Bn	13.6	39.2	73.6	54.6	39.0
30	75ad	6-F	Bn	20.2	25.4	31.7	40.1	33.1
31	75ae	6-Cl	Bn	13.7	5.3	12.9	14.7	5.1
32	75af	6-CH3	Bn	10.7	0	24.0	19.6	11.9
33	75ag	7-Br	Bn	6.8	0	14.1	9.5	5.1
34	Carbendazim	-	-	97.1	65.3	100	100	84.3
35	Boscalid	-	-	83.7	35.7	81.9	17.5	-

. Most flavonoid-based derivatives exhibited excellent broad-spectrum antifungal activity. Notably, the EC₅₀ values of several compounds against Rhizoctonia solani were below 4 μg/mL. Compounds 75m, with bromine at the 6 position of the chromone fragment and a propyl group in the sulfonamide part (EC₅₀ = 0.49 μg/mL), 75o, with chlorine at the 6 position of the chromone fragment and a propyl group in the sulfonamide part (EC₅₀ = 0.37 μg/mL), and 75s, with bromine at the 6 position of the chromone fragment and an isopropyl group in the sulfonamide part (EC₅₀ = 0.37 μg/mL), displayed the highest potency, surpassing that of the reference fungicide Carbendazim (EC₅₀ = 0.52 μg/mL). Structure–activity relationship analysis revealed that electron-withdrawing groups, such as bromine or chlorine, at the 6 position of the chromone fragment improved activity compared with methyl groups. For substituents located on the sulfone moiety, it was observed that large or branched groups also enhanced antifungal activity. The length of the alkyl chain (n) in the sulfonamide part was also important; longer chains (n = 4) in brominated derivatives improved activity compared with shorter counterparts (n = 3).

A series of 2,5-disubstituted 1,3,4-thiadiazole derivatives synthesized by Li and coworkers (12a–r, Figure 12) [51] was also evaluated for antifungal activity against Colletotrichum gloeosporioides, Nakazawaea ishiwadae, Gilbertella persicaria, and Fusarium sp. Inhibition rates at a concentration of 100 μg/mL are summarized in

Table 45. Antifungal activity of compounds 12a–r.

Entry	Compound	R1	R2	Inhibition Rate (%)
				C. gloeosporioides	N. ishiwadae	G. persicaria	Fusarium sp.
1	12a	CH3	CH3	62	74	85	65
2	12b	CH3	CH2CH3	83	74	81	59
3	12c	CH3	CH2CH2CH3	87	66	77	56
4	12d	CH3	CH2C6H5	85	75	79	74
5	12e	CH3	CH2C6H4-4-F	60	60	65	45
6	12f	CH3	CH2C6H4-4-Cl	76	62	72	49
7	12g	CH3	CH2C6H4-3-Cl	70	51	74	35
8	12h	CH3	CH2C6H4-2-Cl	56	46	64	34
9	12i	CH3	CH2C6H3-2,4-diCl	63	70	59	59
10	12j	CF3	CH3	64	74	75	57
11	12k	CF3	CH2CH3	79	55	65	43
12	12l	CF3	CH2CH2CH3	66	63	77	51
13	12m	CF3	CH2C6H5	68	34	60	23
14	12n	CF3	CH2C6H4-4-F	78	61	74	51
15	12o	CF3	CH2C6H4-4-Cl	74	71	72	63
16	12p	CF3	CH2C6H4-3-Cl	90	37	41	54
17	12q	CF3	CH2C6H4-2-Cl	77	51	74	39
18	12r	CF3	CH2C6H3-2,4-diCl	73	66	70	49
19	Prochloraz	-	-	87	98	95	91

. Compound 12b, containing a methyl group at the R¹ site and an ethyl group at the R² site, exhibited remarkable activity against Gilbertella persicaria, with an EC₅₀ value of 6.71 μg/mL, significantly exceeding that of the reference fungicide Prochloraz (EC₅₀ = 22.03 μg/mL).

Fu’s group synthesized a series of substituted 5-aryl-2-amino-1,3,4-thiadiazoles bearing a benzamide moiety (76a–t, Figure 63) [102]. The compounds were tested for antifungal activity against Rhizoctonia solani, Botrytis cinerea, Stemphylium lycopersici, Curvularia lunata, and Pythium aphanidermatum (

Table 46. Antifungal activity of compounds 76a–t.

Entry	Compound	R	EC50 (95% Cl, μmol/L)
			R. solani	B. cinerea	S. lycopersici	C. lunata	P. aphanidermatum
1	76a	2-F	0.0285	0.8424	0.1432	0.4226	---
2	76b	3-F	2.4311	---	0.0374	0.0237	---
3	76c	4-F	0.1297	7.7911	1.2300	---	---
4	76d	2-Cl	0.1586	1.2954	---	---	---
5	76e	3-Cl	0.0466	---	0.0105	0.0051	0.1603
6	76f	4-Cl	0.0415	0.1422	1.0919	1.1636	0.7236
7	76g	2-Br	---	0.0045	---	---	0.2273
8	76h	4-Br	---	---	0.0590	0.0359	0.2717
9	76i	2-CH3	0.1303	0.1169	1.8938	0.2065	---
10	76j	3-CH3	0.0107	---	3.6669	0.3369	---
11	76k	4-CH3	0.0328	0.9588	---	---	0.0334
12	76l	2-OCH3	1.5904	0.0024	2.9342	0.4719	-
13	76m	3-OCH3	0.0079	0.9994	0.0993	0.0119	0.0355
14	76n	4-OCH3	0.0857	---	---	-	0.0180
15	76o	3-CF3	0.0160	0.1465	0.0481	0.3058	0.0270
16	76p	4-CF3	0.0028	---	0.0359	1.0790	1.9811
17	76q	2,4-diCl	0.0547	0.0347	0.2990	0.2019	-
18	76r	2,4-diF	0.0155	0.1419	8.2794	-	0.4482
19	76s	3,4-diOCH3	0.0189	0.1708	0.2725	0.0318	3.5046
20	76t	3,4,5-triOCH3	1.8869	1.1614	0.1054	0.0198	1.1971
21	Difenoconazole	-	0.0019	0.0042	0.0124	0.0014	0.0001

). Most derivatives demonstrated excellent in vitro fungicidal efficacy. Notably, compound 76p, containing a 4-trifluoromethylphenyl substituent, exhibited the highest activity against Rhizoctonia solani (EC₅₀ = 0.0028 μmol/L), 76l, with a 2-methoxyphenyl group, was most effective against Botrytis cinerea (EC₅₀ = 0.0024 μmol/L), and 76e, bearing a 3-chlorophenyl substituent, showed potent activity against both Stemphylium lycopersici and Curvularia lunata (EC₅₀ = 0.0105 μmol/L and 0.005 μmol/L, respectively). The authors made general observations regarding the influence of structural features of the synthesized compounds on their antifungal activity against specific strains. As shown in the results, the order of activity against Rhizoctonia solani was as follows: 3-CF₃ > 4-OCH₃ > 4-CH₃ > 2,6-diF > 4-CF₃ > 2-F. In the case of Botrytis cinerea, it was observed that only the presence of methoxy OCH₃ and bromine Br substituents exerted a notable antifungal effect. Against Stemphylium lycopersici, only the 4-chloro-substituted compound displayed the same activity as the positive control drug.

Durairaj’s group synthesized a series of 1,3,4-thiadiazole derivatives directly linked to a pyrimidine scaffold (77a–j, Figure 64) [103]. The compounds were evaluated for antifungal activity against Aspergillus niger, Penicillium species, and Candida albicans. As shown in

Table 47. Antifungal activity of compounds 77a–j.

Entry	Compound	Zone of Inhibition (mm)
		C. albicans	P. species	A. Niger
1	77a	-	-	-
2	77b	-	9	5
3	77c	6	5	6
4	77d	-	6	-
5	77e	-	8	-
6	77f	-	8	-
7	77g	7	10	8
8	77h	-	5	-
9	77i	-	-	-
10	77j	-	8	-

, 5-(1,3,4-thiadiazol-2-yl)-3,4-dihydropyrimidin-2(1H)-one containing a dimethylamino group at the R¹ site (77c) and 5-(1,3,4-thiadiazol-2-yl)-3,4-dihydropyrimidine-2(1H)-thione containing chlorine at the R¹ site (77g) demonstrated promising activity against all three fungal strains.

Dróżdż et al. evaluated four 1,3,4-thiadiazole derivatives (78a–d, Figure 65) [104] for antifungal activity against Candida albicans and Candida parapsilosis. The determined minimum inhibitory concentrations (MIC) are summarized in

Table 48. Antifungal activity of compounds 78a–d.

Entry	Compound	MIC (μg/mL)
		C. albicans	C. parapsilosis
1	78a	128	64
2	78b	>128	>128
3	78c	>128	16
4	78d	128	64

. For compounds 78a, containing a methyl group at the R² site, and 78d, containing chlorine at the R¹ site and a naphthalen-1-ylmethyl group at the R² site, the MIC₁₀₀ values—defined as the minimum concentration required to completely inhibit fungal growth—ranged from 64 to 128 µg/mL, depending on the strain. As a further aspect of the study, selected thiadiazole derivatives were tested in combination with Amphotericin B, revealing strong synergistic antifungal interactions.

Panwar’s group synthesized a series of [2-phenyl-1-(p-tolyl)pyrido[3,2-f]quinazolin-4(1H)-yl]-1,3,4-thiadiazolyl]-4-piperazine derivatives (13a–i, Figure 13) [52]. In addition to antibacterial assessment, the compounds were tested for antifungal activity against Aspergillus fumigatus, Candida albicans, and Candida krusei.

