Promising Norlabdane-Heterocyclic Hybrids: Synthesis, Structural Characterization and Antimicrobial Activity Evaluation

Abstract

The terpeno-heterocyclic molecular hybrids are a new and promising class of modern organic and medicinal chemistry, because their molecules exhibit high and selective biological activity, natural origins, and good biocompatibility, and, usually, they are less toxic. The reported norlabdane-heterocyclic hybrids were synthesized by classical and new, original, and environmentally friendly methods, which include coupling reactions of norlabdane derivatives (such as carboxylic acids, acyl chlorides, or bromides) with individual heterocyclic compounds, as well as heterocyclization reactions of certain norlabdane intermediates like hydrazides, thiosemicarbazones, or hydrazinecarbothioamides. The aforementioned norlabdanes were derived from (+)-sclareolide 2, which is readily obtained from (−)-sclareol 1, a labdane-type diterpenoid extracted from the waste biomass of Clary sage (Salvia sclarea L.) that remains after essential oil extraction. All synthesized compounds were tested against various fungal strains and bacterial species, with many exhibiting significant antifungal and antibacterial activity. These findings support the potential application of the synthesized compounds in the treatment of diseases caused by fungi and bacteria. Additionally, the use of plant-based waste materials as starting resources highlights the economic and ecological value of this approach. This review summarizes experimental data on the synthesis and biological activity of norlabdane: diazine, 1,2,4-triazole and carbazole, 1,3,4-oxadiazole, 1,3,4-thiadiazole, 1,3-thiazole, 1,3-benzothiazole and 1,3-benzimidazole hybrids performed by our research group covering the period from 2013 to the present.

1. Introduction

The rapid proliferation of microbial infections in recent decades has become a pressing issue in the realm of global public health [1,2]. This trend has driven the search for new molecular structures with antimicrobial properties, aiming to develop effective medicinal agents for their treatment. Among the most promising sources of new biologically active compounds are natural products, due to their biocompatibility, selective biological activity, and typically low toxicity. Terpenes and terpenoids are diverse classes of natural compounds, particularly notable for their wide range of applications in medicine, pharmaceuticals, cosmetics and agriculture [3,4]. Particular attention is paid to terpene compounds, which exhibit multiple biological activities, including anticancer [5], antioxidant, antimicrobial [6], antifungal [7], antimalarial [8], and antidiabetic properties [9]. Some studies have shown that the introduction of heteroatoms, especially nitrogen and sulfur, into terpene structures often enhances their biological activity. Furthermore, the incorporation of heteroatomic functional groups, specific molecular fragments, or heterocyclic units can significantly increase the therapeutic potential of these compounds [10,11].

The synthesis of hybrid molecules—structures that integrate multiple pharmacophores—has emerged as a powerful strategy in drug design. These hybrid compounds often exhibit superior bioactivity compared to existing drugs [12]. In this context, the functionalization of norlabdane derivatives of (−)-sclareol 1, including sesquiterpenoids (pentanorlabdanes), is of considerable interest. Modifications performed either on the side chain or at the C₇ position of the B ring have successfully yielded products containing heterocyclic units, paving the way for novel bioactive agents.

The main goal of the syntheses presented in this review was the synthesis of norlabdane-heterocyclic molecular hybrids from (−)-sclareol 1, a readily available natural product that serves as the foundation for these compounds [13,14,15,16,17]. (+)-Sclareolide 2 as an oxidation product of (−)-sclareol 1 [18,19], and its derivatives, such as ketones 3–6, served as starting materials in the syntheses of di- and trinorlabdane-heterocyclic hybrids (Figure 1).

In other cases, the syntheses of penta- and tetranorlabdane-heterocyclic hybrids started from carboxylic acids 7, 8 and 10–13 or ketones 9, 14 and 15, derived from compounds 1 via (+)-sclareolide 2 (Figure 2 and Figure 3).

Some series of penta- and tetranorlabdane-heterocyclic hybrids were obtained through heterocyclization reactions via hydrazides 16–19, derived from compounds 1 and 2 (Figure 3).

Figure 3. Penta- and tetranorlabdane intermediates and hydrazides 13–19 from (−)-sclareol 1.

Figure 3. Penta- and tetranorlabdane intermediates and hydrazides 13–19 from (−)-sclareol 1.

It should be noted that the methods for obtaining the norlabdanic intermediates depicted in Figure 1, Figure 2 and Figure 3 from compounds 1 and 2 are mentioned in the corresponding subchapters.

For convenience, the syntheses of norlabdane: diazine, 1,2,4-triazole and carbazole, 1,3,4-oxadiazole, 1,3,4-thiadiazole, 1,3-thiazole, 1,3-benzothiazole, and 1,3-benzimidazole hybrids are organized into separate Section 2.1, Section 2.2, Section 2.3, Section 2.4, Section 2.5, Section 2.6 and Section 2.7.

2. Results

2.1. Synthesis of Norlabdane–Diazine Hybrids

Many drugs have been designed based on diazine compounds. Diazines have proven highly valuable in medicine, pharmaceuticals, and cosmetics due to their broad spectrum of biological activities, including antimicrobial, antifungal, antitubercular, antiviral, anti-HIV, and anticancer properties [20]. In this subsection, the synthesis of norlabdane–diazine hybrids will be discussed.

The synthesis of the mentioned hybrids was carried out based on (+)-sclareolide 2, via ∆^8,13-bicyclohomofarnesenoic 7, 13,14,15,16-tetranorlabd-6,8(9)-dien-12-oic 11 and drimenoic 13 acids which were obtained in 5 to 6 steps with total yields of 60%, 94%, and 55%, respectively. Next, the condensation reactions of their acyl chlorides 20, 21 and 22, obtained in situ after treatment with oxalyl chloride in anhydrous benzene, with corresponding amines 23a–c (2-aminopyrimidine, 2-aminopyrazine and 4-aminopyrimidine) were performed [21].

As a result, from bicyclohomofarnesenoyl chloride 20 a series of new hybrid amides 24a–c and 25 were obtained (Scheme 1) [21,22]. It should be noted that in the case of 2-aminopyrazine 23b and 4-aminopyrimidine 23c, only monoacylamides 24b (15%) and 24c (60%) were formed, while for 2-aminopyrimidine 23a, in addition to monoacylamide 24a (16%), bis-acylamide 25 (54%) was also produced.

The molecular structure of hybrid 25 was established spectrally and its stereochemistry was confirmed based on single-crystal X-ray diffraction. It was determined that the bis-acylation process occurs only in the case of 2-aminopyrimidine.

