2-Phenyl-4-heteroarylaminomethylene-5(4H)-oxazolones 3, which were prepared from the corresponding N,N-dimethyl-N'-heteroarylformamidines 1 and hippuric acid 2 in acetic anhydride, react with amino acids giving dehydropeptide derivatives 4, 5, and 6 as products. Dehydration of N-protected peptides 7–10, containing glycine at the C-terminal, followed by the reaction with formamidines 1 gave 2-subst