Table 49. Antifungal activity of compounds 13a–i.

Entry	Compound	R	Zone of Inhibition (mm)
			A. fumigatus	C. albicans	C. krusei
1	13a	H	6	0	0
2	13b	2-Cl	6	16	16
3	13c	3-Cl	10	12	14
4	13d	4-Cl	12	16	18
5	13e	3-NO2	10	12	12
6	13f	4-NO2	0	10	10
7	13g	3-OCH3-4-OH	0	10	8
8	13h	4-CH3	6	6	8
9	13i	2-OH	8	12	16
10	Fluconazole	-	0	29	20

presents the zones of inhibition recorded at a concentration of 250 μg/mL. Several derivatives exhibited antifungal efficacy against Aspergillus fumigatus, a strain intrinsically resistant to Fluconazole.

As previously mentioned, Blaja et al. synthesized tetranorlabdane derivatives bearing 1,3,4-thiadiazole units (14a–c, Figure 14) [53]. The compounds were evaluated for antifungal activity against five fungal strains: Alternaria alternata, Aspergillus niger, Penicillium chrysogenum, Penicillium frequentans, and Fusarium solani. The results, summarized in

Table 50. Antifungal activity of compounds 14a–c.

Entry	Compound	MIC (μg/mL)
		A. niger	F. solani	P. chrysogenum	P. frequentans	A. alternata
1	14a	0.125	0.125	0.125	0.125	0.125
2	14b	>32	>32	>32	>32	>32
3	14c	>32	>32	>32	>32	>32
4	Caspofungin	0.25	0.25	0.25	0.25	0.25

, indicate that only 5-(((8aS)-2,5,5,8a-tetramethyl-4a,5,6,7,8,8a-hexahydronaphthalen-1-yl)methyl)-1,3,4-thiadiazol-2-amine (14a) exhibited pronounced antifungal activity, with an MIC value of 0.125 μg/mL.

Shi et al. synthesized a series of phenylmethanol-linked 1,3,4-thiadiazole thioethers (79a–ad, Figure 66) [105]. The compounds were evaluated for antifungal activity against Alternaria solani, Gibberella saubinetii, Verticillium dahliae, Gibberella zeae, and Thanatephorus cucumeris (

Table 51. Antifungal activity of selected compounds 79a–ae.

Entry	Compound	MIC (μg/mL)
		G. saubinetii	A. solani	V. dahliae	G. zeae	T. cucumeris
1	79c	102.8	91.9	-	-	31.3
2	79d	84.2	119.0	-	-	15.3
3	79i	86.8	62.5	-	87.3	11.8
4	79j	23.3	50.0	88.4	63.0	9.7
5	79k	28.0	45.9	89.2	54.9	18.7
6	79m	43.3	42.3	58.7	104.0	35.4
7	79n	52.2	37.1	73.4	112.7	27.7
8	79o	11.4	36.6	69.6	167.5	46.1
9	79p	92.7	37.0	-	-	-
10	79q	90.1	63.8	-	108.1	25.7
11	79s	47.6	56.4	66.2	55.3	51.2
12	79t	37.6	42.1	72.1	49.1	20.0
13	79u	23.6	41.2	72.9	33.7	24.3
14	79w	69.2	70.5	-	-	-
15	79x	37.1	49.6	110.6	81.4	34.9
16	79y	47.7	76.4	102.9	119.9	32.7
17	79z	43.9	35.8	-	93.8	60.6
18	79aa	31.4	47.1	95.8	86.3	32.3
19	79ac	49.2	53.2	175.8	92.3	116.4
20	79ad	91.0	48.2	-	112.9	53.9
21	Triadimefon	14.8	45.3	2.9	16.9	11.0

). Bioassay results revealed that compound 79j exhibited excellent efficacy against Thanatephorus cucumeris, with an EC₅₀ value of 9.7 μg/mL. Further studies demonstrated that (5-((2-methylbenzyl)thio)-1,3,4-thiadiazol-2-yl)(phenyl)methanol (79j) not only significantly inhibited Thanatephorus cucumeris mycelial growth but also suppressed sclerotia formation and exhibited substantial in vivo protective (61.1%) and curative (67.9%) effects at 200 μg/mL.

The modified chitosan–thiadiazole conjugate developed by Ibrahim et al. (15, Figure 15) [54], previously tested for antibacterial properties, was also evaluated for antifungal activity against Candida albicans. At a concentration of 50 μg/mL, the compound exhibited a zone of inhibition measuring 8 mm, indicating moderate antifungal potential.

Zhou et al. synthesized a series of flavanol derivatives containing a 1,3,4-thiadiazole moiety (80a–v, Figure 67) [106]. The compounds were screened for antifungal activity against Rhizoctonia solani, Botrytis cinerea, Fusarium graminearum, Colletotrichum gloeosporioides, Sclerotinia sclerotiorum, Phytophthora capsici, Alternaria brassicae, Fusarium oxysporum f. sp. cucumerinum, Fusarium oxysporum f. sp. capsicum, and Phomopsis sp. Inhibition rates obtained from biological assays are summarized in

Table 52. Antifungal activity of compounds 80a–v.

Entry	Compound	Inhibition Rate (%)
		BC	PS	CG	RS	PC	SS	FcU	FG	AB	FcA
1	80a	55.5	32.2	47.7	54.0	30.0	40.6	20.5	33.2	44.4	44.4
2	80b	61.3	40.4	46.0	55.5	38.4	38.2	18.9	36.5	46.0	33.9
3	80c	65.1	37.2	47.7	59.3	34.5	63.6	32.3	24.9	51.7	48.8
4	80d	36.6	34.1	42.7	38.6	41.8	43.8	35.0	38.5	51.0	40.3
5	80e	38.7	60.2	41.8	43.8	36.3	44.6	32.7	31.6	49.8	42.3
6	80f	60.1	51.7	32.2	48.5	28.7	10.7	41.7	32.8	35.6	42.3
7	80g	74.6	36.8	30.1	55.3	32.1	51.2	29.1	35.7	32.6	39.1
8	80h	62.2	53.5	35.6	51.5	31.2	52.6	30.7	28.3	42.9	42.7
9	80i	54.4	55.0	36.0	52.2	49.4	53.8	31.1	31.6	39.8	47.6
10	80j	56.7	37.2	38.9	47.1	51.9	45.0	30.3	33.2	28.7	41.5
11	80k	33.3	27.1	24.3	57.4	35.7	45.7	46.5	33.5	36.8	47.6
12	80l	91.7	59.6	36.0	55.5	24.5	54.2	37.8	35.7	39.5	45.6
13	80m	83.2	42.8	41.8	54.0	34.2	47.0	46.5	35.7	39.8	52.4
14	80n	69.1	73.8	54.0	65.6	16.8	71.5	40.9	20.2	47.9	50.8
15	80o	58.8	59.0	36.0	40.1	32.9	57.4	39.4	51.6	34.5	47.6
16	80p	50.4	50.2	23.9	40.8	32.5	48.8	39.4	31.2	30.3	51.6
17	80q	87.3	66.1	46.4	65.2	22.4	65.3	34.3	28.4	48.3	50.4
18	80r	96.2	65.3	45.2	63.4	22.4	58.7	29.5	18.0	47.1	51.2
19	80s	60.1	49.4	41.0	45.6	28.3	47.0	41.3	32.0	40.2	46.4
20	80t	65.9	42.8	34.3	45.2	49.8	45.0	26.0	26.6	31.4	46.4
21	80u	58.4	72.8	45.2	46.3	46.8	66.9	35.4	44.3	42.5	48.4
22	80v	52.7	38.0	37.7	52.8	52.7	41.7	28.7	37.7	36.8	44.4
23	Azoxystrobin	80.7	58.1	56.1	75.7	75.5	71.9	48.0	36.6	24.5	60.9

RS: Rhizoctonia solani; BC: Botrytis cinereal; FG: Fusarium graminearum; CG: Colletotrichum gloeosporioides; SS: Sclerotinia sclerotiorum; PC: Phytophthora capsica; AB: Alternaria brassicae; FcU: Fusarium oxysporum f. sp. Cucumerinum; FcA: Fusarium oxysporum f. sp. Capsicum; PS: Phomopsis sp.

. Several derivatives demonstrated excellent antifungal efficacy. Notably, compounds 80l, 80m, 80q, and 80r, bearing 4-methylphenyl, 4-methoxyphenyl, 4-fluorophenyl, and 3-fluorophenyl substituents at the flavanol core, showed inhibition rates against Botrytis cinerea of 91.7%, 83.2%, 87.3%, and 96.2%, respectively, all exceeding the activity of the reference fungicide Azoxystrobin (80.7%). Additionally, compounds 80n and 80u, characterized by the presence of one or two methoxy groups, displayed superior activity against Phomopsis sp., with inhibition rates of 73.8% and 72.8% compared to 58.1% for Azoxystrobin. In summary, antifungal activity was higher for derivatives unsubstituted at the 5 position of the 1,3,4-thiadiazole ring (R¹ = H) than for those bearing an amino group (R¹ = NH₂). Compounds with electron-withdrawing groups on the phenyl ring of the flavanol scaffold displayed stronger fungicidal effects, with an order of efficacy of 80r (R³ = 3-F) > 80l (R³ = 4-CH₃) > 80m (R³ = 4-OCH₃). Furthermore, substitution at R³ with 3-F conferred a greater inhibitory effect than substitution with 4-F. Overall, compound 80r (R¹ = H, R² = H, R³ = 3-F) demonstrated markedly superior performance relative to the other target compounds and Azoxystrobin.