The coupling reactions of 13,14,15,16-tetranorlabd-6,8(9)-dien-12-oyl chloride 21 with amides 23a–c were carried out under the same conditions, in methylene chloride while stirring. In the case of 2-aminopyrazine 23b and 4-aminopyrimidine 23c, only monoacylamides 26b and 26c were obtained, with yields of 35% and 40%, respectively (Scheme 1). In the case of 2-aminopyrimidine 23a, two compounds were similarly obtained: monoacylamide 26a (69%) and bis-acylamide 27 (25%) [23].

All N-substituted amides, derived both from Δ^8,13-bicyclohomofarnesenoic acid 7 and from 13,14,15,16-tetranorlabd-6,8(9)-dien-12-oic acid 11, are obtained through the condensation of primary amines with acyl chlorides 20 and 21. Secondary amides are also capable of undergoing further reaction with acyl chlorides; however, experimental data indicate that only the monoacylamides 24a and 26a undergo bis-acylation, affording the corresponding bis-acylamides 25 and 27.

This selectivity is likely attributable to the delocalization of the nonbonding electrons of nitrogen into the adjacent carbonyl group (amide bond resonance), which decreases the overall nucleophilicity of the amide nitrogen and thereby reduces its reactivity toward further acylation. Moreover, in the case of amides 24a–c and 26a–c, additional resonance delocalization into the aromatic substituents further modulates their electronic environment, suggesting that the aryl groups exert a pronounced influence on the observed reactivity. Reaction time may also play a critical role in enabling the formation of the bis-acylated derivatives.

Drimenoic acid 13 was synthesized from (+)-sclareolide 2 in six steps, with a total yield of 55%, according to the method developed by the authors [24].

Further synthetic efforts were directed toward the preparation of amides of drimenoic acid 13. Unexpectedly, instead of the anticipated products, isodrimenoic acid amides 28a and 28b were isolated in yields of 16% and 14%, respectively. In addition, the reaction of the intermediate acid chloride 22 with 4-aminopyrimidine 23c furnished a mixture of amides 29 and 30 in a 2:3 ratio, with a total yield of 28% [24].

It should be noted that during the reaction of drimenoic acid 13 with oxalyl chloride, the isomerization of the double bond ∆^7,8 in the position ∆^8,9, occurs. As a result, the subsequent reaction of acyl chloride 22 with 2-aminopyrimidine 23a, 2-aminopyrazine 23b, and 4-aminopyrimidine 23c affords the isodrimenoic acid amides 28a, 28b, and 29, as well as the albicanoic acid amide 30.

The in vitro antimicrobial activities of the synthesized homodrimane sesquiterpenoids, both with and without a diazine skeleton, were systematically evaluated. The tested compounds exhibit excellent antibacterial activity against Gram-positive strains of S. aureus and B. cereus. Structure-activity relationship correlations reveal that homodrimane sesquiterpenoids with diazine skeleton possess a better antibacterial activity compared to those without a diazine skeleton. Additionally, in the homodrimane sesquiterpenoids with a diazine skeleton series, the 2-aminopyrimidine 23a derivatives demonstrate better activity than the 2-aminopyrazine 23b ones and, the mono-acyl amides show better activity than the bis-acyl amides. Compounds 24a–c and 25 from this series exhibited good antibacterial activity, but none of the tested compounds showed antifungal activity [22].

As a continuation of previous research on the synthesis of new compounds incorporating both terpene and 1,2-diazine fragments, a series of norlabdane derivatives containing a pyridazinone structural unit were synthesized [25,26].

For the preparation of these norlabdane-based compounds, the methyl ester of 7-oxo-13,14,15,16-tetranorlabd-8-en-12-oic acid 9 was used as the starting material. This intermediate can be obtained from (+)-sclareolide 2 in two steps, with an overall yield of 77%.

The ketoester 9 was subsequently brominated using N-bromosuccinimide (NBS, 1.5 equivalents), resulting in the formation of the corresponding tetranorlabdane bromide 31 (Scheme 2).

Subsequently, ketobromide 31 was coupled with 6-(p-tolyl)-3(2H)-pyridazinone 36 in a basic medium (K₂CO₃) using N,N-dimethylacetamide (DMAA) as the solvent, leading to the formation of the tetranorlabdane–pyridazinone hybrid compound 37. This reaction proceeds efficiently due to the conjugated unsaturated nature of pyridazinone 36, which enhances the acidity of the N–H bond, making it readily deprotonated under basic conditions (Scheme 2). The coupling was conducted under both conventional thermal conditions and under microwave irradiation [25].

Next, through a series of transformations starting from the methyl ester of 7-oxo-13,14,15,16-tetranorlabd-8-en-12-oic acid 9, two pentanorlabdane ketone intermediates were obtained. These ketones were then treated with N-bromosuccinimide (NBS) in carbon tetrachloride, leading to the formation of several brominated derivatives: 11-bromo-drim-8(9)-en-7-one 32, 12-bromo-drim-8(9)-en-7-one 33, 11,12-dibromo-drim-8(9)-en-7-one 34, and 11,12-dibromo-drim-5(6),8(9)-dien-7-one 35. The ratio of these brominated products in the reaction mixture can be adjusted by altering the amount of NBS and the reaction time (Scheme 2).

The brominated compounds 32–35 were subsequently subjected to coupling reactions with 6-(p-tolyl)-3(2H)-pyridazinone 36 in a basic medium (KOH) using N,N-dimethylacetamide (DMAA) as the solvent. These reactions were carried out under both conventional heating and microwave irradiation conditions. A chromatographically inseparable mixture of bromides 32 and 33 (in a 3:2 ratio) was reacted with 6-(p-tolyl)-3(2H)-pyridazinone 36, yielding the monosubstituted hybrids 11-p-tolyl-pyridazinyl-drim-8(9)-en-7-one 38 and 12-p-tolyl-pyridazinyl-drim-8(9)-en-7-one 39. Unlike the starting bromides, these products could be separated by silica gel column chromatography. The combined overall yield of compounds 38 and 39 was 85%. Individual dibromides 34 and 35, when treated with pyridazinone 36 under the same reaction conditions, produced 11,12-di(p-tolyl-pyridazinyl)-drim-8(9)-en-7-one 40 and 11,12-di(p-tolyl-pyridazinyl)-drim-5(6),8(9)-dien-7-one 41, respectively.

The structures of the newly synthesized compounds were confirmed using spectroscopic methods, including ¹H and ¹³C NMR, and IR spectroscopy. Additionally, the stereochemistry of compound 41 was established by single-crystal X-ray diffraction analysis [26].

Scheme 2. Synthesis of tetra- and pentanorlabdane-p-tolyl-pyridazinone hybrids. Data from [25,26,27].

