Synthesis and Thione–Thiol Tautomerism of 5-Thioxopyrano[3,4-c][1,2,4]triazolo[4,3-a]pyridines

A method for the synthesis of derivatives of 8-hydrazino 6-thioxopyrano[3,4-c]pyridine has been developed. Derivatives of 8-hydrazino-6-(methylsulfanyl)pyrano[3,4-c]pyridines and 5-(or 3)-(alkylsulfanyl)pyrano[3,4-c][1,2,4]triazolo[4,3-a]pyridines have been synthesized. The thione-thiol tautomerism of 5-thioxopyrano[3,4-c][1,2,4]triazolo[4,3-a]pyridine has been studied. In a basic medium, the equilibrium shifts toward the thiol form, which has allowed synthesis of S-alkyl-substituted triazolo[4,3-a]pyridines.