Condensation Reactions of 2-Aminothiophenoles to Afford 2-Substituted Benzothiazoles of Biological Interest: A Review (2020–2024)

Abstract

Several benzothiazole (BT) derivatives have recently been explored in medicinal chemistry, and they are frequently reported in the literature. The interest in this kind of heterocyclic compounds and their structural hybrids has been increasing, as shown by several reviews reported over the last decade. In this context, we found that about 70 articles related to the synthesis of BT derivatives that studied their biological activities were published in the last five years. From this, we prepared a review on the synthesis and biological activity studies about this topic. In this bibliographic review it was found that medicinal chemists also explore BT derivatives in search of anticancer and anti-Alzheimer’s candidates. This review comprehends 70 articles, published between 2020 and 2024, related to the synthesis of BT derivatives with the purpose of assessing their biological activities. On the other hand, BT derivatives have been explored as molecular species that perform two or more biological actions, called multifunctional drugs. Some accounts related to the structure–activity relationship which provide a framework for drug discovery and design are also discussed. The synthetic methods of BT synthesis include the use of biocatalysts, solvent-free conditions, photocatalysts, and catalysts supported on nanoparticles. Studies also explore renewable energy sources such as microwave, UV, and visible-light and mechanochemical sources.

1. Introduction

Benzothiazole (BT) is an aromatic heterobicyclic system containing a benzene ring fused to a thiazole ring. This small molecule is frequently present in compounds that possess a wide range of biological activities. BT derivatives are rarely found in natural products; however, some of them have been found in terrestrial and marine natural products as part of simple or more complex structures. For instance, in the 1940s, the simple firefly d-luciferin structure [1,2,3], the more complex rifamycins P and Q, and thiazinotrienomycin F and G were isolated (Figure 1) [4,5]. Aromatic constituents of cranberries and tea leaves and flavor compounds such as the benzothiazole (BT) alkaloids nordercitine, dercitamide, dercitamine, and cyclodercitin, from the Dercitus sp. and Stellata sp. sponges, were also isolated (Figure 1) [6,7]. On the other hand, since riluzole (6-trifluoromethoxy-2-benzothiazolamine) has been clinically used to diminish amyotrophic lateral sclerosis progression and as an anticonvulsant drug [8,9,10], medicinal chemists have been interested in developing synthetic methodologies to afford BT derivative compounds.

From these findings, BT cores have been found in several synthesized drugs. From 1970 to the present, BT derivatives have been found to possess antiviral [11,12,13], antibacterial [13,14,15,16], antimicrobial [17,18,19], fungicidal [20,21], antiallergic [22,23,24], antidiabetic [25,26,27], antitumoral [28,29,30,31], anti-inflammatory [32,33], and anthelmintic [34,35,36] properties and have recently been shown to possess anticonvulsant and antioxidant [37,38,39,40,41,42] biological activities. For instance, zopolrestat has been used since 1991 in treating diabetes (Figure 2a) [43], compound 2-(3,4-dimethoxyphenyl)-5-fluoroBT (PMX 610, NSC 721648) has shown antitumor activity (Figure 2b) [44,45], and the Schiff bases shown in Figure 2c were found to be useful in the treatment of Alzheimer’s disease [46] (Figure 2).

On the other hand, the BT nucleus has been found in molecules used as ligands for catalysis [47,48]. In this context, we found that the investigation of the synthesis and biological activity of BT derivatives in medicinal chemistry has increased. In the last five years we found twenty-one article reviews about this topic, from 2020 to 2024 [49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70], several of them related to the use of BT derivatives as anticancer agents [51,52,53,54,55,56,57,58,62,63,64]. Also, we found in this period about 53 articles related to the condensation reaction of 2-aminothiophenol (ATP) to synthesize BT derivatives. In some of these, the synthesized compounds were studied concerning their biological activity. By conducting this search, we prepared a bibliographic review on the synthesis and biological activity studies about this topic. In this review it was found that medicinal chemistry also explores BT derivatives in search of anticancer [53,54,55,56,57], anti-Parkinson’s [71,72], and anti-Alzheimer’s candidates [73,74,75]. In addition, some accounts of the structure–activity relationship that provide a framework for drug discovery and design were also discussed. On the other hand, BT derivatives were also explored as molecules capable of two or more biological actions, known as multifunctional drugs [76]. Some recent examples of this kind of BT-based compounds that have been clinically approved are halethazole (antibacterial) [77], thioflavin-T (amyloid imaging agent) [78], dimazole (antifungal) [79], ethoxzolamide (treatment of glaucoma) [80], phortress (antitumoral) [81], riluzole (antidepressant) [82], flutemetamol (radiopharmaceutical) [83], and frentizole (immunosuppressive) [84] (Figure 3).

2. Condensation of 2ATPs with Carbonyl Compounds

One of the best-known reactions that affords 2-substituted benzothiazoles (BTs) is the condensation of carbonylic compounds with 2-aminothiophenoles (2ATPs). The followed pathway consists of three stages: (a) imine thiophenol (ITP) formation, (b) cyclization to benzothiazolidine (BTI), and (c) reduction to BTs (Scheme 1). These reactions have been carried out with and without the use of catalysts.

2.1. Condensation with Aldehydes

In one step, the condensation reaction of 2ATP with several benzaldehydes using sodium hydrosulfite (Na₂S₂O₄) as an oxidizing agent in refluxing a mixture of water–ethanol as the dissolvent for 12 h was carried out to afford 2-arylBTs 1a–k in 51–82% yields (

Table 1. Compounds 1a–k, 2b–k and 3b–k.

	1a–k	2b–k	3b–k	1–3	a	b	c	d	e	f
R1 =	H	6CO2H	6SO2NH2	R2 =	Ph	2OHPh	3OHPh	4OHPh	4MeOPh	2,4OHPh
			% Yields		60	80,71,78	71,55,78	78,67,84	82,82,80	68,53,74
					g	h	i	j	k
					2,5OHPh	2,3OHPh	2OH,3MePh	3,5OHPh	2,4,5OHPh
			% Yields		58,75,55	70,69,69	65,69,78	58,59,63	51,63,51

) [85]. On the other hand, the condensation of 5-substituted 2ATPs as hydrochlorides or disulfides with benzaldehydes under refluxion for 36 h afforded the BTs 2b–k in 53–82% yields and 3a–k in 51–84% yields.

Compounds were evaluated as sun-filtering, antioxidant, antifungal, and antiproliferative agents [85]. Compound 1g was the most effective blocker of hERG potassium channels expressed in HEK 293 cells, resulting in 60.32% inhibition, with an IC₅₀ = 4.79 μM. Compounds 1g, 1h, 1k, 2d, 2g, 3d, and 3g were considered broad-spectrum and ultraviolet A filters (UVA filters) because their λc ≥ 370 nm and the ultraviolet A-protector factor (UVA-PF) values were greater than one-third of their sun protection factor (SPF) values. Compounds 1d and 1j were regarded as candidates for UVB protection. All compounds were photostable, especially 2e, which showed a degradation rate of less than 2%.

It was found that the wavelength of maximum absorption (λ_max) was related to stereo electronic effects by substituents on the 2-aryl or BT moiety, which alter the electron density of compounds. The -OH group at the para position of the 2-aryl group leads the resonance interaction of the lone pair of oxygen and the π cloud of 2-aryl and the BT group. This shift increases if -OH is at the ortho position, due to the formation of an intramolecular hydrogen bond between the nitrogen of BT and the hydrogen of the -OH group. On the other hand, the order of λ_max along the series was -H ≪ -SO₂NH₂ < -COOH. Additionally, the -COOH or -SO₂NH₂ groups on the BT ring increased the photostability of the compounds.

Compounds 1i,j, 2g,h, and 3r,s showed a good antioxidant profile. Compound 1h had the highest antifungal activity against the dermatophytes Trichophyton tonsurans, Epidermophyton floccosum, and Microsporum gypsum. Significant activities were found for compounds 1f and 1g against Epidermophyton floccosum and Microsporum gypseum. Compounds 1g and 1k exhibited high antiproliferative activity against CEM and SK-Mel 5 tumor cells, 1g (6-fold) and 1k (25-fold) being more selective for CEM and SK-Mel 5 cells, respectively. Because of their various biological activities, some 2-arylBTs could be applied as drugs in the treatment of neoplastic diseases such as melanoma, childhood leukemia, and pancreatic cancer. However, based on the experimental results and SAR studies on positions 2 and 6 of 2-arylBTs, compounds 1g and 1k were proposed as powerful candidates for designing multifunctional drugs.

2ATP was condensed with substituted benzaldehydes by using biodegradable rice husk chemically activated carbon (RHCAC) as the catalyst at room temperature in ethanol–water to afford 2-arylBTs 4a–f in 93–98% yields (

Table 2. Compounds 4a–f.

4	a	b	c	d	e	f
R1 = H, R2 =	Ph	4MeOPh	4ClPh	4NO2Ph	4OH-5MeOPh	4BrPh
% Yield	98	94	93	96	97	90

) [86]. The catalyst was recovered by filtration, washed, dried, and recycled for eight runs, unlike a corrosive and toxic acid catalyst that was difficult to recover and separate.

No significant effects of electron donation were observed, while OMe, CH₃, and Cl groups and electron-withdrawing NO₂ groups were found, and the reactions were completed in short times (5–10 min), giving high yields of the desired products (93–98%).

A series of 2-arylBTs 5a–f were synthesized from the condensation reaction of 2ATP with substituted benzaldehydes in 1,4-dioxane under an O₂ atmosphere in the presence of [PhI(OH)OTs] (Koser’s reagent) as a catalyst at room temperature (

Table 3. Compounds 5a–f.

5	a	b	c	d	e	f
R1 = H, R2 =	pMeOPh	3,4MeOPh	3,4,4MeOPhPh	4CNPh	4FPh	3NO2Ph
% Yield	84	88	90	895	87	80

) [87]. The features of this protocol included short reaction times (15 min), a broad substrate scope, 80–90% yields, low catalyst loading, and scalability.

Six p-substituted 2-arylBTs 6a–f were obtained in 70–90% yields from the condensation of 2ATP with p-benzaldehydes catalyzed with Cu₂O and DMSO as oxidants in string at room temperature for 3–5 h (

Table 4. Compounds 6a–f.

6	a	b	c	d	e	f
R1 = H, R2 =	Ph	pClPhPh	pFPh	pMePh	pNO2Ph	4BrPh
% Yield	85	80	85	90	88	70

) [88]. The reaction tolerated a wide range of functional groups. The compounds 6c and 6e showed equal antifungal activity against A. niger and the highest activity against C. albicans, compared to voriconazole as a reference.

Three green synthetic protocols were used to synthesize 6-substituted 2-arylBTs, 7a–f and 8a–f, in 62–89% yields (

Table 5. Compounds 7a–f and 8a–f.

7a–f	8a–f	7,8	a	b	c	d	e	f
R2 = oOHPh	R2 = oMeOPh	R1 =	H	NO2	CN
	% Yield	=	62,70	59,78	71,79	66,74	63,83	89,82

) [89]. Method A: A stirred suspension of 4-substituted-2ATP and the 2-hydroxybenzaldehyde was heated in glycerol at 110–170 °C for 1 to 24 h to afford compounds 7a–c in 62–71% yields. Method B: A stirred suspension of the 5-substituted 2ATP disulfide and 2-hydroxybenzaldehyde or 2-methoxybenzaldehyde was heated in glycerol at 160–175 °C for 30–60 min to afford compounds 7a–c in 74–80% yields and 8a–c in 70–79% yields. Method C: The corresponding 2-substituted benzaldehyde was added to a stirred solution of 4-amidino-2ATPate in glacial acetic acid, and the mixture was heated under nitrogen for 4 h, then alkalinized with NaOH (pH 10–11); the resulting free base was mixed with 2-propanol and methanesulfonic acid and stirred at room temperature for 2 h to afford compounds 7d, 7e, 8d, and 8e in 62–83% yields. Method D: A solution of SnCl₂ dihydrate in conc. HCl and methanol was added to the corresponding 6-nitro-substituted BTs 7b and 8b, then refluxed for 30 min. The mixture was alkalinized with 20% NaOH (pH 8–9) to afford compounds 7f and 8f in 89.3 and 81.6% yields, respectively. The influence of the hydroxy and methoxy groups and substituents on C6 on the 2-arylBT scaffold against antibacterial, antitumor, and antioxidant activities was studied. The amidino derivatives 7d and 8d showed modest activity against Gram-negative and Gram-positive bacterial strains. At the same time, compounds 7b, 7c, 7e, 7f, and 8f resulted in potent and selective antiproliferative activity towards tumor cells, without activity against human skin fibroblasts. Compounds 7f and 8f were the most selective against the growth of HeLa cell lines. Compound 7f resulted in the most promising radical scavenging activity. HIF-1α hydroxylated protein was upregulated by treatment of HeLa cells with compounds 7f, 8c, and 8f, which were considered potent suppressors of the hypoxia-induced HIF-1 protein. Compounds 7f and 8f were proposed to lead compounds for further rationalized design of the BT skeleton. Compound 7f with the OH group on the 2-arylBT core had the most promising antioxidative activity as evaluated by DPPH, ABTS, and FRAP in vitro assays. The presence of the amino protonated group attached at the BT moiety was essential for the antiproliferative and antioxidant activities observed, exerted through a change in the levels of the reactive oxygen species-modulated HIF-1 protein. The BTs 7a–f and 8f showed good antioxidant activity (38 to 117 µM), while BTs 8a–e had a low ability for stabilization of ABTS^•+ radicals (>200 µM).