Scheme 2. Synthesis of tetra- and pentanorlabdane-p-tolyl-pyridazinone hybrids. Data from [25,26,27].

All synthesized compounds were evaluated in vitro against five fungal strains and two bacterial species.

Compound 41, featuring a quinone-analogue skeleton in combination with two diazine units, exhibits high antibacterial and antifungal activity. Its antifungal activity was recorded at an MIC of 5 × 10⁻³ μg/mL, while its antibacterial activity (MIC = 3.2 × 10⁻² μg/mL) was approximately 90 times more active than Kanamycin (3 μg/mL) [27]. The enhanced activity of compound 41 can be attributed to the synergistic effect of the quinone moiety, which is known to participate in redox cycling and disrupt microbial metabolic pathways, together with the two diazine rings that may improve molecular interactions with biological targets.

Compound 39, which includes only a diazine ring at C₁₂ of the norlabdane core and lacks the analogous quinone moiety, exhibits moderate antifungal activity (MIC = 15 × 10⁻¹ μg/mL), but it is six times lower compared to the reference compound Caspofungin (0.25 μg/mL). Compounds 38 and 40 do not exhibit inhibitory activity against the aforementioned strains of bacteria and fungi.

Thus, for the first time, based on (+)-sclareolide 2, syntheses of penta- and tetranorlabdane–diazine hybrids were achieved, one of which exhibited exceptional antimicrobial activity that was patented [27].

2.2. Synthesis of Norlabdane-1,2,4-triazole and Carbazole Hybrids

In the previous subchapter, molecular hybridization methods applied to obtain homodrimane sesquiterpenoids with diazine units were described [21]. Although none of the hybrids depicted in Scheme 1 were active against fungi, some demonstrated high antibacterial activity [22]. This fact directed the attention of our research team toward other heterocyclic units suitable for molecular hybridization, such as 1,2,4-triazole units.

This structural core is widely represented in both natural products and therapeutic agents, and its substituted derivatives are regarded as privileged pharmacophores in compounds with anticancer, antimicrobial, and antiviral activities [28,29,30,31].

The first attempt to obtain N-substituted tetranorlabdane-1,2,4-triazole hybrids was made by the authors [32]. The products obtained resulted in a substantial increase in the antioxidant activity of the biomass of certain cyanobacterial species, which were patented together along with cultivation methods [33].

Next, the results of the syntheses of some norlabdane hybrids with 1,2,4-triazole units via the corresponding acyl chlorides and hydrazinecarbothioamides, the structures and biological properties of which have been elucidated, will be reported.

The synthesis of amides 44 and 47 of ∆^8,13-bicyclohomofarnesenoic acid 7 and amides 45 and 48 of 13,14,15,16-tetranorlabd-6,8(9)-dien-12-oic acid 11, which include 1,2,4-triazole and carbazole rings, was carried out according to Scheme 3 [23,24].

Upon interaction of acids 7 and 11 with (COCl)₂, the acyl chlorides 20 and 21 were obtained in situ. As a result of their reaction with 3-amino-1,2,4-triazole 42 and N-aminocarbazole 43, the amides 44, 45, 47 and 48 were formed. In the case of drimenoic acid 13, contrary to expectations, albicanoic acid 46 and isodrimenoic acid 49 amides were obtained [24]. It should be noted that under the conditions of the reaction of drimenoic acid 13 with oxalyl chloride, isomerization of the ∆^7,8 double bond to the ∆^8,9 position occurs, which leads to the subsequent interaction of the acyl chloride 22 with N-aminocarbazole 43 forming albicanoic acid 46 and isodrimenoic acid 49 amides. Spectral analysis of albicanoic acid amide 46 revealed that 3-amino-1,2,4-triazole 42 participated in the reaction in its tautomeric form, resulting in the incorporation of an amino group and the preservation of the semicyclic double bond Δ^8,13 within the amide structure.

The structure and stereochemical configuration of N-(isodrimenoylamino)carbazole 49 were unambiguously established by single-crystal X-ray diffraction analysis.

In the continuation of the research in the field of norlabdane compounds with biologically active heterocyclic fragments, a series of norlabdane-1,2,4-triazole hybrids was synthesized by an alternative method [34].

For the preparation of the aforementioned derivatives, 8α-hydroxy-homodrim-11-hydrazide 16 was employed as the key starting material. This compound was obtained in a single step from commercially available (+)-sclareolide 2 with a yield of 85%, as illustrated in Scheme 4 [34]. Next, hydrazide 16 was coupled with substituted aryl isothiocyanates in ethanol, resulting in hydrazinecarbothioamides 50a–d. The synthesis of hydrazine carbothioamides 50a–d by the classical method was carried out at room temperature for 270–300 min, which has a disadvantage. For these reasons, unconventional methods were used, and the synthesis was carried out by microwave irradiation at a constant power of 200 W. According to the experimental data obtained from microwave irradiation, the reaction time decreases considerably (from several hours to 5 min), and the yields increase slightly. Therefore, the coupling reactions of hydrazide 16 with aryl isothiocyanates through microwave irradiation are considered environmentally friendly [34].

Hydrazine carbothioamides 50a–d were treated with aqueous NaOH (8%) at 70 ˚C to afford the corresponding norlabdane derivatives bearing a triazole moiety 51a–d in yields of 70–83%. Subsequent coupling of triazoles 51a–d with bromoacetophenone in acetone, in the presence of triethylamine (Et₃N), furnished the S-substituted 1,2,4-triazoles 52a–d.

Theoretically, the N–H functional group of the triazole moiety can undergo condensation with a halogenated aromatic derivative such as bromoacetophenone. However, in compounds 51a–d the triazole ring exists in two resonance forms, one of which corresponds to an aromatic zwitterion with a positively charged N1 and a negatively charged sulfur atom. The excess electron density on sulfur renders it nucleophilic, enabling preferential reaction with bromoacetophenone to give the S-substituted products.

The structures of these compounds were elucidated on the basis of spectral data (¹H and ¹³C NMR, IR), while the stereochemistry of compounds 51c and 51d was unambiguously confirmed by single-crystal X-ray diffraction.

The antibacterial and antifungal activities of the synthesized compounds with hydrazide 16, hybrid norlabdane and carbothioamide 50a–d, or triazole skeletons 51a–d and 52a–d were tested in vitro on pure cultures of bacteria and fungi.

