Regioselective synthesis of new 2,5,6-trisubstituted 5,6-dihydro-2H-pyrazolo[3,4-d]thiazoles from 5-dimethylaminoethylene-thaozolidin-4-thiones

Thiazolidin-4-ones assembled through one-pot three-component (carbonyl compound, amine and ethyl mercaptoacetate) reactions in the presence of dicyclohexyl carbodiimide, on simultaneous treatment with dimethyl formamide dimethylacetal and Lawesson's reagent furnished 5-dimethylaminomethylene thiazolidin-4-thiones. These thiones on condensation with hydrazine derivatives afforded 2,5,6-trisubstituted-5,6-dihydro-2H-pyrazolo3,4-d]thiazoles 4 and not their possible 1,5,6-trisubstituted regioisomers 5. Results of single-crystal X-ray diffraction studies of thiazolidin-4-one 1b have been reported. The structures of all the synthesized compounds were established by spectral data. DFT studies on regioisomers were carried out using B3LYP with the 6-31G** basis set and carbon NMR shifts of 4 show good correlation with experimental values.