In a two-step protocol, the corresponding p-bromophenyl-ITP intermediate, accessed from the condensation of 2ATP and p-bromobenzaldehyde, suffered a Pd-catalyzed cyclization to 2-bromophenyl-BT 9 in [BMIM][BF₄] or [BMIM][PF₆] as a recycled IL solvent. Compound 9 was functionalized by cross-coupling reactions (Suzuki, Heck, or Sonogashira) and catalyzed by Ni or Pd to afford a series of substituted 2-aryl-/heteroaryl-BTs, 10a–e, 11a–d, and 12a–d, in acceptable yields (55–86%) under mild reaction conditions (Scheme 2) [90]. The yields were from moderate to good for 10a–e (60–81%, 12–14.5 h), 11a–d (63–80%, 16.5–17 h), and 12a–d (55–80%, 14.5–16 h).

2-substituted BTs, 14a,b (25, 34%) and 17a,b (52, 60%), were designed and synthesized from BTs 13 (60%) and 15 (34%), which were obtained from the condensation of 2-aminobenzothiazole (2ABT) with 3-bromobenzaldehyde and lactic acid, then oxidation of the derived alcohol with MnO₂, to be tested as antiproliferative cancer cell lines (Scheme 3) [91]. Compounds 217a–b were obtained from compound 15 by (a) cyclization of 2ABT with 3-bromobenzaldehyde to afford compound 13, and the BTs 14a,b were obtained from compound 13 by (b) Buchwald–Hartwig amination with anilines. Compounds 17a–b were obtained from (d) α-bromination with CuBr₂, then (e) reaction with the corresponding substituted thiourea. Antiproliferative assays using cancer cells from the breast (MCF7) and prostate (22Rv1 and PC3) were carried out for all synthesized BTs. However, these compounds had worse activity than JG-98 as a reference (IC₅₀ values from ~0.7 to 13 µM).

A photo-assisted radical cyclo-condensation of 2ATP with a series of aromatic and aliphatic aldehydes was designed using wosin Y as a photocatalyst and K₂CO₃ or Et₃N as basic media and tert-butyl hydroperoxide (TBHP) (70% in water) in acetonitrile to afford the 2-aryl(alkyl)BTs 18a–j in 70–92% yields (

Table 6. Compounds 18a–j.

18	a	b	c	d	e	f	g	h	i	j
R1 = H, R2 =	pMePh	pClPh	pBrPh	pFPh	Ph	ptBuPh	pMeOPh	Furan-2-yl	Py-4-yl	nHexyl
% Yield	92	88	92	88	88	70	91	82	85	90

) [92]. Unfortunately, the designed method required harsh reaction conditions, a long reaction time (24–36 h), and flash column chromatography and allowed no recovery of the catalyst.

A reasonable mechanistic process has been proposed beginning with the in situ formation of the corresponding imine and the excitation of eosin Y under blue LED irradiation, then the reaction of eosin Y* with TBHP to generate an (eosin y)^.-radical anion and a t-Bu^. radical, which undergo hydrogen atom abstraction with the imine to produce the Ar-O^. imine-fenoxy radical. Then, there occurs an intramolecular cycloaddition to the corresponding benzothiazoline, which is transformed to a benzothiazoline^.+ radical cation that reacts with the eosin Y^.-radical anion to afford the corresponding BT. This method tolerates several aldehydes, including aliphatic, aromatic, cyclic, and heteroaromatic aldehydes, but has several limitations, such as the requirement of a transition metal, a high temperature, and a pre-synthesized catalyst/ligand scaffold.

As previously reported, three series of substituted 2-arylBTs, 21–23, were synthesized as VEGFR-2/FGFR-1/PDGFR-β multi-angiokinase inhibitors targeting breast cancer (Scheme 4) [93]. The compounds 21 and 22 were obtained from the condensation of 2ATP and vanillin in refluxing DMF to afford the 2-arylBT 19, which was subsequently reacted with methyl bromoacetate, followed by hydrazinolysis with hydrazine hydrate in refluxing ethanol to afford the hydrazide 20, which through acid catalysis with aldehydes or ketones afforded the Schiff bases 21a–n and 22a–c in 40% to 88% yields. On the other hand, the reaction of hydrazide 20 with isocyanates and isothiocyanate in refluxing ethanol afforded the corresponding ureas 23a,b in 84 and 72% yields and thiourea 23c in a 61% yield.

Compounds 21d, 21f, 21i, and 21k showed high inhibitory activity against multiple kinases: VEGFR-2 (IC₅₀s of 0.19, 0.18, 0.17, and 0.13 μM, respectively), FGFR-1 (IC₅₀s of 0.28, 0.37, 0.19, and 0.27 μM, respectively), and PDGFR-β (IC₅₀s of 0.07, 0.04, 0.08, and 0.14 μM, respectively). Additionally, the substituted 2-arylBTs exhibited promising cytotoxic activity against the MCF-7 cell line comparable to that of sorafenib as a reference drug (IC₅₀ = 4.33 μM). The most potent arylBTs were 21d and 21i (IC₅₀s of 7.83 and 6.58 μM). Also, 21d and 21i had VEGFR-2 inhibitory activity in MCF-7 cells of 81% and 83% compared with sorafenib (88%). Molecular docking of compounds in the VEGFR-2 and FGFR-1 active sites showed that the 2-phenylBT moiety was placed in the hinge region of the tyrosine kinase (RTK)-binding receptor site. In contrast, the amide moiety showed hydrogen bonding interactions with the amino acids, directing the aryl group to the hydrophobic allosteric back pocket of the RTKs in a type II-like binding mode. Additionally, the aryl BTs exhibited good ADME properties, facilitating further optimization in drug discovery.

An acetic acid-mediated three-component condensation reaction of 2ATPs and α, β-unsaturated aldehydes in the presence of thiophenols was developed to afford 2-thioalkyl BTs 24a–t (Figure 4) [94]. Metal-free conditions were used in the reaction, with oxygen as an oxidant. This method was regioselective, tolerant of several functional groups, and provided access to various kinds of BTs with modest to good yields (34–81%).

On comparing the yields of compounds 24a (70%), 24b (42%), and 24d, the position of the methyl/phenyl group (R⁴) on the α, β-unsaturated aldehydes affected the reaction yield. However, α, β-unsaturated aldehydes bearing an electron-donating group or electron-withdrawing group like Me, Cl, or Br at the para position of the phenyl ring produced the compounds 24e–g in moderate yields (54–57%). Also, moderate yields (51–59%) were observed when Me, Cl, and F substituents were located at different positions on the benzene of the BT ring. Thiophenols containing methyl groups at the para, meta, or ortho positions did not show an influence and efficiently reacted in moderate yields. However, electron-donating groups such as ^tBu and OMe at the para position of thiophenols afforded BTs in 71% (24k) and 80% (24l) yields, respectively. Halogen-containing thiophenols such as F, Cl, and Br successfully afforded the thioalkyl-substituted BTs 24a, 24m, and 24n in 53–70% yields. Various disubstituted thiophenols resulted in good substrates for this reaction to afford the BTs 24o–q in 56, 80, and 81% yields, respectively.

The eco-friendly biocatalytic oxidant system Laccase/DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) was applied to condense 2ATP with aromatic aldehydes to afford 2-arylBTs 25a–x in 65–98% yields using O₂ pure or from the air as an oxidant in aqueous media at room temperature in 1 h (

Table 7. Compounds 25a–x.

25	a	b	c	d	e	f	g	h	i	j
R1 = H, R2 =	Ph	4MePh	4MeOPh	4,5MeOPh	4NMe2Ph	4FPh	2ClPh	4ClPh	5,6ClPh	4,6ClPh
% Yield	95	90	98	96	80	97	87	98	92	94
25	k	l	m	n	o	p	q	r	s
R1 = H, R2 =	4BrPh	3BrPh	3Br,4OHPh	4OHPh	2MeOPh	3MeOPh	3FPh	3NO2Ph	5Mefuran-2-yl
% Yield	98	96	95	90	90	93	95	75	95
25	t		u		v		w		x
R1 = H, R2 =	naphtalen-1-yl		antracen-10-yl		naphtalen-2-yl		CH=CHPh		4BT-2-ylPh
% Yield	86		65		85		70		72

) [95]. Two steps were required in the aerobic oxidative cyclization: (1) chemical cyclization and (2) chemoenzymatic oxidation. Benzaldehydes with electron-donating and electron-withdrawing groups, heterocyclic and α,β-unsaturated aldehydes, 1-naphthaldehyde, 2-naphthaldehyde, 9-anthraldehyde, and terephthaldehyde were successfully applied to prepare the corresponding BTs. The advantages of this method are as follows: (1) the use of air or O₂ as an environmentally benign, inexpensive, and abundant oxidant agent and the formation of H₂O as a non-toxic by-product; (2) the synthesis of BTs in good to high yields in aqueous media at room temperature; (3) its superiority with respect to other available methods and its attractiveness to the pharmaceutical industry owing to its being free from any toxic and expensive transition metals and halide catalysts; (4) its conforming to several of the guiding principles of green chemistry.

In a one-pot reaction, 2ATP was condensed with benzaldehydes and aromatic heterocyclic aldehydes under solvent-free conditions using ZnCl₂ nano-flakes supported on nano-hydroxyapatite (HAp) as a catalyst to afford the 2-arylBTs 26a–l in 65–95% yields in 15–90 min (

Table 8. Compounds 26a–l.

	26	a	b	c	d	e	f	g
R1 = H	R2 =	Ph	4MeOPh	1,2,3MeOPh	3MeO,4OHPh	4FPH	4ClPh	3ClPh
% Yield		95	80	67	80	90	85	90
	26	h	i		j	k	l
		4NO2Ph	Benzo[1,3]dioxole		Py-3-yl	Indol-3-yl	5Mefuran-2-yl
% Yield		75	80		65	80	85

) [96]. The reaction was fast, eco-friendly, and efficient. The molecular docking validation with transpeptidase and 14 α-demethylase enzyme inhibitors showed that five 2-substituted-BTs gave good energy values. Also, antioxidant studies showed that four BTs were promising in relation to ABTS (inhibition %) compared with ascorbic acid. Substituted and disubstituted benzaldehydes with electron-donating groups on R², such as 4MeO and OH groups, and electron-withdrawing groups, such as F, Cl, and NO₂ groups, gave BTs 26a,b,d–h with excellent yields (75–80%); however, 1,2,3-OMe-substituted and 3-pyridil-substituted benzaldehydes gave BTs 26c and 26j in 67 and 65% yields, respectively.

The condensation reaction of substituted 2ATPs with several aromatic and aliphatic aldehydes was catalyzed with sulfated tungstate (ST) under solvent-free, room temperature, and ultrasound irradiation conditions to afford 2-substituted BTs, 27a–z and 28a–d, in excellent yields (90–98%) (

Table 9. Compounds 27a–z and 28a–d.

27	a	b	c		d		e	f	g	h	i	j
R1 = H, R2 =	Ph	pMePh	pMeOPh		pNMe2Ph		pFPh	pClPh	pBrPh	oBrPh	oClPh	oNO2Ph
% Yield	98	95	94		90		96	97	96	90	93	90
27	k	l	m		n		o	p		q	r
R1 = H, R2 =	pCF3Ph	pCNPh	pNO2Ph		Napht-1-yl		Napht-2-yl	Py-4-yl		Py-3-yl	Furan-2-yl
% Yield	96	95	96		95		94	94		95	94
27	s	t	u	v	w	x	y	z
R1 = H, R2 =	Thiophen-2-yl	Me	iPr	cPr	cHex	Bn	(CH2)2Ph	CH2naphthalen-1-yl
% Yield	93	93	92		92		95	94	92
28	a	b	c		d
R1 = 5Cl, R2 =	Ph	pFPh	pClPh		pCF3Ph
	95	95	96		96

) [97]. ST is considered a mildly acidic, easy-to-prepare, non-toxic, recyclable, efficient, and heterogeneous green catalyst. This method is very good for the synthesis of 2-substituted-BTs from several functionalized aromatic, aliphatic, and heteroaromatic aldehydes. The advantages of this method are as follows: easy handling, functional group compatibility, short reaction times (5 min), high catalytic activity and recyclability, chemoselectivity, very good yields, no column purification, low corrosiveness, and environmental compatibility.

