Anhydro-2-hydroxyoxazolo[3,2-a]pyridinium hydroxide, a mesoionic oxazolone

Abstract

Treatment of 1,2-dihydro-2-oxopyridine-1-acetic acid with acetic anhydride and perchloric acid yields 2,3-dihydro-2-oxo-oxazolo[3,2-a]pyridinium perchlorate, which is deprotonated by triethylamine in dichloromethane to give the highly labile anhydro-2-hydroxyoxazolo [3,2-a]pyridinium hydroxide in solution. Stable acyl and azo-derivatives of this mesoionic compound are obtained by electrophilic substitution reactions; amines open the oxazolone ring with the formation of amides of 1,2-dihydro-2-oxopyridineacetic acid. The oxazolopyridinium per chlorate condenses with aromatic aldehydes to give coloured arylidene derivatives—the salicylidene compound readily rearranges to a coumarin. Coumarins are also obtained by reaction of the mesoionic base with o-hydroxyarenecarbaldehydes. The dimeric decomposition product of the mesoionic oxazolone is shown to be its 3-[(1,2-dihydro-2-oxo-1-pyridyl)acetyl] derivative.