Compounds 51c and 51d displayed remarkable antimicrobial activities, with MIC values of 0.125 μg/mL and 0.094 μg/mL for antifungal activity and 0.064 μg/mL and 0.047 μg/mL for antibacterial activity, respectively. Notably, the antibacterial potency of triazole 51d was 63-fold greater than that of the reference antibiotic Kanamycin (MIC 3.5 μg/mL), while its antifungal effect was threefold stronger than that of Caspofungin (MIC 0.24 μg/mL). In contrast, the S-substituted triazoles 52a–d, in which the thiol group at position 2 of the triazole ring is replaced by an acetophenone moiety, were devoid of antimicrobial activity.

In addition, tetranorlabdane hydrazinecarbothioamides 50c and 50d were subjected to cytotoxicity testing on human ovarian carcinoma cell lines A2780 and A2780cis, as well as on the non-cancerous human renal embryonic cell line HEK293 [34], both demonstrating moderate activity in the micromolar IC₅₀ range.

Thus, for the first time, based on (+)-sclareolide 2, efficient syntheses of biologically active homodrimane hybrids with hydrazinecarbothioamide fragments or 1,2,4-triazole and carbazole rings have been achieved with high yields, both by classical methods and by microwave irradiation.

2.3. Synthesis of Norlabdane-1,3,4-oxadiazole Hybrids

1,3,4-Oxadiazole is a versatile heterocyclic ring, frequently used in pharmaceutical chemistry for the development of novel therapeutic agents [35]. Various 1,3,4-oxadiazole derivatives have attracted significant interest due to their broad range of biological activities, including antimitotic, antifungal, antibacterial, sedative-hypnotic and anticonvulsant activities among others [36,37,38,39]. Due to their great medical significance, numerous synthetic routes for 1,3,4-oxadiazoles have been developed, some of which have been described by the authors [40].

Next, the synthesis and biological properties of new molecular norlabdane-1,3,4-oxadiazole hybrids will be described [41], which is a continuation of our research in the field of compounds with cumulative biological potential [22,26,32].

The synthesis of the reported compounds was carried out based on 8α-hydroxy-homodrim-11-hydrazide 16, previously obtained from commercial (+)-sclareolide 2 in one step with a yield of 85% [41].

Treatment of hydrazide 16 with substituted aryl isothiocyanates in EtOH afforded the corresponding hydrazinecarbothioamides 50a–c in 83–86% yields, as shown in Scheme 5 [34].

Being treated with N,N′-dicyclohexylcarbodiimide (DCC) in a mixture of MeOH and Me₂CO [42], carbothioamides 50a–c formed tetranorlabdane hybrids with substituted 2-amino-1,3,4-oxadiazole units 53a–c in 76–81% yields.

Next, hydrazide 16 was treated with cyanogen bromide (BrCN) in aqueous dioxane [43], yielding unsubstituted 5-(8α-hydroxydriman-11-yl)-1,3,4-oxadiazol-2-amine 54 in 80% yield.

The reaction of hydrazide 16 with 1,1′-carbonyldiimidazole (CDI) in tetrahydrofuran (THF) in the presence of triethylamine led to 5-(8α-hydroxydriman-11-yl)-1,3,4-oxadiazol-2(3H)-one 55 in 74% yield according to Scheme 5 [43].

A study was conducted on the reaction between hydrazide 16 and varying amounts of tetramethylthiuram disulfide (TMTD), by heating at 90 °C in dimethylformamide (DMF) according to the known procedure [44].

The use of 1 equivalent of TMTD under the mentioned conditions proved to be quite efficient and led to 2-thio-5-(11-homodrim-8α-ol)-1,3,4-oxadiazole 57 in 86% yield and a small amount of a 1,3,4-thiadiazole hybrid which will be described in Section 2.4 [41].

Next, 1,3,4-oxadiazoles 55 and 57 were subjected to coupling reactions with bromoacetophenone in acetone in the presence of Et₃N to form 3-N-acetophenone-5-(11-homodrim-8α-ol)-1,3,4-oxadiazol-2-one 56 in 80% yield and 3-N-acetophenone-5-(11-homodrim-8α-ol)-1,3,4-oxadiazole-2-thione 58 in a 91% yield (Scheme 5) [45].

For the preparation of tetranorlabdane-1,3,4-oxadiazole hybrids, hydrazide 18 of Δ⁸,⁹-bicyclohomofarnesenoic acid was used as the starting material. This compound was synthesized from commercially available (+)-sclareolide 2 in seven steps, affording an overall yield of 40% [46] (Scheme 6).

The reaction of hydrazide 18 with allyl or phenyl isothiocyanate in ethanol led to the formation of hydrazine carbothioamides 59a and 59b, which subsequently, upon treatment with N,N′-dicyclohexylcarbodiimide (DCC) at reflux in methanol, are transformed into allylamino- and phenylamino 1,3,4-oxadiazole derivatives 60a and 60b substituted in position 2, in depicted yields (Scheme 6).

Following the interaction of hydrazide 18 with cyanogen bromide (BrCN) in aqueous dioxane, 2-amino-5-(Δ^8,9-bicyclohomofarnesenyl)-1,3,4-oxadiazole 61 was obtained with a yield of 91%.

The 2-thio-5-(Δ^8,9-bicyclohomofarnesenyl)-1,3,4-oxadiazole 62 was prepared under standard conditions by heating hydrazide 18 at 90 °C in dimethylformamide (DMF) with tetramethylthiuram disulfide (TMTD) in 70% yield.

The reaction of hydrazide 18 with 1,1′-carbonyldiimidazole (CDI) in tetrahydrofuran (THF) in the presence of triethylamine led to 5-(Δ^8,9-bicyclohomofarnesenyl)-1,3,4-oxadiazole-2(3H)-yl 63 in 92% yield (Scheme 6).

To make a comparative study of the structure-activity relationship (SAR) with tetranorlandane molecular hybrids, we synthesized a series of pentanorlabdane 1,3,4-oxadiazoles, starting from the hydrazide of drimenoic acid 18, prepared according to standard procedure that includes the in situ preparation of drimenoic acid 13 acyl chloride, followed by its interaction with hydrazine hydrate (N₂H₄∙H₂O) (Scheme 7).

Subsequently, starting from hydrazide 19, under the conditions described above, a series of novel pentanorlabdane 1,3,4-oxadiazole hybrids 65a (83%) and 65b (89%), 66 (89%), 67 (72%) and 68 (91%) were obtained (Scheme 7).

Spectral analyses (¹H and ¹³C NMR, IR) were employed to establish the structures of the synthesized compounds, and the structure as well as the stereochemistry of compound 67 were further validated by sin64gle-crystal X-ray diffraction.

The antibacterial and antifungal activities of the synthesized tetranorlandane 53–58, 59–63 and pentanorlabdane 64–68 1,3,4-oxadiazole hybrids were tested in vitro on pure cultures of bacteria and fungi.