2ATP and substituted aryl aldehydes were condensed using ruthenium silicate (RS-1) zeolite as a catalyst in a hydrothermal process to afford the 2-arylBTs 29a–i in 85–93% yields (

Table 10. Compounds 29a–i.

29	a	b	c	d	e	f	g	h	i
R1 = H, R2 =	pClPh	Ph	oClPh	pMeOPh	pMePh	oOHPh	pOHPh	pNO2Ph	2OH,4MePh
% Yield	93	93	91	91	92	90	91	93	85

) [98]. The benefits of this protocol include mild reaction conditions, short reaction times (30 min), and high thermal catalyst and catalysis recyclability.

Substituted BTs 30a–t were synthesized in 82–94% yields in 3 h by the visible-light-induced condensation–cyclization reaction of substituted 2ATP with substituted aromatic aldehydes (

Table 11. Compounds 30a–t.

30a–j, R1 = H; 30a–t, R1 = Cl
30a–t	a(k)	b(l)	c(m)	d(n)	e(o)	f(p)	g(q)	h(r)	i(s)	j(t)
R2 =	Ph	oBrPh	mBrPh	pBrPh	pFPh	pClPh	pMePh	oMePh	pMeOPh	3,4MeOPh
%Yield	92(89)	90(90)	91(92)	88(93)	94(91)	86(84)	91(89)	92(87)	87(90)	82(83)

) [99]. Fluorescein was used as a photocatalyst, a blue LED lamp was used as the light source, and atmospheric molecular oxygen was required for the reaction to proceed.

Substituted aromatic aldehydes, such as 4-Br, 4-Cl, and 4-F (electron-withdrawing) and 2-Me, 4-Me and 4-OMe (electron-donating) substituents, afforded BTs in high to excellent yields: 30d–f (88–94%) and 30g–i (87–92%). Notably, the position of the substituents on the aromatic aldehydes had little effect on the yield of the BTs 30b–d (88–91%) and 30g,h (91, 92%). In addition, two electron-donating groups on the aromatic aldehydes gave a lower yield (82%) of the BT 3j. The reactions were smooth, with good functional group tolerance. Additionally, the catalytic system eliminates the need for an oxidant or metal catalyst, aligning with the principles of green chemistry.

The 2-aryl-BTs 31a–t were synthesized in 64–99% yields in 10 min by combining enzymatic (trypsin) and visible-light (450 nm) catalysis under an air atmosphere (

Table 12. Compounds 31a–t.

31	a	b	c	d	e	f	g	h	i	j	k
R1 = H, R2 =	oNO2Ph	Ph	pMePh	mMePh	oMePh	pCF3Ph	pFPh	pClPh	pBrPh	pCNPh	mClPh
% Yield	99	96	96	98	96	96	98	95	89	99	95
31	l	m	n	o	p	q	r		s		t
R1 = H, R2 =	mBrPh	pNMe2Ph	oClPh	oBrPh	pNEt2Ph	pMeOPh	Thiophen-2-yl		Napht-1-yl		Bn
% Yield	83	75	89	80	64	92	75		78		80

) [100]. The method consisted of biocatalytic condensation of 2ATP with aromatic or aliphatic aldehydes to afford the corresponding benzothiazoline BTI as an intermediate. Subsequent visible-light-induced oxidization produced the 2-arylBTs in approximately 10 min. Additional oxidants or metals were not required in this protocol, which was environmentally benign.

It was found that the electronic effect on the substituents on the benzene ring influences the reaction. Aldehydes containing electron-deficient groups afforded the BTs 31a, 31g, and 31j, which were obtained in 98–99% yields, while an electron-rich pair group afforded the BT 31m in 75% yields. If the electron-richness group of the aldehyde is increased, the yield of BTs decreases, as in the series of BTs 31g–i, whose yields were 98%, 95%, and 89%, and the yield in BT 31p was reduced to 64%. In the case of steric hindrance of methyl-substituted benzaldehydes, the yield is negligible, as in the BTs 31c–e (96–98%). With a larger volume of 1-naphthaldehyde, the yield of the BT 31s decreases to 78%.

An ionic liquid immobilized on silica-coated cobalt–ferrite magnetic nanoparticles (CoFe₂O₄@SiO₂@PAF-IL) was formed. This hybrid nanostructure was used as a catalyst in the condensation reaction of 2ATP with aromatic aldehydes to synthesize the 2-aryl-BTs 32a–i in 83–91% yields (

Table 13. Compounds 32a–i.

32	a	b	c	d	e	f	g	h	i
R1 = H, R2 =	Ph	oMePh	pMeOPh	Py-2-yl	pFPh	pNO2Ph	pClPh	pOHPh	pNMe2Ph
% Yield	87	90	85	83	90	90	91	83	83

) [101]. The reaction was carried out under heating in solvent-free conditions at 70 °C for 10 min. The advantages of this procedure included the use of solvent-free conditions, the simple work-up, the short reaction times, and the environmentally benign conditions. The nano-catalyst could be reused several times without loss of catalytic activity and was easily separated.

Substituted 2ATP disulfides or dihydrochlorides were condensed with the corresponding aldehydes in refluxing glycerol without a catalyst for 45 min to afford the previously designed 6-cyano- (33a, 83%; 33d, 64%) and 6-amidino- (33b,c,e,f, 22–56%) 2,5-disubstituted furane-BTs 33a–f to be screened for antimicrobial and antitumor activities (Figure 5) [102]. The antitumor activity was tested on the human lung cancer cell lines A549, HCC827, and NCI-H358, with MTS cytotoxicity and in vitro BrdU proliferation assays performed using 2D and 3D cell culture methods. Compounds 33b,c,e resulted in possible antitumor activity to stop the proliferation of cells. Broth microdilution testing was used to evaluate the antimicrobial activity on Gram-negative E. coli and Gram-positive S. aureus and S. cerevisiae as eukaryotic model organisms, according to Clinical Laboratory Standards Institute (CLSI) guidelines. The BTs 33b,c,e and 233f showed promising antibacterial activity. All compounds were more activated in the 2D than in the 3D assays on the three cancer cell lines and in the antimicrobial assays. Compounds with the amidine group at the 6-position of the benzothiazole ring, 33b and 33e, gave low yields (22 and 28%, respectively).

Compound 4-(BT-2-yl)-2-methoxyphenol 34, obtained from the condensation of 2ATP with the corresponding benzaldehyde, was transformed to compound 2-[4-BT-2-yl)-2-methoxyphenoxy]acetohydrazide 36 through ethyl 2-(4-BT-2-yl)-2-methoxyphenoxy) acetate 35, then reacted with chlorine acetyl chloride to afford the 2-(4-BT-2-yl)-2-methoxy-phenoxy)-N′-(2-chloroacetyl)acetohydrazide 38 (Scheme 5) [103]. The acetohydrazides 36 and 38 were reacted with anhydrides and amino acids to afford the BT derivatives 37a–f in 65–90% yields and 39a–k in 65–90% yields, respectively. The in vitro anticancer activity of all the compounds exhibited promising potency against hepatocellular carcinoma HepG-2 (IC₅₀s from 0.7 ± 0.4 to 1.0 ± 0.7 μM) and very good potency against breast cancer cells MCF-7 (IC₅₀s from 2.5 ± 2.5 to 3.5 ± 3.4 μM) compared with the standard drug doxorubicin (IC₅₀s = 1.0 ± 0.8 μM and 2.9 ± 1.9 μM, respectively). The highest cytotoxic activity was observed for compounds 37a–c, 39c, and 39f. Also, these compounds were further evaluated for their EGFR inhibitory activity compared with the reference drug erlotinib. Molecular docking studies of the promising compounds 39a–c were carried out to interpret their enzymatic activities. Moreover, compounds 39a and 39b exhibited considerable pre-G1 and G2/M cell cycle arrest compared with the untreated MCF-7 cells. Additionally, 39a and 39b increased the levels of Bax, p53, and caspase-3 levels while decreasing the levels of Bcl-2, which are oncogenic parameters. The results of these BT-based derivatives proposed an excellent framework for detecting new potent antitumor leads.

The condensation reaction of (un)substituted 2ATP, 2ATP-disulfide, or 2ATP-hydrochloride with furfural, pyrrole-2-carboxaldehyde, or 2-thiophenecarboxaldehyde occurred under stirring in ethanol at room temperature, then unsubstituted 2ATP was reacted under refluxion for 12 h and substituted under refluxion for 36 h in the presence of an aqueous solution of sodium hydrosulfite as a catalyst to afford the 2-substituted-BTs 40a–i in 69, 48, 87, 34, 30, 48, 31, 29, and 54% yields (Figure 6) [76]. The synthesized BTs were investigated for their photoprotective, antioxidant, antiproliferative, and antifungal activities. Compounds 40d and 40g exhibited a multifunctional profile with an excellent filtering UVB capacity, which was higher than that of PBSA, which served as a reference and is currently used as a UV sunscreen filter. These compounds were the best at inhibiting the growth of dermatophytes and C. albicans, and 40g showed good antioxidant activity. Furthermore, 40d was effective on melanoma tumor cells (SK-Mel 5). These compounds are proposed as new skin preventive and protective agents.

2ATP was cyclo-condensed with a series of aromatic/aliphatic aldehydes in the presence of bovine serum albumin (BSA) as a biocatalyst in water under stirring at room temperature for 8 h to afford a series of 2-substituted BTs, 41a–v, in 79–93% yields (

Table 14. Compounds 41a–v.

41	a	b	c	d	e	f	g	h	i	j
R1 = H, R2 =	pMeOPh	Ph	3,4MeOPh	pNMe2Ph	napht-1-yl	pOHPh	pNO2Ph	pCF3Ph	pCNPh	pBrPh
% Yield	93	92	86	80	86	88	90	82	88	83
41	k	l	m	n	o	p		q
R1 = H, R2 =	Thiophen-2-yl	Py-2-yl	Indol-3-yl	Quinolin-2-yl	chexyl	3MeO,4(propene-2-ylO)Ph		3MeO, 4(propine-2-ylO)Ph
% Yield	83	85	79	85	85	81		83
41	r	s		t		41		u		v
R1 = H, R2 =	3MeO,4AcOPh	CinnamoylPh		pMeOcinnamoylPh		R1 = 6CF3, R2 =		CinnamoylPh		pNMe2Ph
% Yield	88	88		89		% Yield		81		84

) [104]. The advantages of this protocol were its excellent yields, high atom economy, gram scalability, operational simplicity, and recyclability.

The condensation–cyclization reaction of 2ATP with aromatic benzaldehydes was carried out in a self-neutralizing acidic CO₂–alcohol system to afford the BTs 42a–n in 55–87% yields. Practicality, economic viability, and environmental friendliness were the advantages of this protocol (

Table 15. Compounds 41a–n.

42	a	b	c	d	e	f	g
R1 = H, R2 =	Ph	pMePh	oMeOPh	3,4MeOPh	pOHPh	pNMe2Ph	pFPh
% Yield	87	60	62	55	60	58	62
42	h	i	j	k	l	m	n
R1 = H, R2 =	pClPh	pBrPh	pCNPh	oOHPh	Furan-2-yl	Bn	R1 = Cl, R2 = Ph
% Yield	58	55	56	57	60	62	72

) [105]. The alkyl carbonic acid formed CO₂, and methanol was proposed as an intermediate in the reaction mechanism, generating hydrogen cations to catalyze the reaction. 2-PhenylBT 42a gave the highest yield (87%), while in the case of the 2-substituted BTs 42b–m the yields decreased to 55–62%. However, in 5-chloro-2-phenylbenzothiazole 42n the yield increased to 72%.

2ATP was condensed with aromatic aldehydes in the presence of 0.05 mol% of bacterium-derived hemoglobin Vitreoscilla (VHb) as a biocatalyst, tert-buthyl hydroperoxide (TBHP) as an oxidant, and dimethyl sulfoxide (DMSO) as a solvent at room temperature to afford 2-arylBTs 43a–n in only 5 min with excellent yields (85–97%) (

Table 16. Compounds 43a–n.

43	a	b	c	d	e	f	g	h	i
R1 = H, R2 =	Ph	pMePh	pMeOPh	pFPh	pClPh	pBrPh	mMePh	mClPh	oMePh
% Yield	92	92	88	97	96	90	89	93	87
43	j		k	l		m		n
R1 = H, R2 =	oClPh		Py-3-yl	Thiophen-2-yl		Furan-2-yl		Naphthalen-2-yl
% Yield	91		93	94		95		85

) [106]. The advantages of this protocol were its mild reaction conditions and high efficiency, with a wide substrate scope.

It was found that electron-rich groups on R² resulted in decreased yields (43b,c,g,i; 92, 88, 89, and 87% yields, respectively) compared with 2PhBT (43a), whereas electron-poor moieties within the aromatic aldehydes (43d–f,h,j; 91–97%) were the best substrates. Aldehydes with heteroaromatic groups produced the BTs 43k–m in high yields (93–95%) with a longer reaction time (20 min). The yield decreased significantly when the benzaldehyde contained the naphthalyl group, affording the BT 43n in an 85% yield, due to the steric effect.

