Peptides—XI

Union of α-methylalanyl residues in a peptide chain is severely sterically hindered. This hindrance can be overcome by employing either the mixed anhydride (I) of pivalic acid and benzyloxycarbonyl-α-methylalanine or, more generally, oxazolones (II), including 4,4-dimethyl-2-trifluoromethyloxazolone (II; R = CF3). 4,4-Dimethyloxazolone (II; R = H) has been obtained in solution and characterized by its infra-red spectrum; it is attacked by α-methylalanine methyl ester at the methine group, in contrast to the normal reaction with cyclohexylamine at the carbonyl group.