According to these, 1,3,4-oxadiazoles 56, 57 and 63 exhibit significant antifungal activity at a minimum inhibitory concentration (MIC) of 2 μg/mL, 1.3 μg/mL and 0.125 μg/mL, respectively, compared to the reference compound Caspofungin (MIC 0.24 μg/mL). Compound 56 also exhibits pronounced antibacterial activity with an MIC of 0.50 μg/mL, which is seven times higher compared to the reference compound, Kanamycin (MIC 3.5 μg/mL).

Thus, for the first time, based on (+)-sclareolide 2, syntheses of corresponding 1,3,4-oxadiazole hybrids were achieved through its tetra- and pentanorlabdanic derivatives. By varying the reagents and molecular ratios, the optimal conditions for the heterocyclization reactions were established. All the synthesized compounds were tested in vitro, and five of them showed antimicrobial activity.

2.4. Synthesis of Norlabdane-1,3,4-thiadiazole Hybrids

1,3,4-Thiadiazole, as well as its derivatives, contains a universal heterocyclic nucleus that has attracted increased attention in medicinal chemistry in the search for new therapeutic agents. This five-membered heterocyclic compound is widely used as a structural element in various biologically active molecules, including drugs [47], because 1,3,4-thiadiazole derivatives have a broad spectrum of biological activity, including antitumor, antibacterial, antifungal, antituberculosis, anti-inflammatory, antiviral and antileishmanial activities [48].

As a starting material for the synthesis of the reported norlabdane 1,3,4-thiadiazoles, 8α-hydroxy-homodrim-11-hydrazide 16 was also used (Scheme 8) [41].

As mentioned in Section 2.3, the reaction of 8α-hydroxy-homodrim-11-hydrazide 16 tetramethylthiuram disulfide (TMTD) in DMF yields a mixture of two compounds: 2-thio-5-(11-homodrim-8α-ol)-1,3,4-oxadiazole 57 and 2-mercapto-5-(11-homodrim-8α-ol)-1,3,4-thiadiazole 69 (Scheme 5 and Scheme 8). The ratio of oxadiazole 57 to thiadiazole 69 hybrids is strongly dependent on the stoichiometry of TMTD used in the reaction [41].

When 0.5 mol of TMTD is used in the reaction with 1 mol of hydrazide 16 only 46% of the initial hydrazide reacts, forming only 1,3,4-oxadiazole 57. Using equimolar amounts of hydrazide 16 and TMTD produces a mixture of oxadiazole 57 and thiadiazole 69, with oxadiazole 57 predominating. Increasing the TMTD to 1.5 mol relative to 1 mol of hydrazide 16 results in the same product mixture, but with thiadiazole 69 prevailing (70%). The formation of 1,3,4-thiadiazole 69 was unexpected; however, its structure was fully confirmed by spectral analysis, and the stereochemistry of compound 69 was definitively established by single-crystal X-ray diffraction.

Next, thiadiazole 69 was subjected to coupling reactions with bromoacetophenone in acetone in the presence of Et₃N to form 2-S-acetophenone-5-(11-homodrim-8α-ol)-1,3,4-thiadiazole 70 in 85% yield (Scheme 8).

Following the strategy for synthesizing norlabdane 1,3,4-thiadiazole hybrids, another synthetic method was employed, involving the interaction of hydrazide 16 with isothiocyanate derivatives, without isolating intermediate compounds. This reaction was carried out in the presence of triethylamine (Et₃N) in water, resulting in 2-amino-1,3,4-thiadiazoles 71a–c with yields of 70–78% (Scheme 8) [41].

The antibacterial and antifungal activity of the synthesized norlabdane 1,3,4-thiadiazole hybrids 69, 70 and 71a–c was tested in vitro on pure cultures of bacteria and fungi. Thiadiazole 69 exhibits pronounced antifungal activity (at MIC 0.032 μg/mL) and antibacterial activity (at 0.094 μg/mL) [41]. The antibacterial activity of compound 69 is thirty times higher than that of the reference compound, Kanamycin (MIC 3.5 μg/mL). As an antifungal agent this compound is eight times more active than the reference compound Caspofungin (MIC 0.24 μg/mL). Thiadiazole 71a exhibits antifungal activity (MIC 0.25 μg/mL) and pronounced antibacterial activity (MIC 0.5 μg/mL). The antibacterial activity of compound 71a is six times higher compared to the reference compound, Kanamycin (MIC 3.5 μg/mL).

Prolonged reflux of hydrazides 18 and 19 with allyl isothiocyanate or phenyl isothiocyanate in water in the presence of Et₃N afforded tetra- and pentanorlabdane 1,3,4-thiadiazole hybrids, 72a and 72b, 73a and 73b [41] (Scheme 8).

For the synthesis of novel tetranorlabdane derivatives bearing thiosemicarbazone fragments or 1,3,4-thiadiazole rings, 13,14,15,16-tetranorlabd-6(7),8(9)-dien-12-oic acid 11 was employed as the starting material. This acid was obtained from commercially available (+)-sclareolide (2) in five steps, with an overall yield of 47% [49]. Subsequently, the coupling of acid 11 with allyl- or phenylthiosemicarbazides (molar ratio 1:1.2) was carried out in dichloromethane using EDCI as the coupling reagent, affording the tetranorlabdane thiosemicarbazone derivatives 74a–c in 73–85% yields (Scheme 8).

In continuation, the heterocyclization reaction of thiosemicarbazones 74a–c was carried out in the presence of Et₃N and H₂O, yielding the tetranorlabdane 1,3,4-thiadiazole hybrids 75 and 76a,b in 75% and 67–84% yields, respectively (Scheme 8).

The antibacterial and antifungal activities of the synthesized tetranorlabdane hybrids with thiosemicarbazone or 1,3,4-thiadiazole moieties were tested in vitro on pure cultures of bacteria and fungi [50].

Thus, for the first time, based on (+)-sclareolide 2, through its tetra- and pentanorlabdanic derivatives, syntheses of hybrids with 1,3,4-thiadiazole units were achieved and the mechanism of formation of some compounds was explained. In vitro tests revealed the increased activity of hybrids 69 and 75 and which are of interest to the pharmaceutical industry, and the activity of compounds 69 and 75 was patented [51,52].

