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

<div><p>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-pyrazolo[3,4-d]thiazoles <b>4</b> and not their possible 1,5,6-trisubstituted regioisomers <b>5.</b> Results of single-crystal X-ray diffraction studies of thiazolidin-4-one <b>1b</b> 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 <b>4</b> show good correlation with experimental values.</p></div>