Unstable mesoionic oxazolium-5-oxides

Abstract

Deprotonation of seven 2,3-disubstituted 5-oxo-Δ²-oxazolinium perchlorates yielded solutions of the corresponding mesoionic oxazolones. These compounds are too unstable to be isolated; they were trapped as stable 4-acyl derivatives by treatment with aroyl chlorides or trifluoroacetic anhydride and as pyrroles by cycloaddition to dimethyl acetylenedicarboxylate. It is shown by i.r. spectroscopy that the oxazolium-oxides decompose by a process of self-acylation to form dimeric mesoionic 4-acylaminoacyl derivatives, but in many cases the isolated products were symmetrical 1,3-di(acylamino)propan-2-ones. The relative rates of dimerisation of eight oxazolium-oxides indicate that the mesoionic system is stabilised by aryl substitution at C-2, the presence of a methyl group on the nitrogen atom, and fusion to a pyridine ring.