Action of oxazol-5(4H)-ones and of 4,5-dihydro-5-oxo-oxazolium perchlorates on enamines

Abstract

The anomalous reaction of enamines with the acylating agent 2-phenyloxazol-5(4H)-one has been shown to give the corresponding amides of the alkylidenehippuric acids. A mechanism for the reaction involving attack at the α-carbon atom of the enamine and elimination of the secondary amine is proposed based on evidence from the use of an immonium salt, of 4,5-dihydro-2-methyl-5-oxo-oxazolium perchlorate, and also from spectroscopic data. Similar results were obtained from 2-phenylthiazol-5(4H)-one, 4,5-dihydro-2-phenyl-5-oxo-oxazolium perchlorate, and from a dihydro-oxo-oxazolium salt having a methyl substituent on the nitrogen atom. The unsaturated amides obtained gave unsaturated oxazolones on pyrolysis and were regenerated from the latter with the appropriate amine. With a condensed dihydro-oxo-oxazolium salt, 2,3-dihydor-2-oxo-oxazolo[3,2-a]pyridinium perchlorate (46), on the other hand, the enamines were N-acylated with the expulsion of the alicyclic portion giving amides of 1,2-dihydro-2-oxopyridine-1-acetic acid.