Thanh et al. synthesized a series of 1,3,4-thiadiazole derivatives incorporating a thiourea scaffold (16a–i, Figure 16) [55]. In addition to antibacterial testing, the compounds were evaluated for antifungal activity against Aspergillus niger, Aspergillus flavus, Candida albicans, Saccharomyces cerevisiae, and Fusarium oxysporum (

Table 53. Antifungal activity of compounds 16a–i.

Entry	Compound	MIC (μg/mL)
		A. niger	A. flavus	C. albicans	S. cerevisiae	F. oxysporum
1	16a	200	50	25	12.5	25
2	16b	50	25	0.78	6.25	25
3	16c	100	6.25	1.56	25	6.25
4	16d	12.5	1.56	3.125	1.56	200
5	16e	6.25	200	6.25	100	6.25
6	16f	400	400	400	400	400
7	16g	50	0.78	3.125	3.125	1.56
8	16h	3.125	12.5	200	400	1.56
9	16i	0.78	3.125	12.5	0.78	0.78
10	Miconazole	1.56	1.56	3.125	3.125	3.125
11	Fluconazole	1.56	0.78	0.78	0.78	0.78

). Compounds 16b, bearing a methyl substituent at the R site, and 16c, with the ethyl substituent, selectively inhibited the growth of Candida albicans, exhibiting strong activity with MIC values of 0.78 and 1.56 μg/mL, respectively. More broadly, compound 16i, bearing an isopentyl substituent at the R site, showed potent activity against Aspergillus niger, Saccharomyces cerevisiae, and Fusarium oxysporum, with MIC values of 0.78 μg/mL, superior to those of the reference drugs Miconazole and Fluconazole. The authors concluded that the presence of long chains (16i) at the R site and branching (16g, 16i) significantly enhanced antifungal activity compared to the corresponding unbranched analogues (16f, 16h).

The previously discussed series of thiadiazole azetidin-2-one derivatives synthesized by Kumar et al. (20a–g, Figure 19,

Table 54. Antifungal activity of compounds 20a–g.

Entry	Compound	R1	R2	MIC (µM)
				T. harzianum	A. niger
1	20a	Br	4-OH	7.15	28.60
2	20b	Br	4-NH2	28.67	28.67
3	20c	OCH3	3,5-diCl	14.17	28.34
4	20d	OCH3	4-NO2	14.99	14.99
5	20e	OCH3	4-Br	6.93	13.86
6	20f	Cl	4-NO2	3.71	29.69
7	20g	Br	4-Cl	6.87	3.43
8	Fluconazole	-	-	5.10	5.10

) [58] was also evaluated for antifungal activity against Trichoderma harzianum and Aspergillus niger. Additional testing revealed that compound 20g, containing bromine at the R¹ site and a 4-chlorine substituent at the R² site, exhibited remarkable efficacy against Aspergillus niger, with an MIC value of 3.42 µM, while derivative 20f, containing chlorine at the R¹ site and a 4-nitro group at the R² site, was the most active against Trichoderma harzianum, outperforming the reference drug Fluconazole (MIC = 3.71 µM).

As previously mentioned, Alqahtani et al. synthesized a series of compounds bearing a benzothiazolotriazole scaffold linked to a 1,3,4-thiadiazole ring (24a–c, Figure 23) [62]. In addition to antibacterial evaluation, the compounds were tested for antifungal activity against Candida albicans, Aspergillus fumigatus, and Penicillium chrysogenum (

Table 55. Antifungal activity of compounds 24a–c.

Entry	Compound	R	MIC (μg/mL)
			C. albicans	A. fumigatus	P. chrysogenum
1	24a	H	64	128	64
2	24b	Br	8	16	8
3	24c	CH3	16	32	16
4	Fluconazole	-	4	8	8

). Among them, compound 24b, bearing 4-bromophenylamino moiety on the 1,3,4-thiadiazole ring, exhibited the highest potency, with MIC values of 8 μg/mL against Candida albicans and Penicillium chrysogenum and 16 μg/mL against Aspergillus fumigatus.

Dou et al. synthesized a series of acetophenone derivatives containing 1,3,4-thiadiazole-2-thioether moieties (81a–ae, Figure 68) [107]. The compounds were evaluated for antifungal activity against Gibberella saubinetii, Verticillium dahliae, Alternaria solani, Gibberella zeae, and Thanatephorus cucumeris (

Table 56. Antifungal activity of selected compounds 81a–ae.

Entry	Compound	EC50 (100 μg/mL)
		G. saubinetii	V. dahliae	A. solani	G. zeae	T. cucumeris
1	81a	30.5	48.1	61.7	59.8	32.8
2	81b	21.9	45.4	67.5	42.8	22.2
3	81c	21.5	41.6	63.4	37.3	39.6
4	Triadimefon	14.8	2.9	45.3	16.9	11.0
5	Tebuconazole	0.4	0.1	1.3	0.4	0.6

). Preliminary bioassay results indicated that compounds 81a–c exhibited inhibitory effects against all five tested fungal strains. Notably, the EC₅₀ value of (5-(ethylthio)-1,3,4-thiadiazol-2-yl)(phenyl)methanone (81b) against Thanatephorus cucumeris was 22.2 μg/mL, while 81c showed an EC₅₀ of 21.5 μg/mL against Gibberella saubinetii.

Pham and coworkers synthesized a series of 5-substituted-2-amino-1,3,4-thiadiazole derivatives (25a–l, Figure 24) [63]. In addition to the previously discussed antibacterial evaluation, the compounds were tested for antifungal activity against Candida albicans and Aspergillus niger (

Table 57. Antifungal activity of compounds 25a–l.

Entry	Compound	MIC (μg/mL)
		C. albicans	A. niger
1	25a	-	-
2	25b	1024	512
3	25c	512	>1024
4	25d	128	128
5	25e	-	-
6	25f	-	-
7	25g	8	64
8	25h	-	-
9	25i	-	-
10	25j	128	128
11	25k	1024	>1024
12	25l	512	512
13	Fluconazole	4	256

). Compound 25g, bearing a 3,4-dichlorophenyl group on the 1,3,4-thiadiazole ring, exhibited the highest potency among the synthesized derivatives, with MIC values of 16 μg/mL against Candida albicans and 64 μg/mL against Aspergillus niger, outperforming Fluconazole in both cases.

Zou et al. synthesized a series of 1,3,4-thiadiazole-amide derivatives incorporating a gem-dimethyl-cyclopropane ring (82a–u, Figure 69) [108]. The compounds were evaluated for antifungal activity against Fusarium oxysporum f. sp. cucumerinum, Cercospora arachidicola, Physalospora piricola, Alternaria solani, Gibberella zeae, Rhizoctonia solani, Bipolaris maydis, and Colletotrichum orbiculare (

Table 58. Antifungal activity of compounds 82a–u.

Entry	Compound	Inhibition Rate (%)
		FO	CA	PP	AS	GZ	RS	BM	CO
1	82a	19.1	23.0	33.1	57.6	31.2	12.2	19.5	14.5
2	82b	23.6	31.7	48.5	43.3	28.2	12.2	19.5	14.5
3	82c	34.1	64.3	94.8	56.2	56.6	19.8	31.3	31.6
4	82d	28.2	14.3	48.5	57.6	25.2	38.3	19.5	14.5
5	82e	14.5	14.3	25.4	48.1	40.3	14.3	19.5	14.5
6	82f	28.2	31.7	33.1	24.3	25.2	12.2	33.8	28.2
7	82g	23.6	14.3	17.7	52.9	31.2	31.7	19.5	14.5
8	82h	32.7	40.4	33.1	67.1	31.2	47.0	33.8	37.3
9	82i	69.1	83.9	79.2	76.7	70.6	86.1	71.9	73.6
10	82j	23.6	31.7	17.7	57.6	34.2	62.2	29.0	32.7
11	82k	28.2	23.0	17.7	52.9	25.2	53.5	24.3	23.6
12	82l	28.2	14.3	17.7	57.6	25.2	31.7	19.5	23.6
13	82m	23.6	23.0	17.7	52.9	28.2	31.7	29.0	37.3
14	82n	35.9	64.3	33.9	45.9	51.4	17.3	35.5	33.5
15	82o	28.2	57.8	48.5	52.9	52.4	53.5	33.8	32.7
16	82p	14.5	23.0	17.7	33.8	40.3	31.7	19.5	23.6
17	82q	23.6	14.3	17.7	43.3	31.2	42.6	15.5	23.6
18	82r	46.4	66.5	48.5	67.1	52.4	70.9	43.3	46.4
19	82s	18.2	14.3	33.1	43.3	43.3	62.2	33.8	32.7
20	82t	18.2	14.3	33.1	57.6	31.2	25.2	33.8	23.6
21	82u	13.6	14.3	17.7	38.6	43.3	42.6	29.0	23.6
23	Chlorothalonil	100	73.3	75.0	73.9	73.1	96.1	90.4	91.3

FO: Fusarium oxysporum f. sp. Cucumerinum; CA: Cercospora arachidicola; PP: Physalospora piricola; AS: Alternaria solani; GZ: Gibberella zeae; RS: Rhzioeotnia solani; BM: Bipolaris maydis; CO: Colleterichum orbicalare.

). Preliminary bioassay results indicated that compound 82i, containing a 4-bromophenyl substituent at the amide moiety, exhibited broad-spectrum antifungal activity against the tested strains.

