Appliance Features of 4-Amino-1,2,4-triazole-3-thiols in the Synthesis of 3,6-Disubstituted [1,2,4]Triazolo[3,4-b] [1,3,4]thiadiazoles

4-Amino-5-substituted-4H-1,2,4-triazole-3-thiols are versatile synthons for constructing of various biologically active heterocycles. This is provided by the close proximity of the amino and mercapto groups, which serve as readily accessible nucleophilic centers for the preparation of N-bridged heterosystems. One of the possible and convenient directions of using 4-amino-4H-1,2,4-triazole-3-thiols in heterocyclic synthesis based on their utilization in reaction with various carboxylic acids in the presence of dehydrating reagents, most often phosphorus oxochloride. Synthesized as follows [1,2,4]triazolo[3,4-b][3,4-b]thiadiazole derivatives may differ by various substituents in positions 3 and 6 such as alkyl-, aryl-, aryl(oxy)alkyl-, heteryl- etc. In this review, we presented the synthetic strategies and subsequent chemical transformations of the resulting triazolo[3,4-b]thiadiazoles, providing certain important class of functionalized compounds.