Substituted 2ATPs were condensed with a series of aldehydes at 80 °C using an environmentally friendly, inexpensive, and easily accessible Zn(OAc)₂·2H₂O (5 mol%) in a solvent-free reaction to afford a series of substituted 2-arylBTs, 44a–p, in 67–96% yields in 30–60 min (

Table 17. Compounds 44a–p.

44	a	b	c	d	e	f	g		h	i
R1 =	H	6Me	6MeO	6Br	6CF3	H	H		H	H
R2 =	Ph	Ph	Ph	Ph	Ph	pMePh	pMeOPh		pClPh	pOHPh
% Yield	94	92	94	89	91	89	96		90	85
44	j	k		l	m		n	o	p
R1 =	H	H		H	H		H	H	H
R2 =	pNMe2Ph	pCNPh		mNO2Ph	iBuPh		Py-4-yl	Furan-2-yl	Thiophen-2-yl
% Yield	85	90		94	67		79	81	84

) [107]. The reaction involving heterocyclic aldehydes interestingly afforded the corresponding BTs 44m–o in good yields (79–84%), and with alkyl aldehydes, such as isobutyraldehyde, the corresponding BT 44p was produced in a moderate yield (67%). The BTs 44a–o were obtained in 79–96% yields.

The nanocomposite MNPs-phenanthroline-Pd (Fe₃O₄@SiO₂-diPy-Pd) was synthesized to be used as a catalyst for the cyanation of aryl halides at 80 °C for 6 h in the synthesis of BTs 45a–h in 89–98% yields (

Table 18. Compounds 45a–h.

45	a	b	c	d	e	f	g	h
R1 = H, R2 =	Ph	pMePh	pMeOPh	pClPh	pNO2Ph	pBrPh	pEtPh	mBrPh
% Yield	94	93	96	98	92	95	92	89

) [108]. The optimal conditions for the reaction were 20 mg of the MNPs-phenanthroline-Pd nano-catalyst and K₂CO₃ in DMF under reflux conditions. The catalyst was highly efficient and easily recovered and reused, without losing the morphology and dispersion of the particles. Aryl halide compounds with electron-donating groups at the p-position of the aryl group afforded the BTs 45c,d,f in higher yields (96%, 98%, and 94%) than that of aryl with electron-withdrawing groups (BT 45e, 92% yield), whereas aryl bromide in the m-position decreased the yield (BT 45h, 89% yield).

The eco-friendly DABCO-based dicationic acidic ionic liquid [C₄H₁₀-DABCO][HSO₄]₂ (DABCO = 1,4-diazabicyclooctane) was synthesized, characterized, and successfully used as a catalyst in the condensation of aryl aldehydes under H₂O reflux for 30–50 min to afford 2-phenylBTs 46a–h in 85–91% yields (

Table 19. Compounds 46a–h.

46	a	b	c	d	e	f	g	h
R1 = H, R2 =	pBnOPh	pClPh	pMePh	Ph	pNO2Ph	mNO2Ph	oOHPh	pMeOPh
% Yield	90	87	82	85	90	91	86	85

) [109]. The advantages of this protocol were its short reaction time; easy work-up; moderate to excellent yields; the use of green solvents; the use of non-metal, inexpensive catalysts; and catalyst recyclability. Aldehydes with electron-withdrawing groups at the m- or p-position of the aryl group afforded BTs 46b,e,f in higher yields (87%, 90%, and 91%) than that of aldehyde with an electron-donating group (BT 45c, 82%).

Six 1-aryl-4-BTyl-1,2,3-triazoles 47a–f were prepared from condensation of 2ATP with aldehydes (Figure 7) [110]. The synthesized donor–acceptor molecules exhibited optical properties based on charge transfer emission depending on the substituent in the 1,2,3-triazole moiety. Compounds 47a and 47e with moderately electron-donating groups (CH₃ and OCH₃) or 47b and 47c with electron-withdrawing substituent groups (F and CF₃) in the triazole fragment were blue-shifted, in contrast to compounds 47d and 47f with strong electron-withdrawing (-NO₂) and electron-donating (-N(CH₃)₂) substituents, respectively, which showed red-shifted maxima. Compounds 5a–d had low fluorescence quantum yields, 5d being almost non-emissive with a ΦF less than 0.01 due to the –NO² group. However, compound 5f had the highest quantum yield, which increased with the decreasing polarity of the solvent.

Some of these compounds were active in human A2780, HeLa, and A549 cancer cell lines, exhibiting IC₅₀ values in the low micromolar range and low cytotoxicity in healthy CHO cells. Interestingly, compound 47f showed cytoplasmic staining determined by confocal fluorescent microscopy.

The fluorescent probe 51 based on BT was synthesized in a 53% yield by condensing 2ATP with 5-methyl salicylaldehyde in DMF and Na₂S₂O₅ as an oxidant to afford BT 48 in an 80.5% yield, followed by three steps through BT 49 in a 62.5% yield and 50 in a 72% yield (Scheme 6) [111]. The excellent selectivity enabled the detection of Cu²⁺. The probe exhibited a cyan blue fluorescence emission peak at 487 nm under 360 nm UV excitation with fluorescence quenching after adding Cu²⁺. The detection limit of Cu²⁺ of the probe was determined as 3.08 × 10⁻⁷ M. According to a Job plot and high-resolution mass spectrometry, the complexation ratio of the probe to Cu²⁺ was 2:1. An excited intramolecular proton transfer (ESIPT) was confirmed as the sensor mechanism. Under alkaline conditions, the probe showed cyan blue and green fluorescence under acidic conditions. On the other hand, due to the excellent membrane permeability and low cytotoxicity, the probe could detect Cu²⁺ in water samples and recognize Cu²⁺ in human breast cancer cells (MCF-7).

2ATPolate, 2ATP-disulfide, or 2ATP-hydrochloride were condensed with the corresponding aldehyde using two methods to afford the series of benzo[b]thienyl and 2,2′-bithienyl-BTs 52a–d and 53a–d to study the in vitro antitrypanosomal and antiproliferative activities (Figure 8) [112]. Specifically, the amidine group substitutions and the type of thiophene backbone impacted the biological activity. All synthesized BTs were active as both antiproliferative and antitrypanosomal agents.

The 2,2′-bithienyl-BTs with unsubstituted and 2-imidazolinyl amidine showed the most selective antitrypanosomal activity. The 2,2′-bithiophene BTs showed the most selective antiproliferative activity, whereas all 2,2′-bithienyl BTs were selectively active against lung carcinoma. The BT with an unsubstituted amidine group also produced strong antiproliferative effects. The pronounced antiproliferative activity of the BTs was attributed to different cytotoxicity mechanisms. Cell cycle analysis and DNA binding experiments provided evidence that BTs are localized in the cytoplasm and do not interact with DNA.

All amidino-substituted BTs, 52a–d and 53a–d, were tested in vitro for their antiproliferative activity against human cancer cell lines, including HCT116 and SW620 (colon carcinomas), H460 (lung carcinoma), MCF-7 (breast carcinoma), PC3 (prostate carcinoma) and HeLa (cervical carcinoma), and HEK 293 (human embryonic kidney) cells. The 2,2′-bithiophene-BT 53a displayed very potent and selective activity against H460 cells, with an IC₅₀ value of 0.02 μM, which was twice the magnitude of the IC₅₀ values for the other cancer cells tested. All other 2,2′-bithienyl-BTs, 53b, 53c, and 53d, also showed selective activity against H460 cells, with IC₅₀ values of 0.2 μM, compared with doxorubicin (IC₅₀ = 0.04 ± 0.01).

4NO₂-2ATP was condensed with 4-N-substituted benzaldehydes to afford the targeting fluorescent 2-(N,N-dimethyl/diphenyl-aminophenyl)-5-substituted-BTs 54a,b in 73.5% and 69.1% yields, respectively. Then, the nitro group of compounds 54a,b was reduced to the amine derivatives 55a,b (81.3 and 78.8%, respectively) with Pd(OAc)₂ in ethanol and reacted with the corresponding aldehyde under stirring in methanol to afford the imines 56a,b in 73.8 and 70.2% yields, respectively (Scheme 7) [113]. The compounds were analyzed for their photophysical properties, absorbance, and fluorescence in different organic solvents. The photophysical properties of BTs 54a,b and 56a,b were strongly impacted by the solvent used. In DMSO, all BTs had higher fluorescence intensities, showing that solvent polarity has a great impact on their excited-state properties. For BT 54a, the λmax values ranged from 337 nm to 389 nm, and BT 54b showed λmax values ranging from 373 nm to 394 nm, with the highest absorption observed in DMSO, whereas BTs 56a and 56b presented λmax values ranging from 387 to 417 nm and 393 nm to 423 nm, respectively. For BT 54a,b, the fluorescence emission maxima ranged from 495 to 522 nm and 512 to 531 nm, respectively, with their highest values observed in ethanol. Conversely, BTs 56a,b displayed emission maxima ranging from 518 to 537 nm and 518 to 571 nm, respectively, with the highest fluorescence observed in DMSO and ethanol, respectively.

All BTs were evaluated for cytotoxicity across anticancer cell lines and inhibition against VEGFR-2 kinase using an anti-phosphotyrosine antibody test. BT 54a showed moderate inhibitory activity with little variability (IC₅₀ = 0.38 ± 0.13 μM). Conjugate 54b had a higher potency, with an IC₅₀ of 0.29 ± 0.16 μM, with lower variability. The BT 56a showed considerable inhibitory activity, with an IC₅₀ of 0.31 ± 0.08 μM, with the lowest variability, whereas BT 56b showed an IC₅₀ value of 0.23 ± 0.20 μM. Compounds 54b and 56b resulted in potent cytotoxic activity against MCF-7 cells (IC₅₀ values of 7.06 ± 0.29 and 8.82 ± 0.37 μM, respectively), whereas compound 56b exhibited the greatest potency in VEGFR-2 inhibition (IC₅₀ of 0.23 ± 0.20 μM). Molecular docking studies indicated BT 54b to have a stronger interaction. An ADME study showed BTs 54b and 56b to have enhanced lipophilicity and decreased solubility, which could influence their pharmacokinetic behavior.

2ATP was condensed with aromatic aldehydes in the presence of the prepared Co/Niacin-MOF as an economic, efficient, sustainable, and green stable catalyst under stirring at 60–70 °C for the appropriate length of time (30–60 min) to afford 2-heteroaryl-BTs 57a–f in 75–95% yields (

Table 20. Compounds 57a–f.

57	a	b	c	d	e	f
R1 = H, R2 =	Furan-2-yl	Thiophen-2-yl	Thiazo-2-yl	Py-4-yl	4benzoatePh	4MebenzoatePh
% Yield	75	80	88	85	95	90

) [73]. All compounds were screened as acetylcholine esterase (AChE) inhibitors. Validation in molecular docking studies showed that BTs 57e and 57f gave good results in binding with acetylcholine esterase. The data from in vitro studies showed that compound 57e had a promising value (59.8% inhibition, IC₅₀ = 5.25 μg/mL) compared with the Alzheimer’s reference drug donepezil (74.89% inhibition, IC₅₀ = 4.19 mg/μL).

2.2. Condensation with Carboxylic Acids and Their Derivatives

In a simple trituration method, 2ATP was condensed with N-protected amino acids using molecular iodine as a mild Lewis acid catalyst to synthesize the 2-substituted BTs 58a–f in 66–97% yields and 59a–f in 54–98% yields (Figure 9) [114]. The reaction was carried out in solvent-free conditions for 20–25 min to provide the products with moderate to excellent yields (54–98%).

Graphene oxide (GO) was used as a catalyst in the condensation of 2ATF with α-phenyl glyoxylic acids, and water was used as a solvent; the reaction was conducted at room temperature under heating conditions for 1 h to synthesize the 2-aryl-BTs 60a–d (

Table 21. Compounds 60a–d.

60	a	b	c	d
R1 = H, R2 =	Ph	pClPh	pMePh	pMeOPh

) [115]. Their neuroprotective effects were assayed in the U87 MG cell line under a H₂O₂-induced stressed condition and compared with the breast cancer (MCF-7) and macrophage (RAW264.7) cell lines using a cell viability assay. These 2-aryl-BTs enhanced the neuronal cell viability and protected neuronal cells from the ROS-mediated neuronal damage induced by H₂O₂. Furthermore, these compounds modulated catalase and enhanced the catalase activity by up to 90% during the H₂O₂ exposure in the U87MG cell line. Molecular modeling studies in the AutoDockTool-1.5.6. Lig Plot + program showed these compounds to have strong binding energies of −7.39, −7.52, −6.5, and −7.1 Kcal/mol, as observed by using the potent analogs 60b and 60c and the catalase enzyme, which indicated the presence of hydrophobic interactions in the catalytic site. Furthermore, a simulation of the ligand and catalase protein performed using DESMOND software showed further strengthened ligand and enzyme interactions. An in silico ADMET study revealed the drug-likeliness of these analogs. The BTs 60b,d had potential catalase-modulating activity comparable with valproic acid as a standard drug. These results indicated the use of an in vivo animal model for possible therapy.