Sunitha et al. synthesized a series of azo-imine thiadiazole derivatives (28a–e, Figure 27) [66] and evaluated their antifungal activity against Aspergillus niger, Aspergillus flavus, and Rhizopus stolonifera using Fluconazole as the reference drug (

Table 59. Antifungal activity of compounds 28a–e.

Entry	Compound	Zone of Inhibition (mm)
		A. flavus	A. niger	R. stolonifera
1	28a	8	-	-
2	28b	8	7	8
3	28c	10	16	12
4	28d	8	-	-
5	28e	-	-	-
6	Fluconazole	20	19	21

). Overall, compound 28c, containing a 3-hydroxy-4-methoxyphenyl substituent at the imine fragment, demonstrated moderate inhibitory activity against the tested fungal strains.

Pan et al. synthesized a series of pyrimidine derivatives bearing a 1,3,4-thiadiazole core (83a–t, Figure 70) [109]. The compounds were tested for antifungal activity against Botrytis cinerea, Botryosphaeria dothidea, and Phomopsis sp. at a concentration of 50 μg/mL (

Table 60. Antifungal activity of compounds 83a–t.

Entry	Compound	R1	R2	Inhibition Rate (%)
				B. dothidea	Phomopsis sp.	B. cinerea
1	83a	H	2-CH3	41.8	50.6	73.2
2	83b	H	2-F	63.0	83.2	78.7
3	83c	H	4-F	75.6	89.6	85.1
4	83d	H	2-Cl	57.4	74.6	71.1
5	83e	H	3-Cl	65.9	79.4	79.2
6	83f	H	4-Cl	72.4	84.5	84.9
7	83g	H	2-CN	80.0	88.7	86.1
8	83h	H	4-CF3	82.6	89.2	90.7
9	83i	H	3,4-diCl	70.8	84.6	85.4
10	83j	CH3	2-CH3	36.2	42.9	65.3
11	83k	CH3	4-F	59.0	71.6	74.0
12	83l	CH3	2-Cl	51.5	64.5	65.7
13	83m	CH3	3-Cl	57.4	71.9	73.3
14	83n	CH3	4-Cl	65.4	78.4	80.4
15	83o	CH3	2-CN	73.7	76.7	78.8
16	83p	CH3	2-CF3	68.4	80.3	81.8
17	83q	CH3	4-CF3	75.7	86.8	88.3
18	83r	CH3	2,3-diCl	58.2	69.0	66.5
19	83s	CH3	2,4-diCl	75.6	82.4	83.9
20	83t	CH3	3,4-diCl	65.7	78.0	80.8
21	Pyrimethanil	-	-	84.4	85.1	82.8

). Biological assay results demonstrated that compound 83h containing (4-fluoromethylbenzyl)thio moiety exhibited lower EC₅₀ values (25.9 and 50.8 μg/mL) against Phomopsis sp. compared to the reference fungicide Pyrimethanil (32.1 and 62.8 μg/mL). Further structure–activity relationship analysis showed that more than 80% of the tested compounds exhibited excellent antifungal activity against Phomopsis sp. and Botrytis cinerea. Modification of the R¹ substituent in the pyrimidine ring (H or CH₃) did not significantly enhance antifungal potency; however, for Phomopsis sp., the number of compounds with R¹ = H and activity above 80% was twice that observed for compounds with R¹ = CH₃. Moreover, introducing strong electron-withdrawing groups at R² (e.g., CN or CF₃) within the benzylthio fragment enhanced activity, whereas introduction of an alkyl group (CH₃) had little effect.

Yu et al. synthesized a series of thiochroman-4-one derivatives incorporating carboxamide and 1,3,4-thiadiazole thioether moieties (29a–o, Figure 28) [67], previously reported for their antibacterial activity. The compounds were also evaluated for antifungal activity against Botrytis cinerea, Verticillium dahliae, and Fusarium oxysporum at a concentration of 50 μg/mL (

Table 61. Antifungal activity of compounds 29a–o.

Entry	Compound	R1	R2	Inhibition Rate (%)
				B. cinerea	V. dahliae	F. oxysporum
1	29a	CH3	Cl	0	0	2
2	29b	CH2CH3	Cl	17	8	5
3	29c	CH2CH2CH3	Cl	21	15	12
4	29d	CH2C6H5	Cl	0	2	0
5	29e	CH2C6H4-4-F	Cl	0	0	0
6	29f	CH3	F	0	0	0
7	29g	CH2CH3	F	48	24	16
8	29h	CH2CH2CH3	F	55	36	26
9	29i	CH2C6H5	F	0	0	0
10	29j	CH2C6H4-4-F	F	0	0	0
11	29k	CH3	CH3	0	12	0
12	29l	CH2CH3	CH3	61	45	32
13	29m	CH2CH2CH3	CH3	69	54	40
14	29n	CH2C6H5	CH3	0	0	0
15	29o	CH2C6H4-4-F	CH3	9	6	0
16	Carbendazim	-	-	57	79	100

). Notably, compound 29m, containing a propyl group at the R¹ site and a methyl group at the R² site, exhibited superior activity against Botrytis cinerea compared to the reference fungicide Carbendazim.

As previously mentioned, Shu et al. synthesized a series of galactoside derivatives containing a 1,3,4-thiadiazole moiety (30a–t, Figure 29) [68]. The compounds were tested for antifungal activity against Gibberella zeae, Botryosphaeria dothidea, Phytophthora infestans, Phomopsis sp., and Thanatephorus cucumeris at a concentration of 50 μg/mL (

Table 62. Antifungal activity of compounds 30a–t.

Entry	Compound	Inhibition Rate (%)
		G. zeae	B. dothidea	P. infestans	Phompsis sp.	T. cucumeris
1	30a	28.8	24.5	23.6	49.2	45.2
2	30b	34.5	32.0	28.1	36.3	33.4
3	30c	37.6	31.0	25.6	32.5	56.3
4	30d	45.4	25.8	24.7	36.2	42.6
5	30e	40.1	26.8	25.3	47.5	47.5
6	30f	36.2	21.6	56.4	34.2	43.4
7	30g	47.0	33.2	56.7	55.6	46.3
8	30h	34.2	48.5	56.1	35.2	35.7
9	30i	38.6	54.8	59.8	33.5	45.6
10	30j	43.0	51.6	57.5	37.3	55.4
11	30k	45.4	50.7	57.6	45.1	43.0
12	30l	63.4	50.5	73.5	34.5	59.7
13	30m	53.8	46.4	73.1	48.1	68.3
14	30n	52.3	66.4	77.5	42.6	56.5
15	30o	61.0	65.3	75.1	45.2	56.3
16	30p	52.2	66.0	80.1	58.1	56.5
17	30r	45.2	54.3	79.7	43.2	58.7
18	30s	55.4	55.2	78.0	44.5	65.3
19	30t	57.2	54.7	79.3	48.2	68.4
20	Dimethomorph	74.3	72.3	78.2	69.3	68.3

). Among them, compound 30p, containing a nitro substituent at the meta position, compound 30r, containing a nitro substituent at the para position, and compound 30t, with a trifluoromethyl group at the meta position, demonstrated satisfactory in vitro activity against Phytophthora infestans, with inhibition rates of 80.1%, 79.7%, and 79.3%, respectively. These results were comparable to the activity of the reference fungicide Dimethomorph (78.2%).

Geng et al. synthesized a series of terpene-derived compounds incorporating a 1,3,4-thiadiazole moiety (84a–ab, Figure 71) [110]. The compounds were tested for antifungal activity against Valsa mali (

Table 63. Antifungal activity of compounds 84a–ab.

Entry	Compound	EC50 (μg/mL)	Entry	Compound	EC50 (μg/mL)
1	84a	83.450	16	84p	114.00
2	84b	70.725	17	84q	82.085
3	84c	29.924	18	84r	89.406
4	84d	41.398	19	84s	107.633
5	84e	45.246	20	84t	90.859
6	84f	50.608	21	84u	52.089
7	84g	43.570	22	84v	126.441
8	84h	84.770	23	84w	101.486
9	84i	79.635	24	84x	66.656
10	84j	40.969	25	84y	205.857
11	84k	55.469	26	84z	9.911
12	84l	58.441	27	84aa	3.785
13	84m	50.201	28	84ab	6.049
14	84n	43.448	29	Boscalid	47.356
15	84o	62.553

). The results indicated that several derivatives exhibited satisfactory inhibitory effects, with some outperforming the commercial fungicide Boscalid. The most potent compound, 84aa, containing a (3-fluorophenyl)sulfonyl group, demonstrated a favorable EC₅₀ value of 3.785 μg/mL.

Prasad et al. synthesized a series of quinoline-bridged thiophene derivatives linked to a 1,3,4-thiadiazole ring (31a–e, Figure 30) [69]. In addition to antibacterial evaluation, the compounds were tested for antifungal activity against Aspergillus niger and Aspergillus flavus at a concentration of 10 μg/mL (

Table 64. Antifungal activity of compounds 31a–e.

Entry	Compound	R	Zone of Inhibition (mm)
			A. niger	A. flavus
1	31a	H	6	6
2	31b	Cl	6	10
3	31c	F	12	6
4	31d	NO2	20.5	6
5	31e	Br	9	6
6	Fluconazole	-	19.5	25

). The study revealed that among the tested derivatives, only compound 31d, bearing a nitro group at the R site, exhibited notable activity against Aspergillus niger.

