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, i.e., N-(chloroacetyl)-dehydroamino acids.