Substituted 2ATPs and substituted α-keto acids were reacted by a decarboxylative cross-coupling reaction under blue LED (λ = 435–445 nm) irradiation without using any photocatalyst or metal at room temperature for 8 h to afford unsubstituted and substituted BTs, 61a–p, in moderate to good yields (40–88%) (

Table 22. Compounds 61a–p.

61	a	b	c	d	e	f	g	h
R1 =	H	5Cl	H	5Cl	H	5CF3	H	H
R2 =	Ph	Ph	4FPh	4FPh	2OHPh	2OHPh	Thiophen-2-yl	4MePh
% Yield	88	83	73	87	73	48	82	58
61	i	j	k	l	m	n	o	p
R1 =	5Cl	H	5Cl	6Me	H	H	5Cl	H
R2 =	4MePh	4MeOPh	4MeOPh	Ph	4ClPh	H	H	Me
% Yield	62	74	67	80	77	40	61	53

) [116]. The reaction was carried out without any photocatalyst or metal. An electron donor–acceptor complex (EDA) formed with α-keto acid, and 2ATP drove the formation of the corresponding BT. The BT 61n had the lowest yield (40%), the 5ClBT 61o had a yield of 61%, and the 2MeBT 61p had a yield of 53%; the 2-phBT 61a had the highest yield (88%) yield. The 2-(oOHPh)BT 61e had a yield of 73%), the 2-(pOMePh)BT 61j a yield of 74%, the 2-(pMePhBT) 61h a yield of 58%, the 2-(pClPh)BT 61m a yield of 77%, the 2-(pFPh)BT 61c a yield of 73%, and the 5Cl,2PhBT 61b a yield of 83%).

The 2PhBT 61a had the higher yield (88%), whereas BT 61n had the lowest yield (40%). Substituents in the para position of the phenyl ring just as the compounds 61c,m with strong deactivating effect (F and Cl) with yields of 73 and 77%, and compounds 61h,j with strong activating effect (OH and OMe) with yields of 73 and 77%, respectively), reflected a decrease compared to unsubstituted 2PhBT 61a. Also, the yields of the BTs with activating groups had lower yields than those of BTs with deactivating groups. The 2-alkyl BTs 61n–p had the lowest yields (40, 61, and 53%, respectively). The 4-amino substituted α-keto acid (R³ = p-aminoPh) could not be transformed to the BT 61q in the present reaction condition, whereas from α-ketoglutaric acid, the BT 61s was obtained in traces.

A decarboxylative coupling of α-keto acids with 2ATP was carried out in a one-pot reaction using an aqueous suspension of K₂CO₃ as a base under grinding at room temperature for the appropriate time for the synthesis of the 2-aryl-BTs 62a–h (

Table 23. Compounds 62a–h.

62	a	b	c	d	e	f	g	h
R1 = H, R2 =	Ph	pFPh	pClPh	pBrPh	mNO2Ph	pMeOPh	mMePh	R1 = Cl, R2 = Ph
% Yield	95	97	93	95	92	98	95	91

) [117]. The protocol was carried out with excellent tolerance of functional groups in yields of 95, 97, 93, 95, 92, 98, 95, and 91%. The advantages of this method were short reaction times, a straightforward work-up, and catalyst-free and chromatography-free purification. On comparing BTs 62e (92%) and 62f (98%), it was clear that an electron-withdrawing group at the meta position of the phenyl ring disfavored the yield, whereas electron-donating groups at the para position favored the yield compared with the 2PhBT 62a (95%).

On comparing 3NO₂PhBT 62e (92%) with 2PhBT 62a (95%), it was clear that an electron-withdrawing group at the meta position of the phenyl ring disfavored the yield, whereas an electron-donating in 3MePhBT 62g (95%) favored the yield. However, the best electron-donating groups at the para position were in 4MeOPhBT 62f (98%), the yield of which was more highly favored, surprisingly, and with electron-withdrawing groups at this position in 4FPhBT 62b (97%) the yield also increased, maybe due to the resonance effect of the fluor atom in this case.

The Schiff base BTs 63a–g were synthesized in two steps: (a) 4-minosalicylic acid and 5-aminosalicylic acid were reacted with 2ATP in polyphosphoric acid to produce the intermediates 2-(2-hydroxyaniline)BT and 2-(3-hydroxyaniline)BT. Finally, (b) these intermediates were treated with a series of trisubstituted aldehydes by stirring a few drops of acetic acid in ethanol at room temperature for 8 h to produce the 63a–g BTs in good yields (85–91%) (Figure 10) [118]. These compounds were evaluated for in vitro antibacterial activities on Bacillus sps, S. aureus, K. pneumoniae, and E. Coli strains and antifungal activity on the C. Albicans strain using the ‘micro-broth dilution method’ (MICs in μg/μL). All the compounds displayed very good antibacterial activity for the S. aureus, E. coli, and K. pneumonia strains (MIC = 0.8 μg/mL), compared with ciprofloxacin (2.0 μg/mL), and antifungal activity for C. albicans, compared with fluconazole (16 μg/mL), as standard drugs. Molecular docking studies were performed to understand the binding mechanism. Molecular electrostatic potential (MEP) was used to evaluate the reactivity of the molecules. Antimicrobial activity was correlated with the calculated HOMO–LUMO gap, chemical hardness, and global softness. BT 63e exhibited very good antimicrobial activity, less toxicity, and more chemical reactivity, as confirmed by a smaller HOMO–LUMO gap, a lesser electrophilicity index, a higher global softness, and a lesser chemical hardness. The evaluation of DNA cleavage of 63e against MCF-7 breast cancer cells revealed 85.82% inhibition of cancer cells at 200 μg/μL. In addition, 63e showed less toxicity to normal cells at the concentration required to produce an anticancer effect (IC₅₀ = 973 μg/μL).

2ATP was condensed with the corresponding 2-aryl-1H-benzimidazole-5-carboxylic acid in the presence of polyphosphoric acid (PPA) via heating to 250 °C and 20 bar for 3 min to produce BTs 64a–h in moderate yields (40–72%) (Figure 11) [119]. NIH3T3 (ATCC CRL-1658), Caco-2 (ATCC HTB-37), and A549 (ATCC CCL-185) cell lines were used to test the cytotoxic effects of the BTs. All the BTs exhibited low to moderate activity against the tested cancer cell lines. Compound 64e had the most potent activities against A549 and Caco-2 cells, with IC₅₀ values of 73.76 ± 2.54 μM and 73.15 ± 2.84 μM, respectively. This result suggested that the methyl group of the phenyl ring group may affect the antiproliferative activity to a certain extent. Also, BT 64e displayed antiproliferative activity similar to cisplatin against NIH3T3. On the other hand, BT 64c displayed significant potency against A549 and Caco-2 cells, with IC₅₀ values of 98.83 ± 3.71 μM and 77.80 ± 3.19 μM, respectively. In addition, BT 64e showed a similar selectivity to that of cisplatin.

Aliphatic dicarboxylic acids were condensed with 5-amidino-2ATPs in polyphosphoric acid (PPA) at 180 °C to afford a series of symmetric bis-6-amidino-BTs 64–71 with aliphatic central units, as previously designed, with 24–71% yields (Figure 12) [120]. The BTs were isolated as methanesulfonates by an additional acid–base reaction because these salts are more stable and soluble in water. All BTs were evaluated for their efficacy against bloodstream forms of the African trypanosome Trypanosoma brucei. Most of the tested compounds were more potent than fexinidazole (2.40 μM), with EC₅₀ values against T. brucei ranging from 0.51 to 5.39 μM. With the exception of BTs 68a–c, trypanocidal activity decreased in the following order: unsubstituted amidine > pyrimidine > imidazoline. The aliphatic spacer between the two BTs also influenced the activity. The bis-BTs 70a–c with a cyclohexyl unit had the most pronounced impact, and bis-BT 70a, which has an unsubstituted amidino moiety, displayed a remarkable potency (EC₅₀ = 0.51 nM). BTs 71a–c, which have conformationally constrained ethenyl spacers, showed better activity than the corresponding conformationally unrestricted ethyl spacers 64a–c. All BTs with conformationally unrestricted alkyl chains, with the exception of the most active BT, 68c (n = 6; EC₅₀ = 0.028 ± 0.001 μM), and BTs 64–69, with three to eight different lengths (n = 4 (66a–c; 0.063, 1.14, 0.19 μM)), were generally optimal. On the other hand, BT 70a showed sub-nanomolar in vitro potency with good selectivity over mammalian cells (>26,000-fold). In all the experiments, mice treated with a dose of 20 mg kg⁻¹ were cured of stage 1 trypanosomiasis. Compound 70a displayed a favorable in vitro ADME profile, except for its low membrane permeability, while 70a was also active at low nanomolar concentrations against cultured asexual forms of the malaria parasite Plasmodium falciparum. On these bases, compound 70a was considered a lead with therapeutic potential.

2ATP was condensed with ethyl oxalate to afford the ethyl 2-carboxylateBT 72, which suffered hydrazinolysis to be transformed to the acid hydrazide 73, the precursor used for the preparation of the hydrazone derivatives 72–76 (Scheme 8) [121]. BTs 74a,b were obtained in 87 and 92% yields, respectively, through the interaction of 73 with either 4-chloro or 4-f luorobenzoyl chloride, respectively, in glacial acetic acid. Also, compound 73 was reacted with p-substituted isothiocyanates to afford the respective thiosemicarbazides 75a–d in 84–90% yields. The thiosemicarbazides 75a–d were refluxed in 2N NaOH to suffer cyclization into their 1,2,4-triazole-3-thiols 76a–d in 31–63% yields. Also, compound 73 was reacted with 4-bromo benzenesulfonyl chloride or tosyl chloride in glacial acetic acid to afford the sulfonamides 77a,b in 84 and 78% yields, respectively. Finally, acid hydrazide 73 was used as a precursor for the synthesis of the hydrazones 78a–g in 88–93% yields through a reaction with aromatic and heteroaromatic aldehydes. The antitumor activities of the synthesized hydrazone BTs 73–76 were evaluated. Also, they were tested in vitro against colorectal carcinoma (HCT-116), hepatocellular carcinoma (HepG2), prostate cancer (PC-3), mammary gland cancer (MCF-7), and epithelioid carcinoma (HeLa). The most active compounds were 78e, 78f, 75b, 76c, and 77b, exhibiting IC₅₀ values comparable to that of lapatinib, the reference drug. The most potent compounds for EGFR inhibitory activity were 78e and 78f, with IC₅₀ values of 24.58 and 30.42 nM, respectively, compared to the standard drug lapatinib (17.38 nM). Molecular modeling studies were conducted, including docking into the EGFR active site and surface mapping for all compounds. The hydrazones 78e and 78f showed superior binding with EGFR, showing them to be excellent candidates for targeted antitumor therapy through EGFR kinase inhibition.

The condensation reaction of 2ATP with acyl chlorides or acid anhydrides was performed in the presence of KF-Al₂O₃ as a heterogeneous base catalyst under stirring with acetonitrile at room temperature for 30 min to afford 2-substituted BTs 79a–k in 87–97% yields (

Table 24. Compounds 79a–k.

79	a	b	c	d	e	f	g	h	i	j	k
R1 = H, R2 =	pCNPh	pMeCOPh	pClPh	Ph	Napht-1-yl	pMePh	pOHPh	pMeOPh	Me	Bn	nBu
% Yield	97	95	95	94	91	90	87	87	91	92	90

). The catalyst showed good recyclability [122]. It was observed that electron-withdrawing functionalities gave higher isolated yields (79a,b; 97 and 95%). On the other hand, electron-donating groups (79d–f; 94, 91, 90%) gave lower isolated yields.

2ATP was condensed with nine pyrrolidinone ester derivatives under microwave irradiation in the presence of a heterogeneous eutectic mixture of (CuCl₂ or NiCl₂/urea) and Cu- or Ni-doped TiO₂ nanoparticles to produce N-aryl pyrrolidinone-BTs 80a–i in 83–96 and 78–91% yields, respectively (Figure 13) [123]. Eco-friendliness and short reaction times (5 min) were the advantages of this protocol. All synthesized BTs were screened for their antibacterial activity against Gram-positive and Gram-negative bacterial strains: compounds 80a and 80b were active against E. coli and P. aeruginosa, with an MIC of 12.5 µg/mL. Compounds 80c, 80e, and 80f showed important activity against MRSA and BS, with MIC values ranging from 12.5 µg/mL to 25 µg/mL. Also, compounds 80c and 80f were active against the four tested bacteria strains, with MIC values of 12.5 for EC, 12.5 for PA, 12.5 for BS, and 18.75 μg/mL for MRSA, compared with the antibiotic ciprofloxacin as a standard control.