Acar Çevik et al. synthesized a series of benzimidazole derivatives containing a 1,3,4-thiadiazole scaffold (32a–k, Figure 31) [70]. The antifungal activity of all compounds was assessed by determining their minimum inhibitory concentrations (MIC) against Candida albicans, Candida krusei, Candida glabrata, and Candida parapsilosis (

Table 65. Antifungal activity of compounds 32a–k.

Entry	Compound	MIC (μg/mL)
		C. albicans	C. krusei	C. glabrata	C. parapsilosis
1	32a	1.95	62.5	3.9	62.5
2	32b	15.625	62.5	3.9	62.5
3	32c	15.625	62.5	3.9	125
4	32d	15.625	125	125	125
5	32e	31.25	125	7.81	125
6	32f	15.625	125	31.25	125
7	32g	7.81	62.5	3.9	62.5
8	32h	1.95	7.81	1.95	7.81
9	32i	7.81	125	125	62.5
10	32j	15.625	125	62.5	62.5
11	32k	3.9	125	62.5	62.5
12	Voriconazole	3.9	3.9	1.95	3.9
13	Fluconazole	7.81	7.81	3.9	3.9

). Screening results revealed that compound 32a, containing a methyl group at the R site, and compound 32h, containing an isopropyl group, exhibited the highest potency against Candida albicans, with MIC values of 1.95 μg/mL—twice as effective as the reference drug Voriconazole (3.90 μg/mL) and four times more effective than Fluconazole (7.81 μg/mL). The authors concluded that the enhanced antifungal activity was attributed to the presence of alkylamino groups (R) at the 5 position of the 1,3,4-thiadiazole ring, with small to medium alkyl chains (methyl, isopropyl) providing optimal activity.

Mao et al. synthesized four series of dehydroabietyl-1,3,4-thiadiazole-2-amide and dehydroabietyl-1,3,4-thiadiazole-2-imine derivatives (85a–g, 86a–h, 87a–f, 88a–g, Figure 72) [111]. The antifungal activity of all synthesized compounds was evaluated against Sclerotinia sclerotiorum, Botrytis cinerea, Magnaporthe oryzae, and Fusarium oxysporum. At a concentration of 100 mg/L, the mycelial growth inhibition rates against Sclerotinia sclerotiorum, Botrytis cinerea, and Magnaporthe oryzae were below 20%, indicating weak activity. However, the activity against Fusarium oxysporum ranged from moderate to significant. Notably, compound 85e, substituted with a nitro group on the thiophene ring, demonstrated excellent antifungal efficacy, with an EC₅₀ value of 0.618 mg/L—lower than that of the reference fungicide Carbendazim (0.649 mg/L). In vivo studies further confirmed that 85e provided a protective effect on cucumber plants.

As previously mentioned, Garg and coworkers synthesized a series of 1,3,4-thiadiazole derivatives bearing 2,3-disubstituted thiazolidinone moieties (35a–j, Figure 34) [73]. In addition to antibacterial evaluation, the compounds were tested for antifungal activity against Candida albicans and Aspergillus niger (

Table 66. Antifungal activity of compounds 35a–j.

Entry	Compound	R	Zone of Inhibition (mm)
			A. niger		C. albicans
			250 μg/mL	25 μg/mL	250 μg/mL	25 μg/mL
1	35a	H	16	11	17	13
2	35b	N(CH3)2	20	15	18	12
3	35c	(OCH3)2	15	-	14	10
4	35d	Cl	15	-	14	10
5	35e	NO2	15	10	16	11
6	35f	OH	16	12	15	12
7	35g	OCH3	18	11	17	13
8	35h	CH3	17	12	14	10
9	35i	NH2	19	13	17	12
10	35j	F	14	10	17	11
11	Miconazole	-	19	14	17	14

). The most promising result was obtained for compound 35b, bearing a dimethylamino substituent, which exhibited antifungal activity slightly superior to or comparable with that of the standard drug Miconazole.

Wang et al. synthesized a series of novel nopol derivatives incorporating a 1,3,4-thiadiazole–thioether moiety (89a–w, Figure 73) [112]. The antifungal activity of all compounds was evaluated against Fusarium oxysporum f. sp. cucumerinum, Cercospora arachidicola, Physalospora piricola, Alternaria solani, Gibberella zeae, Rhizoctonia solani, Bipolaris maydis, and Colletotrichum orbiculare at a concentration of 50 μg/mL (

Table 67. Antifungal activity of compounds 89a–w.

Entry	Compound	Inhibition Rate (%)
		FC	CA	PP	AS	GZ	RS	BM	CO
1	89a	62.2	54.2	59.3	40.9	23.8	50.6	31.6	21.6
2	89b	8.1	20.8	33.3	0	23.8	50.6	31.6	29.7
3	89c	29.7	25.0	59.3	22.7	33.3	44.6	28.9	21.6
4	89d	45.9	54.2	40.7	36.4	14.3	62.7	57.9	21.6
5	89e	21.6	29.2	59.3	9.1	23.8	53.0	34.2	35.1
6	89f	54.1	54.2	77.8	63.6	33.3	32.5	23.7	16.2
7	89g	51.4	50.0	29.6	18.2	33.3	53.0	31.6	27.0
8	89h	43.2	45.8	59.3	45.5	19.0	44.6	34.2	32.4
9	89i	37.8	33.3	88.9	27.3	38.1	8.4	13.2	29.7
10	89j	24.3	25.0	51.9	4.5	28.6	8.4	18.4	18.9
11	89k	29.7	33.3	25.9	22.7	33.3	38.6	18.4	21.6
12	89l	21.6	54.2	59.3	0	14.3	14.5	42.1	21.6
13	89m	16.2	25.0	22.2	0	52.4	80.7	60.5	62.2
14	89n	40.5	41.7	28.5	40.9	38.1	38.6	31.6	40.5
15	89o	56.8	54.2	29.6	27.3	19.0	62.7	39.5	45.9
16	89p	56.8	54.2	33.3	40.9	47.6	68.7	50.0	35.1
17	89q	54.1	62.5	77.8	50.0	23.8	36.1	21.1	10.8
18	89r	10.8	16.7	40.7	9.1	47.6	60.2	36.8	29.7
19	89s	27.1	35.7	50.1	24.1	22.8	16.8	24.9	46.7
20	89t	28.6	51.6	47.3	28.6	27.4	46.2	39.5	44.3
21	89u	18.9	45.8	59.3	0	23.8	26.5	21.1	10.8
22	89v	51.4	54.2	29.6	50.0	47.6	62.7	52.6	29.7
23	89w	43.2	66.7	31.1	45.5	33.3	62.7	34.2	29.7
24	Chlorothalonil	100	73.3	75.0	73.9	73.1	96.1	90.4	91.3

FC: Fusarium oxysporum f. sp. Cucumerinum; CA: Cercospora arachidicola; PP: Physalospora piricola; AS: Alternaria solani; GZ: Gibberella zeae; RS: Rhizoeotnia solani; BM: Bipolaris maydis; CO: Colleterichum orbicalare.

). Compounds 89i (R = 3-F), 89f (R = 3-OCH₃), and 89q (R = 3-I) exhibited excellent inhibition rates of 88.9%, 77.8%, and 77.8%, respectively, against Physalospora piricola, surpassing the reference fungicide Chlorothalonil (75%). Additionally, compound 89m (R = 4-Cl) showed strong activity against Rhizoeotnia solani, with an inhibition rate of 80.7%.

Chen et al. synthesized a series of 1,3,4-thiadiazole–thiourea derivatives (90a–r, Figure 74) [113]. All compounds were evaluated for antifungal activity (

Table 68. Antifungal activity of compounds 90a–r.

Entry	Compound	Inhibition Rate (%)
		FC	CA	PP	AS	GZ	RS	BM	CO
1	90a	34.1	64.3	64.3	60.6	54.8	19.8	37.7	35.5
2	90b	37.8	67.1	75.2	72.3	47.9	19.8	29.1	35.5
3	90c	28.5	52.9	86.1	61.3	56.6	17.3	33.4	25.7
4	90d	30.4	50.0	66.5	33.1	47.9	53.9	24.9	29.6
5	90e	28.5	64.3	55.7	65.0	58.3	14.9	24.9	27.6
6	90f	21.1	50.5	68.7	50.9	39.3	17.3	22.8	25.7
7	90g	21.1	70.0	64.3	77.3	56.6	17.3	27.0	27.6
8	90h	30.5	80.6	55.5	69.7	56.8	19.3	36.5	33.6
9	90i	29.8	69.3	80.2	61.3	58.1	22.3	38.4	28.7
10	90j	36.1	66.3	66.3	70.3	65.4	21.8	39.7	49.3
11	90k	32.2	64.3	75.2	69.0	47.9	14.9	35.5	33.5
12	90l	26.7	52.9	51.3	71.0	30.7	19.8	35.5	29.6
13	90m	34.1	67.1	64.3	61.3	27.2	17.3	29.1	31.6
14	90n	34.1	61.4	57.8	72.8	79.0	16.1	31.3	37.5
15	90o	26.7	72.9	53.5	70.9	56.6	17.3	29.1	25.7
16	90p	35.9	72.9	36.1	63.8	51.4	17.3	35.5	37.5
17	90q	34.1	70.0	86.1	60.6	58.3	17.3	22.8	29.6
18	90r	28.5	61.4	75.2	63.8	51.4	14.9	24.9	27.6
19	Chlorothalonil	100	73.3	75.0	73.9	73.1	96.1	90.4	91.3

FC: Fusarium oxysporum f. sp. Cucumerinum; CA: Cercospora arachidicola; PP: Physalospora piricola; AS: Alternaria solani; GZ: Gibberella zeae; RS: Rhizoeotnia solani; BM: Bipolaris maydis; CO: Colleterichum orbicalare.

