Oxazoles and thiazoles have been widely used in synthesis. These compounds undergo nucleophilic addition reactions with acid chlorides although their reactivities vary a lot. A series of acylation reactions of 2,4,5 -trimethyloxazole, 2,4,5-trimethylthiazole and variously substituted benzoxazoles being explored which result in N-substituted beta-keto cyclic ketene acetals. All proceed thorough cyclic ketene N, X intermediate (X=O, S) having seven pi electrons.