Fused mesoionic heterocycles: synthesis of 1,3,4-triazolo[3,2-a]pyridine derivatives

Abstract

A number of mesoionic derivatives of 1,3,4-triazolo[3,2-a]pyridine have been prepared. The 1,3,4-oxadiazolo[3,2-a]pyridylium-2-aminides [(14)–(19)], available from 1,3,4-oxadiazolo[3,2-a]pyridylium-2-olate (12) and N-aryliminotriphenylphosphoranes, undergo base-catalysed isomerization to the corresponding 1,3,4-triazolo[3,2-a]pyridylium-2-olates [(20)–(25)]. 1-Amino-2-methylthio-4,6-diphenylpyridinium iodide (27), reacts under basic conditions with aryl isothiocyanates to yield the corresponding 1,3,4-triazolo[3,2-a]pyridylium-2-thiolates [(36)–(40)]. Compounds [(14)–(17)] react with arylmethylamines to yield the guanidine derivatives [(43)–(46)]; these cyclize on treatment with base giving the mesoionic 1,3,4-triazolo[3,2-a]pyridylium-2-aminides [(47)–(50)]. The reactions of (14) and (16) with aniline and p-chloroaniline respectively, lead directly to the corresponding mesoionic compounds (51) and (52).