The p-nitro benzoyl chloride was condensed with 5-methoxy-2ATP in toluene as a dissolvent upon heating for 1 h to afford the BT derivative 81, which was reacted in ethanol with stannous chloride upon heating to 80 °C for 3 h; then, paraformaldehyde dissolved in fresh NaOMe in MeOH solution was mixed and stirred at room temperature for 4 h. After that, sodium boron hydride was added, and the solution was refluxed for 1.5 h to afford BT 82 in a 33% yield. A solution of 82 and ICl in acetic acid was refluxed upon stirring overnight for 10 h to afford BT 83a in a 35% yield, which was cooled to −78 °C and reacted with BBr₃ upon stirring at room temperature for 16 h to afford 83b in a 37% yield. The neopentylboronic ester 84a could be quantitatively formed from 83a, but the isolation of 84a without hydrolysis was challenging. Partial and full hydrolysis of 84a during isolation efforts led to unsatisfactory mixtures of 84a alongside boronic acid 85a and protodeborylated compound 82. The synthesis of BT derivatives with [²¹¹At]3′-At-PIB-OMe 86a,b was carried out using a Cu-catalyzed astatination protocol via boronic acid precursors 85a,b (Scheme 9) [74]. Compound 86a had a radiochemical yield of 55% and was stable for at least 3 h in phosphate-buffered saline. Studies were carried out to evaluate the binding affinities of 86a and 86b against Aβ(1–40) AD plaques and their in vivo stability, biodistribution, and AD-associated plaque clearance abilities.

A series of substituted 2ATPs were condensed with a variety of aryl/alkyl nitriles under mild reaction conditions (oil bath, 70 °C) using ZnO nanoparticles as a catalyst in solvent-free conditions to produce two series of 2-substituted BTs, 87a–z and 88a–d, in 88–96% yields (

Table 25. Compounds 88a–z and 89a–d.

87	a		b	c		d	e	f	g	h	i		j	k
R1 = H, R2 =	Ph		pMePh	pMeOPh		pNMe2Ph	pFPh	pClPh	pBrPh	oBrPh	oClPh		oNO2Ph	pCF3Ph
% Yield	94		91	90		88	93	95	96	90	92		89	94
87	l		m	n		o	p		q		r		s
R1 = H, R2 =	pCNPh		pNO2Ph	Napht-1-yl		Napht-2-yl	Py-4-yl		Py-3-yl		Furan-2-yl		Thiophen-2-yl
% Yield	94		96	92		91	93		92		90		89
87	t	u	v	w	x	y	z		88		a	b	c	d
R1 = H, R2 =	Me	iPr	cPr	cHex	Bn	(CH2)2Ph	CH2napht-1-yl		R1 = Cl, R2 =		Ph	pFPh	pClPh	pCF3Ph
% Yield	91	90	92	94	93	93	89				94	93	95	96

) [124]. This method was compatible with several functional groups and exhibited excellent catalyst recyclability, easy recovery of materials, and high economy; there is no need to purify the products by recrystallization or column chromatography, and it is an environmentally benign process.

Five 2-[3-(aryl)prop-2-enenitrile]BTs 90a–e were synthesized in 84–96% (4–8 min) yields by a microwave irradiation method from 2-cyano-methylBT 89 in EtOH/AcOH, obtained in a 92% yield from the condensation of 2ATP and malononitrile under microwave irradiation at 40 °C for 10 min and used as an intermediate in the synthesis of 7-amino-6-(BT-2-yl)-5-(aryl)-2-thioxo-2,3dihydropyrido[2,3-d]pyrimidin-4(1H)-ones 91a–e obtained in 84–92% yields (25 min) and 1-amino-2-(aryl)pyrrolo[2,1-b][1,3]BT-3-carbonitrile derivatives 92a–e obtained in 78–90% yields (20–25 min) (Scheme 10) [125]. These compounds were tested as antioxidant, antimicrobial, and anticancer agents.

It was found that pyrimidine-BTs 91a,d were more potent against bacteria, with MICs from 4 to 12 μmol L^–1, than cefotaxime (MIC = 6–12 μmolL^–1); pyrimidine-BTs 91bc,e showed good activity against all the bacterial strains. This high efficacy was attributed to the presence of a BT and thiophene moieties, as in pyrimidine-BT 91a, and to the presence of an electron-withdrawing (fluoro) group in the para position of the phenyl ring attached to the pyridopyrimidine, as in pyrimidine-BT 91d. Pyrimidine-BT 91a had equipotent activity with MICs of 6 and 12 μmol L⁻¹ similar to those of cefotoxime against Bacillus subtilis and Chlamydia pneumonia, respectively, whereas 91e was equipotent (MIC = 8 μmol L⁻¹) with respect to cefatoxime against Salmonella typhi. Pyrrolo-BTs 92a–e effectively inhibited the growth of the tested bacteria strains. PyrroloBT 92a was more potent against Staphylococcus aureus and had equipotent efficacy against Bacillus subtilis and Chlamydia pneumonia compared with cefotaxime; this was attributed to the presence of a thiophene moiety. In addition, pyrrolo-BT 92d (MICs from 4 to 10 μmol L^–1) had extraordinary activity toward all bacteria due to the presence of a p-fluorophenyl substituent which increased the antibacterial potency compared to the reference drug. Also, pyrrolo-BTs 92b,c,e exhibited good inhibition activity against all bacterial strains. The cytotoxic activity of pyrimidine-BTs 91a–d and pyrrolo-BTs 92a–d against the colon cancer HCT-116, lung cancer NCI-H460, and liver cancer HepG2 human cell lines was observed and compared with that of doxorubicin as a reference drug. The series of pyrimidine-BTs 91a–e exhibited higher cytotoxic activity than the pyrrolo-BTs 92a–e. Pyrrolo-BT 91a containing thiophene (IC₅₀s = 0.66, 0.45, and 0.70 μmol L⁻¹), pyrrolo-BT 91d containing p-fluorophenyl (IC₅₀s = 0.28, 0.39, and 0.14 μmol L⁻¹), pyrrolo-BT 92a with thiophenyl (IC₅₀s = 1.10, 0.49 and, 0.89 μmol L⁻¹), and pyrrolo-BT 92d containing p-fluorophenyl (IC₅₀s = 0.45, 0.47, and 0.59 μmol L⁻¹) were found to be more potent and efficacious than doxorubicin as the reference drug (IC₅₀s = 1.98, 0.52, and 1.12 μmol L⁻¹). It was found that the presence of the p-fluorophenyl group improved the antitumor activity more than thiophene. In addition, pyrrolo-BT 91b (IC₅₀s = 2.56, 2.89, and 2.68), and pyrrolo-BT 91e (2.60, 3.00, and 3.08 μmol L⁻¹) exhibited good cytotoxic effects towards the three tumor cell lines. On the other hand, 5-methylfurany 92b (IC₅₀s = 3.90, 4.08, and 4.50 μmol L⁻¹) showed good antitumor activity against all tumor cell lines. Compounds with 5-naphthalenyl side chains in both pyrimidine-BT 91c and pyrrolo-BT 92c showed moderate antitumor activity. From the above structure–activity relationships (SARs), the most selective pyrimidine BT and pyrrolo-BT on the studied cancer cell lines bore p-fluorophenyl and thiophene moieties. Thus, pyrimidine-BTs 91a,d and pyrrolo-BTs 92a,d were considered promising candidates as pharmacophores against cancer cell lines. A series of pyrimidine-BTs 91a–e and pyrrolo-BTs 91a–e were tested for antioxidant activity, as reflected in the ability to inhibit lipid peroxidation in rat brain and kidney homogenates using (3 ethylbenzthiazoline-6-sulfonicacid) an ABTS free radical scavenging assay. Pyrimidine-BTs 91a,d (91.2 and 92.8% yields, respectively) and pyrrolo-BT 92d (90.4%) showed inhibition levels higher than that of Trolox (89.5%), while pyrrolo-BT 92a (88.7%) showed nearly equipotent inhibition activity.

Scheme 10. Condensation of 2ATP with malononitrile to afford 2-[3-(aryl)prop-2-enenitrile]BTs 90a–e as intermediates in synthesizing BT derivatives 91,92a–e by the MWI method in % yields.

Scheme 10. Condensation of 2ATP with malononitrile to afford 2-[3-(aryl)prop-2-enenitrile]BTs 90a–e as intermediates in synthesizing BT derivatives 91,92a–e by the MWI method in % yields.

The nano-catalyst aminopropyl-1,3,5-triazine-2,4-diphosphonium tetrachloroferrate immobilized on halloysite nanotubes [(APTDP)(FeCl₄)₂@HNTs] was prepared to be used as a highly effective agent in the condensation reaction of 2ATP with benzonitriles containing various electron-withdrawing and -donating substituents for the synthesis of several BTs, 93a–o, in excellent yields (90–96%). The reaction was performed in the absence of a catalyst under solvent-free conditions at 80 °C within 4–5 h (

Table 26. Compounds 93a–x.

93	a	b	c	d	e	f	g	h	i
R1 = H, R2 =	Ph	2ClPh	3ClPh	4ClPh	3BrPh	4BrPh	4FPh	4CNPh	2MePh
% Yield	95	90	92	96	92	95	92	95	90
93	j	k	l	m	n	o	p	q	r
R1 = H, R2 =	4MePh	3NH2Ph	4MeOPh	Py-2-yl	Py-3-yl	Bn	4ClBn	4MeOPhBn	4MeBn
% Yield	90	90	91	95	95	90	92	90	90
93	s		t	u	v		w	x
R1 = H, R2 =	CHPh2		3CNPh	3(CH2CN)Ph	3(BT-2-yl)Ph		3(BTCH2)Bn	3,5(BTCH2)2Bn
% Yield	92		90	85	80		78	75

) [126]. Additionally, the use of dinitriles such as 1,3-dicyanobenzene and 1,3-phenylenediacetonitrile, using a 1:1 molar ratio, gave the corresponding mono-benzonitrile-BT 93t and mono-methylphenylacetonitrile-BT 93u in 90 and 85% yields, respectively, using a 1:1 molar ratio. Also, bis- and tris-BTs 93v–x were smoothly synthesized from dinitrile and trinitrile in those conditions in 78 and 75% yields. The advantages of this protocol were its easy work-up, high purity, high catalytic activity, reusability, and the easy recovery of the catalyst. On the other hand, the competitive condensation of 2ATP with an equimolar mixture of an aromatic nitrile (4-methylbenzonitrile) and an aliphatic nitrile (heptanenitrile) was examined under the same conditions. The results showed that 4-methylbenzonitrile was transformed to the corresponding BT 93j (90%) in the presence of heptanenitrile. This outcome indicated the selectivity performance of the catalyst.

2.3. Condensation with Ketones

2ATP was condensed with several 7-benzylidenebicyclo[3.2.0]hept-2-en-6-ones to afford a series of thirteen 2-[2-(2-phenylethenyl)cyclopent-3-en-1-yl]-BTs, 94a–m, in 93–98% yields (Figure 14) [127]. The antiproliferative activities of all compounds were tested against C6 (rat brain tumor) and HeLa (human cervical carcinoma) cell lines using BrdU cell proliferation ELISA assays, with the use of 5-fluorouracil (5-FU) and cisplatin as standard drugs. Compound 94e was the most active against C6 cell lines, with an IC₅₀ = 5.89 μm (5-FU, IC₅₀ = 76.74 μm, and cisplatin, IC₅₀ = 14.46 μm). But compound 94c was the most active against HeLa cell lines, with an IC₅₀ = 3.98 μm (5-FU, IC₅₀ = 46.32 μm, and cisplatin, IC₅₀ = 37.95 μm). On the other hand, computational studies of all compounds using the B3LYP/6-31G+(d,p) level in the gas phase were performed. The calculated results were compatible with the experimental IR and NMR data. Molecular electrostatic potential (MEP) maps allowed the identification of the region in 94c that is biologically active against HeLa and that in compound 94e which is active against C6. Molecular docking analysis was used to determine the biological activities of these molecules. The appropriate target proteins (PDB codes: 1JQH for the C6 cells and 1M17 for the HeLa cells) were used for 94c and 94e, demonstrating high antiproliferative activity against both cell lines. These compounds were proposed as promising candidates for anticancer drugs.

2.4. Miscellaneous Condensations

2ATP and both electron-rich and electron-poor substituted styrenes were condensed in a one-pot methodology for the synthesis of the pharmacologically active 2-arylBTs 95a–f in 30–66% yields (

Table 27. Compounds 95a–f.

95	a	b	c	d	e	f
R1 = H, R2 =	Ph	pMePh	pMeOPh	pBrPh	oClPh	3,5MeOPh
% Yield	60	65	66	35	30	63

). The reaction was carried out with Pd(OAc)₂ in AcOH as a solvent and heating at 90 °C for 12 h in the presence of O₂. The process involved sequential C-C/C-N bond cleavage followed by C-N/C-S bond formation [128]. To further the scope of the reaction, 2ATP was reacted with styrene under standard conditions. However, no desired product was obtained, and the starting materials were isolated.

