Preparation of some thiopyranopyridine derivatives

Abstract

In the first systematic study of thiopyranopyridines in which the sulphur atom is separated from the pyridine ring by one carbon atom, the four isomeric enol esters, ethyl 5-hydroxy-8H-thiopyrano[3,4-b]-pyridine-6-carboxylate (4b), ethyl 8-hydroxy-5H-thiopyrano[4,3-b]pyridine-7-carboxylate (5b), and ethyl 4-hydroxy-1H-thiopyrano[3,4-c]- and [4,3-c]pyridine-3-carboxylate (6b) and (7b), have been synthesised. Improved methods for the preparation of their pyridine precursors are described. With phenylhydrazine, the enol esters (4b)–(7b) give condensed pyrazole derivatives (15)–(18), which have dipolar structures; with hot mineral acid they undergo decarboxylative hydrolysis, to give the corresponding oxothiopyranopyridines (4a)–(7a).