2.5. Synthesis of Norlabdane-1,3-thiazole Hybrids

The 1,3-thiazole moiety represents a key structural element in drug design due to its wide-ranging biological activities. Several thiazole derivatives are known to exhibit anticonvulsant, antimicrobial, anti-inflammatory, antitumor, and other pharmacologically relevant effects [53,54,55,56]. Similarly, compounds containing thiosemicarbazone fragments display a broad spectrum of biological activities, including antitumor, antifungal, antibacterial, antiviral, and antimalarial properties, etc. [57,58,59], and frequently serve as intermediates in the synthesis of compounds with 1,3-thiazole moieties. Unfortunately, there are few mentions in the specialized literature regarding the synthesis of terpenes with 1,3-thiazole units and the evaluation of their biological activity [60,61,62,63].

In continuation, the data regarding the synthesis of tetra- and pentanorlabdane compounds with thiosemicarbazone moieties and 1,3-thiazole units in outside chain or cycle B synthesized by our group will be described [63].

Natural diol (−)-sclareol 1 was used as the starting material for the synthesis of norlabdanic intermediates. Initially, in two consecutive stages it was transformed into unsaturated ketone 15,16-dinorlabd-8(9)-en-13-one 3 in 80% yield (Scheme 9) [64].

The reaction of ketone 3 with thiosemicarbazide or 4-phenylthiosemicarbazide (molar ratio 1:1.1) afforded dinorlabdane compounds with thiosemicarbazone moieties 77 and 78, each as a mixture of two chromatographically inseparable isomers. The reaction of thiosemicarbazones 77 and 78 with 2-bromoacetophenone in ethanol (molar ratio 1:1) led to the formation of dinorlabdane-1,3-thiazole hybrids 79 and 80.

For the synthesis of trinorlabdane-1,3-thiazole hybrids, commercially available (+)-sclareolide 2 was used as a starting material. Its interaction with methyllithium (CH₃Li) gave 8α-hydroxy-14,15,16-trinorlab-12-one 4 in 65% yield according to the methodology [64]. Treatment of hydroxyketone 4 with MeSO₃SiMe₃ in acetonitrile resulted in a mixture of known 14,15,16-trinorlabd-7(8)-en-13-one 5 and 14,15,16-trinorlab-8(9)-en-13-one 6, (ratio 4:1), with a 91% overall yield, which were successfully separated by column chromatography on silica gel (Scheme 9) [65].

Scheme 9. Synthesis of di- and trinorlabdane-1,3-thiazole hybrids. Data from [63,66].

Scheme 9. Synthesis of di- and trinorlabdane-1,3-thiazole hybrids. Data from [63,66].

The reaction of ketones 4–6 with thiosemicarbazide or 4-phenylthiosemicarbazide (molar ratio 1:1.1) gave trinorlabdane compounds with thiosemicarbazone moieties 81, 82 and 85–88. Each of these thiosemicarbazones was obtained as a chromatographically inseparable mixture of two isomers. It should be noted that in subsequent reactions, mixtures of isomeric thiosemicarbazones were used, as it is known that over time, the Z-isomers transform into the more stable E-isomers. The reaction of thiosemicarbazones 81, 82 and 85–88 with 2-bromoacetophenone in ethanol (molar ratio 1:1) led to the formation of trinorlabdane-1,3-thiazole hybrids 83, 84 and 89–92.

Further, ketones 3–6 underwent a condensation-cyclization reaction with thiourea and iodine in ethanol, to afford the corresponding norlabdane-2-amino-1,3-thiazole hybrids. The unsaturated dinorlabdane ketone 3 yielded only the mentioned 2-amino-4-(15,16-dinorlabd-8(9)-en-13-one)-1,3-thiazole 93 with an overall yield of 85%. In the case of trinorlabdane hydroxyketone 4, a mixture of thiazoles 94–96, in a ratio of 1:1.5:2.5 was obtained, with an overall yield of 85%. The formation of this mixture may be explained as hydroxyketone 4 undergoes partial dehydration, which leads to 2-amino-4-(14,15,16-trinorlabd-7(8)-en-13-one)-1,3-thiazole 95 and 2-amino-4-(14,15,16-trinorlabd-8(9)-en-13-one)-1,3-thiazole 96, obtained in 43% and 25% yields, respectively. This fact is confirmed by the formation of minor hydroxylated 2-amino-4-(8α-hydroxy-14,15,16-trinorlabd-13-one)-1,3-thiazole 94, isolated from the reaction mixture in a 17% yield. The condensation-cyclization reaction of unsaturated ketones 5 and 6, under the same conditions, led to trisubstituted 95 and tetrasubstituted 96 trinorlabdane-1,3-thiazoles [66].

As a starting material, for the synthesis of cycle B tetra- and pentanorlabdane-1,3-thiazole hybrids, the methyl ester of 7-oxo-13,14,15,16-tetranorlabd-8-en-12-oic acid 9 was used, which was obtained from (+)-sclareolide 2, in two steps and with a total yield of 76%. Decarboxylation of ketoester 9 was carried out at reflux for 3 h in an alcoholic potassium hydroxide solution, leading to the reference pentanorlabdane intermediate drim-8(9)-en-7-one 14 with a 98% yield (Scheme 10) [67].

Subsequently, in the reaction of ketones 9 and 14 with thio- or 4-phenylthiosemicarbazide in a molar ratio of 1:1.1, tetra- and pentanorlabdane compounds with thiosemicarbazide moieties 97, 98, and 101, 102 were obtained [63,68].

Tetra- and pentanorlabdane-1,3-thiazole hybrids 99, 100, and 103, 104 were obtained via the heterocyclization reaction of thiosemicarbazones 97, 98, and 101, 102 with 2-bromoacetophenone [63].

Scheme 10. Synthesis of tetra- and pentanorlabdane-1,3-thiazole hybrids. Data from [63].

Scheme 10. Synthesis of tetra- and pentanorlabdane-1,3-thiazole hybrids. Data from [63].

The structures of all newly synthesized compounds were established based on spectral data (IR, ¹H, ¹³C, and ¹⁵N NMR).

All newly synthesized compounds were tested in vitro for antifungal and antibacterial activity against pure cultures of five fungal species (Aspergillus niger, Fusarium solani, Penicillium chrysogenum, P. frequentans, Alternaria alternata) and Gram-negative (Pseudomonas aeruginosa) and Gram-positive bacteria (Bacillus polymyxa).

Compounds 78a,b and 81a,b possessed antifungal activity with minimal inhibitory concentrations (MIC = 0.25 and 0.19 μg/mL) comparable to that of the antifungal drug Caspofungin (MIC = 0.25 μg/mL) and also showed antibacterial activity (MIC = 4 and 3 μg/mL, respectively) comparable to that of the antibiotic Kanamycin (MIC = 4.0 μg/mL). Compound 97 possessed moderate antifungal activity with minimal inhibitory concentrations (MIC = 1.5 μg/mL) comparable to that of the antifungal drug Caspofungin (MIC = 0.2 μg/mL) and also showed significant antibacterial activity (MIC = 0.125 μg/mL), which was 24 times more active than the known antibiotic Kanamycin (MIC = 3.0 μg/mL).

