Treatment of N-dichloroacetyl amino acids with acetic anhydride at a temperature between 85° C and 130° C yielded 4-alkylidene-and 4-aralkylidene-2-chloromethyl-5-oxazolones, which were readily hydrolyzed to give 2-(2-chloroacetamido)-2-alkenoic acids, ie, N- (chloroacetyl)-dehydroamino acids.