2ATP disulfide was reacted with terminal alkynes under aerobic oxidative C(sp)–S coupling in a visible-light-driven copper-catalyzed reaction (

Table 28. Compounds 96a–n.

96	a	b	c	d	e	f	g	h
R1 = H, R2 =	Ph	4MePh	4MeOPh	4tBuPh	4ClPh	2ClPh	4BrPh	4CNPh
% Yield	66	68	70	68	64	55	65	58
96	i	j	k	l	m		n
R1 = H, R2 =	4CO2MePh	4PhCOPh	3NO2Ph	Py-2-yl	Napht-1-yl		Fenantren-1-yl
% Yield	61	56	64	59	76		79

) [129]. The photochemical reaction was achieved using a wide range of thiol dimers and alkynes with good chemoselectivity and excellent conversion. The utility of alkynyl sulfide formations as isolated intermediates (35–92%) was demonstrated in the construction of the 2-phenylBTs 96a–n in 55–79% yields via “thia-Wolff rearrangement” in the presence of AgNO₃, using visible light with 9-mesityl-10-methylacridinium ions (Acr⁺–Mes) as a photoredox catalyst system. BTs with electron-donating groups on the benzene ring increased the yield, while BTs 96b–d and BTs with electron-withdrawing groups at the ortho (96f), meta (96k), or para (96e,g,h) positions decreased the yields compared with the 2PhBT 96a. The BT with a pyridinyl ring instead of a benzene ring in 96l (59%) decreased the yield. Curiously, napthalenyl and phenanthrenyl groups, instead of the phenyl ring, resulted in high yields of 96m,n. This reaction failed with aliphatic terminal alkynes.

In a metal-free and environmentally friendly approach, 2ATP was condensed with methyl arenes under visible-light irradiation using eosin Y as a photocatalyst, ethanol–water (1:2) as a green solvent at room temperature, and atmospheric air as an oxidant to synthesize the biologically important BTs 97a–q (

Table 29. Compounds 97a–q.

97	a	b	c	d	e	f	g	h	i	j
R1 = H, R2 =	Ph	4FPh	4ClPh	4BrPh	4NO2Ph	3ClPh	3NO2Ph	3BrPh	2,3ClPh	4MeOPh
% Yield	90	91	90	90	91	85	92	85	85	80
96	k	l	m		n	o		p		q
R1 = H, R2 =	4NMe2Ph	2NO2Ph	Naphth-2-yl		Py-2-yl	Thiophen-2-yl		Indol-2-yl		Furan-2-yl
% Yield	85	92	87		84	84		83		84

) [130]. This methodology demonstrated a broad substrate scope, yielding the desired products in good to excellent yields (83–92%). The advantages of this procedure included its environmental friendliness, low cost, non-toxicity, green solvent, ease of handling, and use of visible light as a renewable energy source. All the BTs were obtained in good yields when the substituent was in the meta and ortho positions (97b–h,j; 85–92%). When the methyl arene had F, Cl, and Br (97b–d; 91, 90, and 90%, respectively) and OMe (97j, 86%) in the p-position, the yield obtained with electron-withdrawing groups was higher than that obtained with electron-donating groups. Additionally, methyl arene with multiple substituents yielded 85% (97i). The use of heteroaromatic methyl arenes (97n–q,v,w) afforded BTs with good yields (83–84%).

2ATP was condensed by an oxidative coupling with substituted aryl methyl amines in the presence of a Ba-doped CoMoO₄ system as a catalyst to synthesize several 2-aryl BTs 98a–k under visible-light irradiation (

Table 30. Compounds 98a–k.

98	a	b	c	d	e	f	g	h	i	j	k
R1 = H, R2 =	Ph	4MeOPh	Napht-1-ylPh	4iPrPh	2MePh	3PhOPh	2OH,4MePh	4ClPh	4FPh	2FPh	3,4ClPh
% Yield	98	98	80	85	83	94	78	86	89	79	79

) [131]. The reaction was carried out under atmospheric air and had good to excellent yields (78–98%). The advantages of this protocol were its atom economy, functional group tolerance, a wide range of substrate scopes, and suitability for scale-up reusability. Aryl methyl amines with an OMe as an electron-donating group at the para position of the benzene ring favored the yield, as in 98b, but an ⁱPr group (98d) decreased the yield due to steric effects. On the other hand, electron-withdrawing groups, such as Cl or F (98h and 981), decreased the yield, compared with 2PhBT 98a. The aryl methyl amines with electron-donating or electron-withdrawing substituents at the para position, BTs 98b,d,h,i, were obtained with good to excellent yields. Electron-donating or electron-withdrawing substituents at the ortho position of the benzene ring decreased the yields (98e,j), but electron-withdrawing substituents decreased the yields more. The OPh group at the meta opposition of the phenyl ring produced only a little decrease (98f).

The condensation of 2ATP with aromatic primary alcohols via the acceptorless dehydrogenative coupling (ADC) method was carried out in the presence of Pd(II)N^N^S as a pincer-type catalyst for the synthesis of the pharmaceutically important BT derivatives 98a–u (

Table 31. Compounds 99a–y.

99	a	b	c	d	e	f	g	h	i	j	k
R1 = H, R2 =	pMeOPh	pMePh	piPrPh	mMeOPh	mMePh	Ph	mFPh	pClPh	pNO2Ph	oBrPh	2,6ClPh
% Yield	93	90	86	87	85	84	69	72	53	58	63
99	l	m	n	o	p		q			r
R1 = H, R2 =	oNH2Ph	oMePh	2,3MeOPh	2,5MeOPh	Py-2-yl		Benzo[1,3]dioxolan-5-yl			Thiophen-2-yl
% Yield	71	47	74	71	69		73			72
99	s	t	u		v		w	x		y
R1 = H, R2 =	CH=CHPh	pPhPh	Naphth-1-yl		Piren-2-yl		Et	iPr		nPentyl
% Yield	66	79	75		56		52	53		48

) [132]. Several primary aromatic alcohols, such as electron-releasing and -withdrawing alcohols, sterically hindered alcohols, heterocyclic alcohols, polycyclic alcohols, and aliphatic alcohols, were coupled with 2ATP. p-substituted benzyl alcohols, including electron-donating 4-methyl and 4-isopropyl benzyl alcohol, were coupled with 2ATP to afford the corresponding BTs 99a,b in 90 and 86% yields, respectively. Interestingly, alcohols containing electron-poor groups, such as 3-fluro, 4-choloro, and 4-nitro benzyl alcohols, were well-tolerated to obtain BTs 99g–i in 69, 72, and 53% yields, respectively. Benzylic alcohols with sterically hindered 2-substituted alcohols, such as 2-Br, 2,5-Cl₂, 2-amino, and 2-NO₂-benzyl alcohols, gave BTs 99j–m with 58, 63, 71, and 47% yields, respectively. Next, benzyl alcohols containing electron-rich groups like 2,3-dimethoxy and 2,5-substituted groups were reacted to afford BTs 99n,o in 74 and 71% yields, respectively. Reactions with heteroaromatic benzyl alcohols such as 2-pyridine methanol, piperonyl alcohol, and thiophene-2-methanol proceeded smoothly to furnish BTs 99p–r in 69, 73, and 72% yields, respectively. Interestingly, the reaction of cinnamyl alcohol and polycyclic alcohols, such as 4-biphenyl benzyl acohol, 1-naphthalene methanol, and pyrene 2-methanol, afforded the corresponding BTs 99s–v in 66, 79, 75, and 56% yields, respectively. The use of aliphatic alcohols such as propan-1-ol, 2-methylpropan-1-ol, and hexan-1-ol gave the BTs 99w–y in 52, 53, and 48% yields, respectively. To understand the electronic effect, an intermolecular competitive experiment involving 4-methyl benzyl alcohol and 4-chloro benzyl alcohol was carried out under the same reaction conditions. It was found that the electron-rich benzyl alcohol is more reactive to give BT 99b with a 53% yield than the electron-deficient benzyl alcohol to give BT 99h with a 32% yield.

The BTs were synthesized via C−S and C−N bond formations, yielding up to 93%. This method was sustainable, with excellent yields, 1 mol% catalyst loading, inexpensive primary alcohol, and only water and hydrogen gas as by-products. A mechanism was proposed via in situ aldehyde formation and dehydrogenation of primary alcohols. The utility of this catalytic procedure was illustrated by the large-scale synthesis of the 2-(4-methoxyphenyl)benzo[d]thiazole 99a.

The treatment of the respective substituted (2-isocyano-phenyl)(alkyl)sulfanes, derived from 2ATP, with a series of diarylphosphine oxides under irradiation with 23W white LED light for 13 h at room temperature in an air atmosphere, using the photocatalyst Rose Bengal and DMF as a solvent, gave the 2-phosphorylBTs 100a–l in 65–89% yields (Figure 15) [133]. On the other hand, electron-poor and electron-rich substituents on the benzene ring in the 2-isocyanoaryl thioethers produced the substituted 2-phosphorylBTs 100m–t in 64–88% yields.

Several H-phosphorus oxides were well-tolerated, regardless of whether the ring of diarylphosphine oxides was substituted with either electron-deficient, electron-rich, or steric-hindered groups; the reactions proceeded to afford BTs 100a–i in medium to good yields (65–87%). Additionally, dimethyl-substituted H-phosphorus oxide was used to give BT 100j in a moderate yield (58%). The same procedure was used with piperonyl and naphthyl phosphine oxide derivatives, affording the BTs 100k,l (79 and 81% yields, respectively). On the other hand, functional groups with electron-rich (OMe and Me) and electron-poor (F, Cl, Br, and –SO₂Me) substituents on the BT ring were also tolerated to afford BTs 100m–t in good yields (64–88%).

A metal-free radical cascade cyclization of substituted 2-isocyanophenyl-methyl-sulfanes with 5.0 mL of the corresponding primary or secondary alcohol in the presence of the radical initiator di-tert-butyl-peroxide (DTBP) was refluxed for 6 h at 120 °C (Figure 16) [134]. The desired 2-hydroxyalkylBTs 101a–i and 102a–v were obtained in yields ranging from 34 to 92%. The advantages of this reaction were its mild reaction conditions, its being a metal-free reaction, the good functional group tolerance, and the broad substrate scope.

This cascade cyclization reaction of 2-isocyanoaryl thioethers proceeded smoothly with alcohols such as isopropanol, ethanol, and 2-butyl alcohol to give the desired BTs 101a,c,f in good yields (68.92%). Other alcohols such as methanol, n-propanol, n-butyl alcohol, cyclopentanol, and cyclohexanol afforded BTs 101b,d,e,g,h with moderate yields (51–68%). Additionally, the benzyl alcohol gave BT 101i in a 34% yield. On the other hand, several 5-substituted 2-isocyanoaryl thioethers reacted well with isopropanol to yield BTs 102a–g in 57–91% yields. It was found that substituents at the C5 position of 2-isocyanoaryl thioethers had little effect on the reaction. Other substituents such as phenyl, 4-chlorophenyl, 4-methylphenyl, naphthalen-2-yl, and phenanthren-9-yl also proceeded smoothly to synthesize BTs 102h–j and 102r,s in good yields (53–86% yields). Substituents at the C5 position were also well-tolerated to afford BTs 102k–m in 76–82% yields. C4 substituents were also tolerated to afford BT 102o in a 73% yield. Moreover, it was found that heteroaryl substituents also gave BTs 102t–v in 64–84% yields. 3-chloro, 4,6-dichloro, and 5,6-dimethyl substituents also reacted with isopropanol to give BTs 102p,q in 68 and 84% yields, respectively.

2-Isocyanoaryl thioethers derived from 2ATPs were cyclized with ethers in an oxidant-free and metal-free visible-light-induced reaction to access the ether-functionalized BTs 103a–v (Figure 17) [135]. The cyclization reaction was carried out with 1,2,3,5-tetrakis-(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a photocatalyst and the irradiation of blue LED light in a nitrogen atmosphere in mild reaction conditions, and the procedure was characterized by operational simplicity. This strategy provides an efficient approach to access various ether-containing BTs in acceptable to good yields.

Both electron-rich groups (−Me and −OMe) and electron-poor groups (−F, −Cl, and −Br) were well-tolerated in the reactions and smoothly afforded BTs 103a–k in moderate to good yields (42–70 %). Notably, the Br substituent at both the 4- and 5-positions of the phenyl rings gave BTs 103e,g,l in relatively low yields (64, 43, and 47% yields, respectively). Moreover, this methodology was extended to 1,3-dioxolane and several substituted isocyanides to afford BTs 103m/m′-p/p′ in 65 (2:1), 61 (2:1), 57 (3:1), and 60% (2:1) yields, respectively. It was observed that the BTs showed a preference for C2 selectivity over the C4 position. The synthesized compounds combine BT with an ether functional group representing key structural motifs of various biologically active molecules of interest in medicinal chemistry.