). Several target derivatives demonstrated superior efficacy against Physalospora piricola, Cercospora arachidicola, and Alternaria solani compared to the commercial fungicide Chlorothalonil at a concentration of 50 μg/mL. Notably, compound 90c, bearing a 3-methylphenyl substituent on the thiourea fragment, compound 90q, with an isopropyl substituent, and compound 90i, with a 4-chlorophenyl substituent, exhibited inhibition rates of 86.1%, 86.1%, and 80.2%, respectively, against Physalospora piricola, clearly outperforming the reference control.

The Tahtacı group synthesized two 1,3,4-thiadiazole derivatives containing halogens (91a, 91b, Figure 75) as intermediates in the development of their imidazothiadiazole analogues [114]. In vitro antifungal activity was evaluated by determining the minimum inhibitory concentration (MIC), minimum fungicidal concentration (MFC), and lethal dose (LD₅₀) values against Alternaria solani, Fusarium oxysporum f. sp. melonis, and Verticillium dahliae (

Table 69. Antifungal activity of compounds 91a,b.

Entry	Compound	(LD50 a/MIC b/MFC c μg/mL)
		A. solani	F. melonis	V. dahliae
1	91a	28.7/<1.25/<40	80.3/0.625/>40	14.3/<0.625/20
2	91b	26.1/<1.25/<40	27.9/<1.25/>40	23.5/<0.625/>20
3	Thiram 80% d	11.9/>0.625/<3000	11.3/>0.625/<3000	14.34/<1.25/<3000

^a LD50: The amount of a compound that causes the death of 50% (one half) of test fungi; ^b MIC: Minimum inhibitory concentration; ^c MFC: Minimum fungicidal concentration; ^d positive control.

). Based on the test results, both compounds exhibited moderate antifungal activity against the tested fungal species.

Laachir et al. synthesized a copper(II) coordination polymer based on 2,5-bis(pyridine-2-yl)-1,3,4-thiadiazole (92, Figure 76) [115]. The antifungal activity of the [Cu92Cl₂]ₙ complex was evaluated against three strains of the plant pathogenic fungus Verticillium dahliae (strains SJ, SH, and SE) and one strain of Fusarium oxysporum f. sp. melonis. At a concentration of 50 μg/mL, the complex exhibited moderate inhibitory activity against Verticillium dahliae strains SH and SE and Fusarium oxysporum f. sp. melonis.

2.2.2. Bicyclic 1,3,4-Thiadiazole Derivatives

Zhan et al. synthesized a series of derivatives containing a fused 1,3,4-thiadiazole ring (93a–am, Figure 77) [116]. The compounds were evaluated for antifungal activity against Rhizoctonia solani, Sclerotinia sclerotiorum, Botryosphaeria dothidea, Fusarium graminearum, Colletotrichum capsici, Phytophthora capsici, and Phomopsis sp. at a concentration of 100 μg/mL (

Table 70. Antifungal activity of compounds 93a–am.

Entry	Compound	Inhibition Rate (%)
		RS	PS	FG	CC	PC	BD	SS
1	93a	53.7	27.0	16.4	19.3	24.6	14.2	34.1
2	93b	6.3	21.8	16.2	24.1	30.6	19.8	33.7
3	93c	10.7	27.4	15.6	26.6	35.8	26.3	27.2
4	93d	20.0	12.9	9.3	12.9	6.9	12.6	12.2
5	93e	11.5	19.8	16.0	21.4	36.2	5.7	39.0
6	93f	11.1	12.5	17.7	31.5	30.6	3.2	35.7
7	93g	22.9	30.2	31.5	30.2	34.9	5.2	48.3
8	93h	27.0	34.7	24.8	25.4	36.6	14.9	41.8
9	93i	22.6	29.4	28.6	19.3	37.1	3.6	36.9
10	93j	28.5	32.7	38.2	24.6	31.0	4.8	49.5
11	93k	26.3	23.0	19.8	25.4	32.0	6.0	35.7
12	93l	21.1	12.5	27.3	21.4	27.6	3.6	33.3
13	93m	27.0	12.9	5.9	22.2	28.5	2.4	41.6
14	93n	21.5	14.1	24.4	17.7	35.4	2.8	36.5
15	93o	26.3	13.7	26.9	19.7	34.	3.6	33.3
16	93p	18.9	5.2	17.7	19.4	30.6	3.5	31.7
17	93q	27.0	6.8	21.0	16.5	25.9	26.7	33.7
18	93r	21.9	23.0	47.5	26.6	25.4	3.2	36.5
19	93s	19.2	15.7	34.5	25.4	31.0	9.7	40.2
20	93t	17.4	18.2	26.5	29.4	31.5	8.1	34.9
21	93u	16.3	14.5	32.4	14.1	28.5	15.3	34.9
22	93v	15.2	20.2	34.9	13.3	26.7	6.0	34.5
23	93w	22.2	20.3	42.4	14.9	31.5	17.0	27.6
24	93x	22.6	19.1	31.1	25.0	32.3	22.2	36.1
25	93y	25.2	12.0	25.6	12.9	33.2	3.2	37.4
26	93z	6.1	10.4	9.3	12.9	6.9	12.5	46.7
27	93aa	23.5	24.5	34.5	37.1	35.4	16.2	12.2
28	93ab	21.3	21.6	30.7	11.7	39.2	27.9	33.3
29	93ac	19.1	27.8	41.2	33.9	53.5	21.4	41.4
30	93ad	22.6	49.0	26.9	29.4	55.1	15.8	47.5
31	93ae	28.3	73.1	15.5	48.4	25.9	28.7	49.5
32	93af	30.4	64.4	27.7	48.4	19.5	17.4	49.9
33	93ag	22.2	29.1	28.6	32.3	34.3	10.1	39.0
34	93ah	19.9	21.6	27.3	20.2	28.7	3.6	47.9
35	93ai	24.3	10.8	27.3	30.5	26.7	3.8	41.8
36	93aj	21.3	17.9	23.1	25.7	36.3	3.8	45.9
37	93ak	19.5	14.1	28.6	27.8	35.8	16.2	47.9
38	93al	23.5	36.1	46.2	27.8	31.9	12.4	50.8
39	93am	22.6	15.4	16.5	32.8	19.5	4.2	50.4
40	Azoxystrobin	58.2	55.6	56.6	52.6	64.7	71.9	73.5

RS: Rhizoctonia solani; SS: Sclerotinia sclerotiorum; BD: Botryosphaeria dothidea; FG: Fusarium graminearum; CC: Colletotrichum capsica; PC: Phytophthora capsica; PS: Phomopsis sp.

). Compounds 93ae, bearing a phenyl group at the R² site, and 93af, with a 4-fluorophenyl group at the same position, exhibited inhibition rates of 64–73% against Phomopsis sp., surpassing the activity of the reference fungicide Azoxystrobin. Structure–activity relationship analysis also revealed that the presence of a methyl group at the R¹ site, together with a tetramethylene linker (n = 4), resulted in enhanced activity against Colletotrichum capsici compared to arrangements containing a shorter trimethylene linker (n = 3). In studies involving the Phytophthora capsici strain, the highest activity was observed for compounds carrying electron-donating groups (4-CH₃, 4-OCH₃) at the R² site.

Singh et al. synthesized a series of derivatives bearing a 1,3,4-thiadiazole ring fused with a 1,2,4-triazole moiety (94a–m, Figure 78) [117]. The compounds were screened for antifungal activity against Fusarium oxysporum and Penicillium citrinum. The results obtained at a concentration of 100 ppm are presented in

Table 71. Antifungal activity of compounds 94a–m.

Entry	Compounds	R	Inhibition Rate (%)
			F. oxysporum	P. citrinum
1	94a	H	42	33
2	94b	4-Br	56	44
3	94c	4-Cl	90	95
4	94d	4-F	51	40
5	94e	4-NO2	48	38
6	94f	3,4-diCl	38	28
7	94g	3-Br	50	38
8	94h	3-Cl	67	64
9	94i	3-F	68	62
10	94j	3-CH3	52	48
11	94k	2-Br	35	26
12	94l	2-Cl	59	61
13	94m	2-CH3	63	42
14	Dithane M-45	-	96	97
15	Griseofulvin		99	98

. Among the tested derivatives, compound 94c, containing a (4-chlorobenzyl)thio moiety attached to the fused core, exhibited fungicidal activity comparable to that of the reference fungicides Griseofulvin and Dithane M-45.

Mahdavi’s group synthesized a series of [1,2,4]triazolo[3,4-b][1,3,4]thiadiazole derivatives (60a–n, Figure 49) [88]. All compounds were evaluated for antifungal activity against Cryptococcus neoformans (

Table 72. Antifungal activity of compounds 60-n.

Entry	Compound	R	MIC (µg/mL)
1	60a	H	>256
2	60b	2-F	>256
3	60c	3-F	>256
4	60d	4-F	>256
5	60e	2-Cl	>256
6	60f	3-Cl	1
7	60g	4-Cl	2
8	60h	3,4-diCl	0.5
9	60i	2-Br	>256
10	60j	3-Br	>256
11	60k	4-Br	>256
12	60l	4-NO2	>256
13	60m	2-CH3	>256
14	60n	3-CH3	>256
15	Fluconazole	-	2

). Among them, the monochloro-substituted isomers 60f (R = 3-Cl) and 60g (R = 4-Cl), as well as the dichloro-substituted derivative 60h (R = 3,4-diCl), exhibited significant activity, with MIC values of 1, 2, and 0.5 μg/mL, respectively, outperforming or matching the efficacy of the reference drug Fluconazole (MIC = 2 μg/mL).