Thus, for the first time, based on (−)-sclareolide 2, efficient syntheses were achieved through its di-, tri-, tetra-, and pentanorlabdane derivatives, including by unconventional methods such as microwave irradiation, of a series of molecular hybrids containing thiosemicarbazone fragments or 1,3-thiazole units. Molecular hybrids 81a and 81b exhibited pronounced antimicrobial properties, while hybrid 97 demonstrated excellent selective antibacterial activity, which was patented [69].

2.6. Synthesis of Norlabdane-1,3-benzothiazole Hybrids

The chemistry of 1,3-benzothiazole and its 2-substituted derivatives has developed into a distinct field of research, driven by their high structural diversity [70,71,72,73,74]. The interest in compounds with 1,3-benzothiazole structural units is fueled by their biological properties, such as anticancer, antimicrobial, antioxidant, anti-inflammatory, antiviral, and other activities [75,76,77,78,79].

Little data is known from the specialized literature about natural and biologically active compounds that include a 1,3-benzothiazole moiety, and even less about terpene compounds [80,81,82].

Currently, researchers’ attention is focused on developing methods for the synthesis of substituted 1,3-benzothiazoles and their derivatives, using different types of catalysts to improve selectivity, purity, and product yield. Structure-activity relationship (SAR) studies particularly reveal that the structure of the substituent at the C₂ carbon atom strongly influences the bioactivity of the compound. Various methods are known to lead to the formation of 2-substituted compounds with 1,3-benzothiazole structural units. The most commonly used synthetic method involves the cyclocondensation reaction of aromatic aldehydes or carboxylic acids, esters, acyl halides with ortho-aminophenol [74] or its disulfides [83].

Thioamides can be obtained by the conversion of amides using Lawesson’s reagent, and the course of the reaction and the yield depend on the structures of the substrates used [84].

As part of modern research focused on the development of new biologically active terpeno-heterocyclic compounds and as a logical complement to the research described in the previous subsections, our team has set a new goal: the synthesis of tetranorlabdane 1,3-benzothiazole hybrids. The data obtained will be presented below [85].

As a starting material for the synthesis of the mentioned compounds, (+)-sclareolide 2 was used, which was transformed into carboxylic acid 10 in three steps with a total yield of 89%. Carboxylic acids 8 and 11 were obtained from (+)-sclareolide 2 in five and six steps, with yields of 81% and 62%, respectively.

The one-pot decarboxylative cyclization reactions of acids 8, 10, and 11 with 2-aminothiophenol, promoted by triphenylphosphine and triethylamine [86], were carried out under reflux for four hours. After purification by column chromatography on silica gel, this afforded 2-homodrimenyl-1,3-benzothiazoles 105–108, with yields as illustrated in Scheme 11.

It should be noted that, in the case of carboxylic acid 8, surprisingly, in addition to the desired compound 107, obtained with a yield of only 5%, the compound 108, with an unexpected structure, was afforded as a major reaction product, with a total yield of 27%. The rearrangement of the carbon skeleton of compound 108 was confirmed by a shift in some signals in the ¹H NMR spectrum compared to the starting acid 8.

Scheme 11. Synthesis of norlabdane-1,3-benzothiazole hybrids. Data from [85].

Scheme 11. Synthesis of norlabdane-1,3-benzothiazole hybrids. Data from [85].

Next, a series of new N-homodrimenoyl-2-amino-1,3-benzothiazoles were prepared, starting from the intermediate carboxylic acids 7, 8, 10, and 11 via their acyl chlorides generated in situ. It should be mentioned that the acid 7 was obtained from the commercially available (+)-sclareolide 2 in six steps, with an overall yield of 60%, according to the known method [22].

The desired N-substituted 2-amino-1,3-benzothiazoles 109–112 were obtained with yields between 40% and 84% through the acylation of 2-amino-1,3-benzothiazole with the mentioned sesquiterpene acyl chlorides under the stated conditions (Scheme 11). According to the NMR spectra, the hybrids involved both heterocyclic and terpene units, and their accurate masses were confirmed by a high-resolution mass spectrometry (HRMS) analysis.

All synthesized compounds were subjected to preliminary screening for their in vitro antifungal and antibacterial activities against pure cultures of fungal species Aspergillus niger, Fusarium solani, Penicillium chrysogenum, Penicillium frequentans, Alternaria alternata, as well as both Gram-positive Bacillus sp. and Gram-negative Pseudomonas aeruginosa bacterial strains. The obtained minimum inhibitory concentration (MIC) values revealed that compounds 105 and 110 possessed a high nonselective antifungal activity (MIC 0.094 and 0.25 μg/mL, respectively) in comparison with Caspofungin. Moreover, compounds 107, 109, and 110, exhibited a promising antifungal activity at MICs in the range from 0.95 to 2 μg/mL, compared to the same standard. At the same time, compound 105 demonstrated high non-selective antibacterial activity (MIC 0.75) relative to the standard Kanamycin. Compounds 107, 108, and 110 exhibited a moderate antibacterial activity. As for compounds 109, 111, and 112, they were biologically inactive.

In conclusion, short synthetic routes to a series of tetranorlabdane–1,3-benzothiazole hybrids were successfully developed starting from acids 7, 8, 10 and 11. The highest antimicrobial activities were observed for homodrimane sesquiterpenoids containing 1,3-benzothiazole units, as well as for derivatives bearing the NCS fragment, which, when rigidly constrained by an additional ring, adopt a spatial configuration that mimics the steric and electronic features of the benzothiazole scaffold.

2.7. Synthesis of Norlabdane-1,3-benzimidazole Hybrids

1,3-Benzimidazole is a valuable structural element in the design of new drugs, due to its diverse biological activity, chemical stability, and ability to interact with a variety of biological targets, which makes it highly relevant to the pharmaceutical industry [87].

Synthetically, benzimidazoles are obtained through the condensation reaction of 1,2-phenylenediamine with aldehydes and carboxylic acids. Their derivatives exhibit low toxicity and high activity against many pathogenic strains, with minimal chances of resistance [88].

There are few studies describing the synthesis and biological evaluation of new hybrid heterocyclic structures. In one of them, molecular hybrids formed by 1,2,3-triazole and benzimidazole units are presented, which reveal significant antibacterial, antifungal, and cytotoxic activities [89]. Other authors reported the synthesis of a new series of hybrid molecules with benzimidazole-1,2,3-triazole units obtained in several steps, including microwave-assisted reactions. The synthesized hybrids showed a moderate inhibition of 30% in the Foa sporulation test [90]. To date, no molecular hybrids with norlabdne-benzimidazoles have been reported.

