Cyclisation of α-acylamino-acids in the presence of perchloric acid to give 5-oxo-Δ2-oxazolinium perchlorates

Abstract

The action of acetic anhydride and perchloric acid on simple α-acylamino-acids yields 5-oxo-Δ²-oxazolinium perchlorates, which can be deprotonated to the corresponding saturated oxazolin-5-ones. N-Benzoyl-L-phenyl-alanine gave the hydroperchlorate of L-4-benzyl-2-phenyloxazolinone. The salt obtained from hippuric acid undergoes ring opening on treatment with water, methanol, and various amines; it condenses with benzaldehyde to give the hydroperchlorate of the labile geometrical isomer of 4-benzylidene-2-phenyloxazolinone. Ten N-substituted oxo-oxazolinium perchlorates were prepared and their reactions with benzaldehyde and amines are reported. The bicyclic perchlorate, obtained from p-nitrobenzoyl-L-proline, is easily racemised, probably via a mesoionic oxazolium oxide; the corresponding diphenylamino-compound is optically stable.