Wu et al. synthesized a series of quinazolin-4(3H)-one derivatives incorporating a 1,2,4-triazolo[3,4-b][1,3,4]thiadiazole moiety (61a–ai, Figure 50) [89]. In addition to antibacterial evaluation, the compounds were tested for antifungal activity against Phytophthora nicotianae, Gibberella zeae, Fusarium solani, Alternaria tenuissima, Colletotrichum gloeosporioides, Sclerotinia sclerotiorum, and Fusarium oxysporum (

Table 73. Antifungal activity of compounds 61a–ai.

Entry	Compound	Inhibition Rate (%)
		PN	GZ	VD	FS	AT	SS	FO
1	61a	26.3	37.2	18.1	10.9	13.1	7.2	23.0
2	61b	33.7	32.6	18.3	0	25.3	20.1	10.0
3	61c	33.5	35.7	26.5	8.3	19.6	25.4	14.3
4	61d	24.7	20.9	15.8	0	12.0	7.6	16.8
5	61e	11.7	45.0	6.0	3.6	15.5	4.8	11.7
6	61f	31.8	37.8	19.0	9.6	14.6	17.9	8.9
7	61g	34.8	41.5	24.2	7.8	16.4	8.7	30.1
8	61h	35.2	17.4	10.4	6.9	19.9	9.8	11.7
9	61i	29.6	24.3	13.6	5.1	18.2	16.1	17.4
10	61j	23.6	25.7	14.5	8.6	16.5	14.8	28.9
11	61k	31.2	23.8	21.4	0	14.2	10.4	0
12	61l	36.4	48.3	24.8	12.6	13.6	9.3	33.5
13	61m	23.5	35.0	14.8	8.1	13.8	21.5	15.7
14	61n	44.0	21.7	8.5	8.8	12.8	23.4	11.5
15	61o	62.7	19.6	41.0	12.1	21.2	24.5	17.9
16	61p	39.0	43.4	33.6	6.6	15.9	26.8	21.9
17	61q	18.8	18.4	11.4	0	9.6	0	11.5
18	61r	26.7	16.5	0	10.4	9.8	8.2	9.7
19	61s	67.2	15.7	15.5	8.9	10.3	5.0	8.3
20	61t	0	12.7	0	9.6	0	7.1	11.3
21	61u	19.9	36.1	13.1	0	12.0	12.6	0
22	61v	47.2	4.4	5.8	6.4	8.3	5.3	13.1
23	61w	52.8	47.0	40.6	11.2	20.5	28.4	35.8
24	61x	6.1	9.4	6.8	8.6	11.9	0	0
25	61y	41.6	19.6	36.2	21.7	33.6	50.4	17.6
26	61z	41.1	16.5	0	7.8	10.5	0	10.7
27	61aa	51.2	22.2	4.8	8.8	18.1	38.6	20.7
28	61ab	50.1	23.9	1.8	10.2	10.1	0	8.3
29	61ac	29.3	25.0	8.5	12.3	13.4	6.2	13.3
30	61ad	48.8	19.6	7.5	8.3	14.0	7.9	10.0
31	61ae	34.4	12.9	0	6.1	8.5	0	11.4
32	61af	11.6	20.2	8.5	7.8	5.9	8.3	8.6
33	61ag	0	12.8	4.4	0	0	0	12.3
34	61ah	0	11.9	5.6	7.7	0	4.9	12.1
35	61ai	0	12.4	3.8	7.4	0	5.2	15.5
36	Chlorothalonil	84.5	75.1	67.1	64.7	73.5	100	67.5
37	Carbendazim	81.1	100	73.5	100	100	100	100

PN: Phytophthora nicotianae; GZ: Gibberella zeae; FS: Fusarium solani; AT: Alternaria tenuissima; VD: Colletotrichum gloeosporioides; SS: Sclerotinia sclerotiorum; FO: Fusarium oxysporum.

). Several derivatives exhibited notable inhibitory effects against specific fungal strains. In particular, compounds 61o (R = 3-CH₃C₆H₄), 61s (R = 4-NO₂C₆H₄), 61w (R = 4-OCF₃C₆H₄), 61aa (R = 3-pyridyl), and 61ab (R = 2-pyridyl), demonstrated inhibition rates above 50% against Phytophthora nicotianae, with compound 61s, bearing a 4-nitrophenyl substituent, showing the highest activity (67.2%).

As previously mentioned, Kamoutsis et al. synthesized a range of fused 1,3,4-thiadiazole derivatives (62a–s, Figure 51) [90]. The compounds were evaluated for antifungal activity against six fungal strains: Aspergillus versicolor, Trichoderma viride, Aspergillus niger, Penicillium verrucosum var. cyclopium, Penicillium funiculosum, and Aspergillus fumigatus (

Table 74. Antifungal activity of compounds 62a–s.

Entry	Compound	MIC/MFC (μg/mL)
		AF	AV	AN	TV	PF	PC
1	62a	5/10	2/5	10/20	2/5	20/40	10/20
2	62b	10/20	5/10	10/20	2/5	10/20	10/20
3	62c	20/40	10/20	15/20	2/5	20/36	20/36
4	62d	2/5	2/5	5/1	5/1	10/20	10/20
5	62e	20/36	10/20	15/20	8/10	33/40	20/40
6	62f	5/10	5/10	5/10	2/5	10/20	10/20
7	62g	10/20	10/20	5/10	5/10	5/10	20/36
8	62h	10/20	10/20	15/20	5/10	10/20	15/20
9	62i	5/10	5/10	10/20	5/10	36/80	30/40
10	62j	32/40	20/40	10/20	5/10	10/20	20/40
11	62k	20/40	5/10	20/32	2/5	2/5	2/5
12	62l	5/20	10/20	15/20	5/10	40/67	20/40
13	62m	40/67	20/36	20/40	2/5	40/80	32/40
14	62n	10/36	10/20	10/20	5/10	30/40	32/40
15	62o	15/20	10/20	20/37	5/10	30/40	30/36
16	62p	20/40	10/20	5/1	5/8	10/20	20/40
17	62q	20/40	20/40	20/32	5/10	30/40	30/40
18	62r	20/32	10/20	10/20	8/10	20/40	30/36
19	62s	20/40	20/36	10/20	8/10	20/40	20/40
20	Ketoconazole	20/500	200/500	200/500	1000/1500	200/500	200/300
21	Bifonazole	150/200	100/200	150/200	150/200	200/250	100/200

AV: Aspergillus versicolor; TV: Trichoderma viride; AN: Aspergillus niger; PC: Penicillium verrucosum var. cyclopium; PF: Penicillium funiculosum; AF: Aspergillus fumigatus.

). Several derivatives exhibited antifungal activity up to 80 times greater than Ketoconazole and 3 to 40 times greater than Bifonazole, both used as reference drugs (MIC: 2–40 μg/mL; MFC: 5–67 μg/mL).

Borthakur et al. synthesized a series of thiadiazolo-thiadiazine derivatives (95a–h, Figure 79) [118]. All compounds were screened for antifungal activity against two fungal species, Rhizoctonia solani and Drechslera oryzae, using Carbendazim as the reference fungicide (

Table 75. Antifungal activity of compounds 95a–h.

Entry	Compound	Inhibition Rate (%)
		R. solani		D. orazae
		Concentration 50 (ppm)	Concentration 100 (ppm)	Concentration 50 (ppm)	Concentration 100 (ppm)
1	95a	33.25	50.15	36.12	52.19
2	95b	46.21	74.80	42.14	76.20
3	95c	31.46	50.12	42.09	57.20
4	95d	35.34	59.84	30.86	68.76
5	95e	40.74	59.35	36.75	49.45
6	95f	46.75	52.76	46.54	61.95
7	95g	47.37	75.38	38.45	55.20
8	95h	38.56	62.24	42.24	60.24
9	carbendazim	96.67	98.56	95.45	98.26

). Among the tested derivatives, only compound 95b, containing a nitro group at the 4 position, compound 95d, containing a methyl group at the 4 position, and compound 95g, containing chlorine at the 4 position, exhibited mild to moderate antifungal activity.

Bhadraiah et al. synthesized a series of bicyclic 1,3,4-thiadiazolo[3,2-α]pyrimidine analogues (63a–i, Figure 52) [91]. In addition to antibacterial evaluation, the compounds were tested for antifungal activity against Aspergillus flavus, Aspergillus niger, Fusarium oxysporum, and Fusarium moniliforme, using Nystatin as the reference drug (

Table 76. Antifungal activity of compounds 63a–i.

Entry	Compound	R1	R2	MFC (μg/mL)
				A. flavus	A. niger	F. oxysporum	F. monaliforme
1	63a	H	H	250	265	270	280
2	63b	H	OCH3	290	285	280	275
3	63c	H	Cl	135	120	125	120
4	63d	OCH3	H	250	220	270	230
5	63e	OCH3	OCH3	135	155	150	165
6	63f	OCH3	Cl	285	275	280	260
7	63g	Cl	H	135	155	140	165
8	63h	Cl	OCH3	215	220	210	215
9	63i	Cl	Cl	120	125	130	130
10	Nystatin	-	-	100	100	100	100

). Among the synthesized derivatives, compound 63c, containing chlorine at the R² site, and compound 63i, bearing chlorines at the R¹ and R² sites, exhibited notable antifungal activity.

