Triazolo[3,4-b]thiadiazoles are a class of heterocyclic compounds, which have attracted great interest in medicinal chemistry owing to their wide range of pharmacological activities. A number of triazoles fused to thiadiazoles are incorporated into a wide variety of therapeutically important compounds possessing a broad spectrum of biological activities. Considering such a significant pharmacological potential, as well as wide synthetic possibilities triazolo-thiadiazoles have received considerable attention from scientific community and are extensively used for construction of prospective drug-likes molecules. In this review, we summarized the literature data about the main synthetic approaches for obtaining condensed heterocyclic compounds based on triazolo[3,4-b][1,3,4]thiadiazole scaffold as promising objects for modern bioorganic and medicinal chemistry.

A series of new 4-(1,3,4-thiadiazol-2-yl)-containing polysubstituted pyrroles 3 a-k has been synthesized by a preparative convenient method from ethyl 5-chloro-4-formyl-1H-pyrrole-3-carboxylates 1 a-e, which were selectively transformed into the corresponding polysubstituted pyrrole-4-carboxylic acids 2 а-е using sodium hypochlorite as an oxidizer. Further, they were transformed into the target compounds with a high yield using the cyclocondensation with N-mono- or N,N-disubstituted thiosemicarbazides in the boiling phosphorus trichloroxide. As seen from the screening of antimicrobial activity, the synthesized compounds exhibit the inhibiting and bactericide activity against some bacteria and fungi. The highest activity has been established for the compounds 3 a, c, e-h, j against the strain Klebsiella pneumoniae (МІС=31.25 µg/mL). The calculated HOMO energy level proves that the compound 3 с is the most reactive ligand for the interaction with a protein receptor. The molecular docking data show that the compound 3 h has the highest affinity to the ThiM Klebsiella pneumoniae kinase.

3-aryl-6-phenyl-7H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazines were obtained with good yields (81-95%) via the heterocyclization reaction of 1-phenyl-2-((5-aryl-1,3,4-oxadiazole-2-yl)thio)ethane-1-ones in acetic acid. The physicochemical characteristics of the synthesized compounds were established, the structures were confirmed by the data of IR, ¹H and ¹³C NMR spectra, as well as the results of X-ray diffraction analysis. The cytotoxic, antibacterial and antifungal properties of these compounds were evaluated. In vitro screening results showed that compounds 8, 9 and 12 significantly inhibit (54-65%) the growth of HeLa, HBL-100 and CCRF-CEM cancer cell lines. It was found that the cytotoxicity of the synthesized compounds increases in the series of oxadiazoltiones (1-4) - S-derivatives (5-8) - triazolothiadiazines (9-12). Compounds 5-16 do not exhibit antimicrobial properties.

In the present work, we presented an efficient synthesis and anticancer activity evaluation of some new 3-R-6-(5-arylfuran-2-yl-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazoles. We have shown that the proposed synthetic protocols provided the possibility to design triazolothiadiazoles diversity with a considerable chemical novelty. The structures of target substances were confirmed by using 1H NMR spectroscopy, mass spectrometry and elemental analysis. The synthesized compounds were selected by the National Cancer Institute Developmental Therapeutic Program for the in vitro cell line screening. Among all the substances tested, three compounds exhibited significant cytotoxic activity.

Proficient routes were devised for coupling different aromatic/aliphatic acids with amines to form amide linkage using various catalysts. Under the optimized reaction conditions, highest conversion was possible without formation of any by-products. All synthesized compounds were purified using column chromatography and characterized by mass spectrometry, nuclear magnetic resonance spectrometry and liquid chromatography-mass spectrometric analysis.

A series of some new pyrazole-substituted 7H-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazines was synthesized in this study. The structures of target substances were confirmed by using 1H and 13С NMR spectroscopy, mass spectrometry and elemental analysis. The synthesized compounds have been evaluated for antimicrobial activity against five bacterial strains (Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Staphylococcus aureus) and two fungal strains (Candida albicans and Cryptococcus neoformans). The antimicrobial screening studies of synthesized substances established that 2 of 12 compounds show pronounced antibacterial activity against the strain Staphylococcus aureus.

Amidoalkylation of secondary heterocyclic amines by N-[5-(alkylsulfanyl)-1,3,4-thiadiazol-2-yl]-2'-chloroacetamide resulted the new compounds 5-10 that contain 1,3,4-thiadiazole-5-thione moiety alongside pyperidine, morpholine, and cytisine fragments. In vitro screening of antimicrobial activity of synthesized compounds showed that N-[5-(amylsulfanyl)-1,3,4-thiadiazol-2-yl]-2'-morpholinacetamide exhibited an appreciable antibacterial activity against gram-negative bacteria of Escherichia coli (inhibition zone diameter of 16 mm) and gram-positive bacteria of Staphylococcus aureus and Bacillus subtilis (10-13 mm).

New series of fused 1,2,4-triazoles, i.e., 3-(4-isopropylphenyl)-6-substituted phenyl-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazoles (5a-5j) have been synthesized via a four-step procedure and using eco-friendly reaction condition at some steps of the synthesis. It was adopted by the formation of hydrazide of methyl 4-isopropylbenzoate (1) followed by reaction with CS2 in basic media to afford potassium salt, which on cyclized to our essential step, 4-amino-5-(4-isopropylphenyl)-4H-1,2,4-triazole-3-thiol (4). The desired adducts (5a-5j) were formed by (4) on reaction with various aromatic acids in POCl3 media. The newly synthesized triazolo- thiadiazoles have been characterized by different spectroscopic techniques and investigated for their in vitro antibacterial and antifungal activity. It was revealed that the compounds 5a, 5c, 5h, and 5i showed interesting antibacterial and antifungal activity compared to the used reference standard.

Readily available tetrachloro-2-aza-1,3-butadienes enter into directed cyclocondensation reaction with N-phenyl-1,2-cyclopentanediamine which leads to regioselective cyclopentane annulation by the 1,3,5-triazepine. The formation of the 1,3,5-triazepine derivatives was confirmed proved by 1H- and 13C-NMR spectral study, elemental analysis and, in one case, single-crystal x-ray crystallographic study.

A bronsted acid-surfactant catalysed and simple, mild, metal catalyst free and chemo-selective method has been developed for synthesis of 1, 3, 5-triaryl benzenes from aryl methyl ketones. The advantages of this protocol subsume green and sustainable reaction medium, mild reaction conditions, easy product recovery and its good yields.