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. 

Following the interaction of 2-chloro-N-(5-aryl-1,3,4-oxadiazole-2-yl) acetamides 1a-b with ammonium thiocyanate in dry acetone, the 5-unsubstituted 2-imino-4-thiazolidinones 4a-b have been synthesized. Compounds 4a-b were subsequently utilized in Knoevenagel condensation with aromatic aldehydes or isatin derivatives to synthesize the series of 5-arylidene/isatinylidene substituted 2-(1,3,4-oxadiazol-2-yl)imino-4-thiazolidinones 5a-h and 6a-d. The structures of target compounds were confirmed by using 1H NMR spectroscopy and elemental analysis. Evaluation of anti-cancer activity in
vitro for the synthesized compounds was performed following the National Cancer Institute protocol against leukemia, melanoma, lung, colon, CNS, ovarian, renal, prostate, and breast cancer cell lines. As a result, the most active compound 5a, namely 2-[5-(4-chlorophenyl)-[1,3,4]oxadiazol-2-ylimino]-5-(4-methoxybenzylidene)thiazolidin-4-one was found to be a highly efficient anti-tumor candidate with average logGI50 and logTGI values of -5.19 and -4.09, respectively