A photo-induced cascade sulfone alkylation/cyclization of 2-isocyanoaryl thioethers and α-iodosulfones was carried out to afford alkyl/benzyl-sulphonyl BTs in up to 97% yields 104a–zf (Figure 18) [136]. This visible-light-triggered reaction not only occurs under extremely mild reaction conditions but also does not require the presence of a photosensitizer. The photocatalytic process is triggered by the photochemical activity of in situ-generated electron donor–acceptor complexes arising from the association of 2-isocyanoaryl thioethers and α-iodosulfones. The radical pathway was confirmed by UV–vis spectroscopy, radical trapping, a Job plot, and on/off irradiation experiments.

A visible-light-induced radical addition/cyclization cascade reaction of 2-isocyanoaryl thioethers was achieved under metal-free and mild conditions (

Table 32. Compounds 105a–r and 106a–k.

105	a	b	c		d	e	f	g		h	i
R1 = H, R2 =	Ph	4MePh	4MeOPh		4FPh	4ClPh	4BrPh	4CF3Ph		4CNPh	4NO2Ph
% Yield	84	83	81		78	80	82	75		77	72
105	j	k	l		m		n	o		p	q	r
R1 = H, R2 =	3ClPh	2ClPh	3,5MeOPh		Naphtha-2-yl		Py-4-yl	Furan-2-yl		Bn	cPr	iPr
% Yield	76	71	79		80		74	76		75	73	70
106	a	b	c	d	e	f	g	h	i	j	k
R2 = Ph, R1 =	6Me	6Et	6MeO	6F	6Cl	6Br	6CF3	6CN	5Cl	4Me	5,7Me
% Yield	82	80	81	76	79	77	73	75	80	71	73

) [137]. This method utilizes hydrazines as radical precursors and efficiently accesses a series of 2-aryl and 2-alkyl BTs, 105a–r and 106a–k, in good yields (71–83% and 70–81%).

In the reaction of 2-isocyanoaryl thioethers, the groups on the benzene ring, electron-donating (Me and MeO) and electron-withdrawing groups (halogens, CF₃, CN, and NO₂), were compatible with the process to afford BTs 105b–k in good yields (71–83% yields). No influence of the position of the substituents was observed on the yields. In addition, the disubstituted substrate reacted well to afford BT 105l in a 79% yield. Instead of aryl rings, β-naphthyl-, 4-pyridyl-, and 2-furanyl-substituted hydrazines were used in this method to afford BTs 105m–o in 74–80% yields. It is worth mentioning that benzylhydrazine, isopropylhydrazine, and cyclopropylhydrazine could furnish BTs 105p–r in good yields (70–75%) when a series of substituted (2-isocyanophenyl)(methyl)sulfanes were used. In general, substrates bearing electron-rich (Me, Et, and MeO) or electron-deficient groups (halogens, CF₃, and CN) were tolerable and afforded BTs 106a–j in good yields (71–82% yields). When a disubstituted substrate was used, BT 106k was formed in a 73% yield.

The 2-guanidinoBT 107, obtained from condensation of 2ATP with cyanoguanidine, was used for the development of a range of synthetic methods with various reagents, such as α,β-unsaturated carbonyl, 2-cyano-three-(dimethylamino)-N-acrylamide, β-diketones, β-keto esters, and (S,S)-ketene dithioacetals, to afford pyrimidine-based BTs 108–115 in 66–72% yields as novel anticancer agents (Scheme 11) [138].

The docking analysis revealed that the most active BTs, 7b, 7c, 13b, 13c, 15c, 17d, 18, and 24, fit inside the active site within the protein tyrosine kinase (PTK). It was observed that these compounds formed a hydrogen donor bond with Met318, except for 15c. Among these compounds, BTs 15c, 17d, 18, and 24 resulted in binding energies closer to that of the cocrystallized ligand 1N1, with values of −7.729, −7.321, −7.681, and −7.5790 kcal mol⁻¹, respectively. Compound 13c had a binding energy of −6.963 kcal mol⁻¹, but it had four more interactions with the active site, including one arene–H interaction with Leu248 and three H-bond donors with Met318 and Thr319. Furthermore, the in silico studies and ADMET properties of the most potent BTs suggested them as promising candidates for further development, with favorable bioavailability and pharmacokinetic profiles. In vitro cytotoxicity studies were carried out against HepG2, HCT116, and MCF7 human tumor cell lines at a concentration 100 μmol mL⁻¹ using the standard 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) bioassay. BTs 108c, 109b, 111d, and 112 showed strong efficacy against the HepG2 cell line, exhibiting cell viability percentages of 61.29, 68.18, 61.04, and 66.85, respectively, compared with the standard drug (64.41%). BTs 108b, 109a, 110c, and 111b exhibited moderate activities against the same cell line, with cell viabilities of 72.70, 72.35, 72.69, and 76.02%. In addition, BTs 108b, 109c, and 110c showed slightly strong efficacy against the HCT116 cell line, with viabilities of 70.62, 67.11, and 65.68%, respectively, in contrast to the standard drug with a cell viability percentage of 55.96. Furthermore, the singular BT 24 exhibited efficacy against the MCF7 cell line, with a cell viability of 69.98%, compared with the standard drug (62.76%). From the IC₅₀ results, it was found that the pyrimidine-based BT 111d (IC₅₀ = 0.41 ± 0.01 μmol mL⁻¹) was the most potent of the tested BTs against HepG2. Also, BT 112 (IC₅₀ = 0.53 ± 0.05 μmol mL⁻¹) was the second most potent compound against HepG2, followed by compound 109b (IC₅₀ of 0.56 ± 0.03 μmol mL⁻¹). Notably, BTs 111d, 112, and 109b showed higher potency, based on their IC₅₀ data, compared with 5-fluorouracil (IC₅₀ of 1.03 μmol mL⁻¹) as the reference drug. Surprisingly, BT 110c (IC₅₀ of 0.02 ± 0.001 μmol mL⁻¹) showed higher efficacy against HCT116 compared with 5-fluorouracil (IC₅₀ of 9 ± 1.7 μmol mL⁻¹). BT 110c not only showed heightened potency related to 5-fluorouracil but also showed notable efficacy together with BTs 108b and 109c (IC₅₀ values of 2.95 ± 0.26 and 1.033 ± 0.06, respectively). In addition, BT 115 showed an IC₅₀ value of 1.485 ± 0.15 μmol mL⁻¹, lower than that of the 5-fluorouracil (IC₅₀ of 7.12 μmol mL⁻¹), against MCF7. These results suggest that the investigated BTs exhibit potential as anticancer agents. The findings revealed that the inclusion of halogen groups, such as F and Cl, on the aryl group bonded with pyrimidine-based BTs 108a–d resulted in an increase in their activity. In addition, the incorporation of CO₂Et, as in 110c, enhanced the potency in comparison to its analogs, 110a,b, containing COCH₃ and COPh, respectively. In the context of pyrimidine-based BTs 111a–d, the presence of a OMe group amplifies the compound’s potency more significantly than those possessing halogen substituents. Notably, BTs containing SCH₃ exhibited the lowest activity levels across the three tested cell lines.

3. Conclusions

In this investigation, we found that BT derivatives are compounds that have currently boomed in the last five years. In this period, at least 21 review articles appeared in the literature, and about 53 articles were found to be related to the condensation reaction of 2ATPs for the synthesis of this kind of compounds, studied for their biological activity, most of them as anticancer agents. In addition, we found that medicinal chemists also explore BT derivatives in searching for anti-Parkinson’s and anti-Alzheimer’s candidates. In some articles, the structure–activity relationships providing a framework for drug discovery and design were discussed. On the other hand, BT derivatives were explored as molecular entities endowed with two or more biological actions, called multifunctional drugs.

The most popular method to synthesize 2-substituted BTs was found to be the condensation of 2ATPs with carbonyl compounds such as aldehydes and carboxylic acids and their derivatives. The majority of these approaches have several drawbacks, such as harsh reaction conditions, high reaction temperatures, multistep processes, the need for an excessive amount of reagents, prolonged reaction times, and the employment of expensive, air-sensitive catalysts, among others. One of the main protocols for preparing BTs documented in the literature is the use of polyphosphoric acid (PPA). This popular method requires heating to high temperatures (110–220 °C) and long reaction times (1–24 h). These harsh conditions depend on the starting material’s stability, which limits the generalization of this condensation. Therefore, as an alternative to PPA, other catalyst-dehydrating agents have been designed to be used in this condensation, such as P₂O₅/MeSO₃H, trimethylsilyl polyphosphate ester (PPSE), triphenylphosphine (PPh₃), tetrabutylammonium bromide (TBAB), Samarium (III) triflate, molecular iodine, N-methyl-2-pyrrolidone (NMP), AlMe₃, KF/Al₂O₃, CuCl₂/K₂CO₃, o-benzene disulfonimide, acetic acid, pyridine, glycerol, DMSO, DMF, etc. These acids were effective for a wide range of aliphatic and aromatic carboxylic acids and derivatives to afford BTs in less exacting conditions and shorter reaction times. However, there is still a need to create a straightforward, mild, highly efficient, and environmentally benign method for the synthesis of BTs without the use of hazardous chemicals or reagents. Sometimes such condensation reactions have been carried out via direct heating or refluxing in solvents from high to low boiling points, improving yields. Due to their low production costs and simplicity of usage, applications of solvent-free synthetic methods to synthesize pharmacologically relevant BT derivatives have grown in favor. In this sense, the use of ionic liquids (ILs) as green solvents in these condensation reactions has gained considerably in importance due to their solvating ability, negligible vapor pressure, and easy recyclability. In addition, they have been shown to promote and catalyze these transformations due to their high polarity. ILs can also be recovered and recycled many times with marginal loss. The use of ILs such as 1-Butil-3-metil imidazolium [BMIM][BF₄] or [BMIM][PF₆] in this type of condensation reaction has been found to increase yields. Another method involving condensation uses microwave irradiation (MI). This method is carried out in short reaction times (5 to 60 min) with good to excellent yields (70–90%). As can be seen in the

Table 33. Novel catalyst systems recently developed.

Entry	Catalyst System	Time	Yields (%)	Ref.
1	Na2S2O4 catalyst as oxidant	12–36 h	51–84	[85]
2	Rice husk chemically activated carbon (RHCAC)	5–10 min	93–98	[86]
3	PhI(OH)OTs (Koser’s reagent)	15 min	80–90	[87]
4	Eosin Y (photocatalyst)/K2CO3 or Et3N/tert-butylhydroperoxide (TBHP)	24–36 h		[92]
5	Biocatalytic oxidant system (Laccase)/DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone)	1 h	65–98	[95]
6	ZnCl2 nano-flake catalyst supported on nano-hydroxyapatite (HAp)	15–90 min	65–95	[96]
7	Sulfated tungstate (ST) catalyst under solvent-free and ultrasound irradiation conditions	5 min	90–98	[97]
8	Ruthenium silicate (RS-1)-zeolite catalyst	30 min	85–93	[98]
9	Fluorescein (photocatalyst), blue LED lamp	3 h	82–94	[99]
10	Biocatalytic enzymatic (Trypsin) catalysis and visible light (450 nm)	10 min	64–99	[100]
11	Nano-catalyst IL on silica-coated/cobalt–ferrite magnetic nanoparticles (CoFe2O4@SiO2@PAF-IL)	10 min	83–91	[101]
12	Biocatalytic bovine serum albumin (BSA) in water	8 h	79–93	[104]
13	Biocatalytic bacterium-derived hemoglobin, Vitreoscilla (VHb)	20 min	85–97	[106]
14	Zn(OAc)2.2H2O heterogeneous catalyst	30–60 min	79–84	[107]
15	Nano-catalytic MNPs-phenanthroline-Pd/K2CO3 in DMF	6 h	89–98	[108]
16	1,4-diazabicyclooctane (DABCO)-based dicationic acidic ionic liquid [C4H10-DABCO][HSO4]2 catalyst	30–50 min	82–91	[109]
17	Co/Niacin-MOF catalyst	30–60 min	75–95	[73]
18	Blue LED (λ = 435–445 nm) irradiation without photocatalyst or metal	8 h	40–88	[116]
19	KF-Al2O3 as a base heterogeneous catalyst	30 min	87–97	[122]
20	Nano-catalytic eutectic mixture of CuCl2 or NiCl2/urea and Cu- or Ni-doped TiO2 heterogeneous nanoparticles and MWI	5 min	78–96	[123]
21	ZnO nanoparticle, solvent-free catalyst	20–25 min	88–96	[124]
22	Nano-catalytic aminopropyl-1,3,5-triazine-2,4-diphosphonium tetrachloroferrate immobilized on halloysite nanotubes [(APTDP)(FeCl4)2@HNTs]	4–5 h	90–96%	[126]

, recently, novel catalyst systems have been developed, such as homogeneous and heterogeneous catalysts, nano-catalysts, biocatalysts, photocatalysts, and combinations of these. Several examples of these systems have been proved to be excellent catalysts that reduce reaction times and increase yields with very good to excellent results (80–99%). Additionally, these methods are mild procedures, environmentally benign with good recyclability of the catalysts, and effective for a wide range of aliphatic and aromatic carbonyl compounds.