The aim of this research was to develop original methods for the synthesis of new terpeno–heterocyclic derivatives based on the readily available natural diterpenoid (−)-sclareol 1, and to design chiral natural molecules of potential interest to the pharmaceutical industry. In this context, we present the results of the synthesis of novel tetranorlabdanes bearing either 2-substituted 1,3-benzimidazole or N-substituted 2-amino-1,3-benzimidazole moieties, along with the evaluation of their antimicrobial activity [91].

The title N-homodrimenoyl-2-amino-1,3-benzimidazoles was synthesized from intermediate carboxylic acids 8, 10, 11, and 12, via their in situ-generated acyl chlorides. The target compounds 113, 114, 115, and 116 were obtained in yields ranging from 66% to 85% by acylating 2-amino-1,3-benzimidazole with the corresponding tetranorlabdane-derived acyl chlorides under the specified conditions [22] (Scheme 12).

NMR spectral analysis confirmed the presence of both heterocyclic and terpene moieties within the hybrid structures, while their molecular formulas were validated by high-resolution mass spectrometry (HRMS).

Multiple attempts to directly synthesize the target hybrid benzimidazoles via heterocyclization of acids 8, 10, 11, and 12 with o-phenylenediamine in the presence of 4N HCl [92], glacial AcOH [93], or BF₃·OEt₂ [94] were unsuccessful. However, treatment with triphenylphosphine and triethylamine [86] afforded the corresponding monoacylated derivatives 117–120 in the yields indicated in Scheme 12. Additionally, diacylated derivatives were obtained in the reactions involving acids 8 and 11.

The structures of the synthesized compounds were confirmed by the ¹H, ¹³C, ¹⁵N, and 2D NMR spectroscopy, by the HRMS analysis, and finally, in the case of amide 120, by the single-crystal X-ray diffraction (XRD).

Subsequently, the cyclodehydration of the resulting monoacylamides 117–120 was carried out using p-toluenesulfonic acid (p-TsOH) in toluene [95]. Monoacylamides 117 and 118 underwent cyclization to afford 2-substituted benzimidazoles 121 and 122 (Scheme 12). The formation of the doubly unsaturated benzimidazole 122 from amides 117 and 118 can be rationalized by the acid-promoted elimination of the C₇-methoxy group from compound 118, followed by proton abstraction at C₅, which results in isomerization of the Δ^6,7 double bond to Δ^5,6. Under analogous conditions, monoacylamides 119 and 120 yielded the same benzimidazole 123 (Scheme 12). In the case of compound 120, the generation of the Δ^8,9 benzimidazole 123 derivative occurs via elimination of the C₈-acetoxy group.

Scheme 12. Synthesis of norlabdane-1,3-benzimidazole hybrids. Data from [91,96,97].

Scheme 12. Synthesis of norlabdane-1,3-benzimidazole hybrids. Data from [91,96,97].

All synthesized compounds were subjected to preliminary screening for their in vitro antifungal and antibacterial activities [50] against pure cultures of the fungal species Aspergillus niger, Fusarium solani, Penicillium chrysogenum, Penicillium frequentans, and Alternaria bacterial strains. The obtained minimum inhibitory concentration (MIC) values revealed that compounds 115 and 120 possess the highest antifungal (MIC 0.064 and 0.05 μg/mL, respectively) and antibacterial (MIC 0.5 and 0.032 μg /mL, respectively) activities, followed by compound 113 (MIC 1.6 and 4.0 μg /mL, respectively), which is comparable to the standard’s activity. Compounds 114, 116, and 118 have shown moderate antifungal activity at MICs in a range from 0.80 to 1.16 μg /mL, and antibacterial activity at MICs in a range from 3.90 to 6.0 μg /mL, against the same standard.

In conclusion, it can be stated that a series of seven tetranorlabdane-1,3-benzimidazole hybrids were synthesized through the decarboxylative cyclization and condensation of the aforementioned acids or their acyl chlorides with o-phenylenediamine and 2-aminobenzimidazole, as well as the p-TsOH-mediated cyclodehydration of the mentioned monoacylamides. The hybrids were evaluated as antimicrobial agents, and six of them demonstrated high to moderate antifungal and antibacterial activities compared to those of the reference drugs. The activity of compounds 115 and 120 has also been patented [96,97].

3. Conclusions

The escalating prevalence of fungal and bacterial infections, compounded by the emergence of antimicrobial resistance, underscores the critical need for the development of novel molecular entities with enhanced antimicrobial efficacy. Natural products, and terpenoids in particular, have emerged as a prolific source of bioactive compounds due to their inherent biocompatibility, selective biological activity, and low toxicity profiles.

The summary of our publications from 2013 to the present shows that norlabdane hybrids bearing diazine, 1,2,4-triazole, 1,3,4-oxadiazole, 1,3,4-thiadiazole, 1,3-thiazole, 1,3-benzothiazole and 1,3-benzimidazole moieties represent valuable bioactive molecules for the design of new drugs.

The mentioned norlabdane-heterocyclic hybrids were synthesized from commercially available (+)-sclareolide 2, derived from the labdane-type diterpenoid (−)-sclareol isolated from industrial Salvia sclarea L. waste via two main strategies: under normal conditions or conventional heating. In some cases, microwave heating was used to improve overall yields and reaction efficiency by reducing reaction times, solvents, and energy consumption, contributing to the greening of the synthetic process.

In the first case, coupling reactions of various terpene derivatives, such as carboxylic acids, acyl chlorides, or bromides, with selected azaheterocyclic compounds were preferred to form new C–N or C–C bonds and thus the desired hybrid molecules.

When heterocyclization reactions were applied, involving the cyclocondensation of key intermediates such as hydrazides, hydrazinecarbothioamides, or thiosemicarbazones, they led to the formation of heterocyclic rings integrated into the side chains or the B-ring of the terpene components.

The in vitro biological evaluation of these compounds against selected fungal strains and bacterial species revealed notable antimicrobial activity, ranging from moderate to pronounced and high, thereby validating their therapeutic potential. This promising biological activity exhibited by the synthesized hybrids highlights their potential as lead candidates for the development of new antimicrobial agents, thus contributing to both the advancement of medicinal chemistry and the sustainable utilization of renewable natural resources.

We believe that this progress in the field of molecular hybrids, especially terpeno-heterocyclic hybrids, will encourage their synthesis and more active exploration of their biological properties.