2.2.3. Multi-Substituted 1,3,4-Thiadiazole Derivatives

Dai et al. synthesized a series of 5-sulfonyl-1,3,4-thiadiazole-substituted flavonoids (68a–ah, Figure 55) [94] initially evaluated for antibacterial activity. The compounds were also tested for antifungal activity against Botrytis cinerea, Alternaria solani, Rhizoctonia solani, Gibberella zeae, and Colletotrichum orbiculare. Inhibition rates at a concentration of 10 μg/mL are presented in

Table 77. Antifungal activity of compounds 68a–ah.

Entry	Compound	R1	R2	R3	Inhibition Rate (%)
					B. cinerea	A. solani	R. solani	G. zeae	C. orbiculare
1	68a	H	CH3	CH3	69.36	47.71	82.53	54.59	46.88
2	68b	6-Br	CH3	CH3	98.57	100.00	100.00	77.32	92.90
3	68c	6-F	CH3	CH3	69.84	70.00	91.67	71.46	86.53
4	68d	6-Cl	CH3	CH3	89.07	100.00	100.00	81.41	94.67
5	68e	6-CH3	CH3	CH3	29.82	48.02	79.13	51.13	65.43
6	68f	7-Br	CH3	CH3	38.37	89.17	100.00	33.37	59.62
7	68g	H	CH2CH3	CH3	43.00	59.94	95.57	54.08	92.21
8	68h	6-Br	CH2CH3	CH3	84.56	100.00	100.00	81.60	100.00
9	68i	6-F	CH2CH3	CH3	42.52	74.01	90.08	67.02	100.00
10	68j	6-Cl	CH2CH3	CH3	78.29	100.00	100.00	68.22	100.00
11	68k	6-CH3	CH2CH3	CH3	31.66	62.44	84.33	46.91	90.87
12	68l	H	CH2CH2CH3	CH3	60.38	56.88	87.48	38.13	78.14
13	68m	6-Br	CH2CH2CH3	CH3	80.82	95.72	100.00	51.11	84.84
14	68n	6-F	CH2CH2CH3	CH3	42.51	90.70	100.00	39.56	97.73
15	68o	6-Cl	CH2CH2CH3	CH3	81.33	92.68	100.00	50.52	92.56
16	68p	6-CH3	CH2CH2CH3	CH3	32.72	25.73	47.28	20.89	32.22
17	68q	7-Br	CH2CH2CH3	CH3	32.51	35.03	60.62	17.83	33.09
18	68r	H	CH3CHCH3	CH3	33.55	65.77	87.77	47.81	67.93
19	68s	6-Br	CH3CHCH3	CH3	72.86	100.00	100.00	50.03	100.00
20	68t	6-F	CH3CHCH3	CH3	5.44	94.47	89.17	50.33	79.39
21	68u	6-Cl	CH3CHCH3	CH3	72.31	100.00	100.00	55.77	97.06
22	68v	7-Br	CH3CHCH3	CH3	22.18	64.92	100.00	70.02	21.12
23	68w	6-F	allyl	CH3	59.09	39.18	77.34	38.37	91.38
24	68x	6-CH3	allyl	CH3	35.77	17.69	61.59	14.74	37.90
25	68y	H	Bn	CH3	14.52	26.04	47.78	21.93	11.03
26	68z	6-Br	Bn	CH3	5.50	39.18	60.28	22.48	13.01
27	68aa	6-F	Bn	CH3	19.13	18.54	62.60	22.51	20.27
28	68ab	6-Cl	Bn	CH3	13.61	18.78	1.54	5.89	0.80
29	68ac	6-CH3	Bn	CH3	14.33	0.00	47.46	7.54	6.88
30	68ad	7-Br	Bn	CH3	11.82	0.00	16.52	3.93	1.38
31	68ae	6-F	CH3	CH2CH3	16.04	71.83	100.00	50.47	87.39
32	68af	6-Cl	CH3	CH2CH3	85.61	100.00	100.00	66.48	94.80
33	68ag	H	i-Pr	CH2CH3	27.38	94.17	94.08	10.62	89.66
34	68ah	6-CH3	i-Pr	CH2CH3	67.48	83.44	69.72	31.83	82.86

. The results indicated that most of the target compounds exhibited significant antifungal activity. Against Botrytis cinerea, six halogen- and alkyl-containing compounds—68b (R¹ = 6-Br, R² = CH₃, R³ = CH₃), 68d (R¹ = 6-Cl, R² = CH₃, R³ = CH₃), 68h (R¹ = 6-Br, R² = CH₂CH₃, R³ = CH₃), 68m (R¹ = 6-Br, R² = CH₂CH₂CH₃, R³ = CH₃), 68o (R¹ = 6-Cl, R² = CH₂CH₂CH₃, R³ = CH₃), and 68af (R¹ = 6-Cl, R² = CH₃, R³ = CH₂CH₃)—showed high inhibition rates exceeding 80%, with the 6-bromine-containing compound 68b demonstrating the strongest effect (98%).

As previously mentioned, Gomha et al. synthesized a series of 1,4-dihydropyridine–1,3,4-thiadiazole hybrids (69a–h, Figure 56) [95]. In addition to antibacterial evaluation, the compounds were assayed in vitro for antifungal activity against Aspergillus niger and Geotrichum candidum at a concentration of 30 μg/mL (

Table 78. Antifungal activity of compounds 69a–h.

Entry	Compound	Zone of Inhibition (mm)
		A. niger	G. candidum
1	69a	21.1	19.8
2	69b	22.3	23.4
3	69c	18.5	23.2
4	69d	19.9	24.1
5	69e	25.3	24.2
6	69f	24.1	23.6
7	69g	23.3	22.3
8	69h	25.5	23.2
9	Amphotericin B	23.3	25.2

), using Amphotericin as the reference fungicide. All tested derivatives exhibited high antifungal activity against the selected fungal strains.

Rashdan and coworkers synthesized a series of 1,3,4-thiadiazole derivatives (70a–e, Figure 57) [96], which, in addition to antibacterial evaluation, were also tested for antifungal activity against Candida albicans. The compounds exhibited low activity, with MIC values of 20 μg/mL, in comparison to the reference drug Nystatin (MIC = 5 μg/mL).

Rashdan and coworkers also evaluated another previously reported series of 1,3,4-thiadiazole derivatives (71a–f, Figure 58) [97] for antifungal activity, alongside their antibacterial assessment. The compounds were screened against the black fungal strain Rhizopus oryzae at a concentration of 20 mg/mL (

Table 79. Antifungal activity of compounds 71a–f.

Compound	71a	71b	71c	71d	71e	71f	Amphotericin
Zone of inhibition (mm)	9.7	10	12	11	14	17	21

). The results indicated that compound 71f, containing a nitro group at the R¹ site and a phenylamino substituent at the R² site, exhibited significantly higher inhibitory potency against the tested strain.

3. Conclusions

Over the last decade, 1,3,4-thiadiazole derivatives have emerged as key scaffolds in medicinal chemistry due to their broad-spectrum biological activities and structural versatility. This review highlights recent advancements (2020–2025) in the development of thiadiazole-based compounds with notable antibacterial and antifungal properties. The antimicrobial activity of these derivatives is often attributed to the presence of a toxophoric N–C–S moiety and their ability to interact with biological targets via hydrogen bonding or metal ion coordination. The mesoionic character and favorable physicochemical properties of the 1,3,4-thiadiazole ring, such as balanced lipophilicity, molecular weight, and hydrogen bonding capacity, contribute to enhanced membrane permeability and bioavailability. These features support their potential as effective enzyme inhibitors and receptor ligands. An important role in the modulation of antimicrobial activity is also played by substituents attached to the thiadiazole ring, either directly or indirectly through an appropriate aromatic or aliphatic linker. Studies have shown that the substituents enhancing biological activity in the analyzed systems include hydroxyl, methoxy, trifluoromethyl, nitro, alkyl groups, and halogens, particularly fluorine and bromine. Biological studies conducted on 10 Gram-negative bacterial strains revealed that 53 novel thiadiazole derivatives exhibited activity exceeding that of the reference drugs or demonstrated a high level of growth inhibition (90–100%). The highest susceptibility was observed for Xanthomonas oryzae pv. oryzae (Xoo), against which 19 compounds showed antibacterial efficacy above 90%. Significant activity was also noted against Escherichia coli (14 compounds) and Pseudomonas aeruginosa (7 compounds). Similarly, studies performed on nine Gram-positive bacterial strains demonstrated that 26 derivatives exhibited activity surpassing the reference drugs or achieving a high level of growth inhibition (90–100%). The highest susceptibility was observed for Enterococcus faecium, with 17 compounds displaying antibacterial efficacy exceeding 90%. Very high activity was also recorded against Staphylococcus aureus, where seven compounds produced excellent results.

In antifungal assays, thiadiazoles were evaluated against 25 fungal species belonging to 15 genera. Among the tested derivatives, 75 novel compounds exhibited growth inhibition surpassing that of the reference drugs or within the 90–100% range. The most susceptible fungi proved to be Rhizoctonia solani, against which 18 compounds displayed fungicidal activity above 90%, followed by Botrytis cinerea (17 derivatives), Colletotrichum orbiculare and Colletotrichum gloeosporioides (14 derivatives), and Aspergillus niger and Aspergillus clavatus (12 derivatives).

In conclusion, 1,3,4-thiadiazole derivatives represent promising candidates for the development of next-generation antimicrobial agents, and ongoing research continues to reinforce their value in both pharmaceutical and agrochemical applications. These results underscore the potential of thiadiazole derivatives as versatile scaffolds for the rational design of novel antimicrobial agents.