2,4-Dimethyl-5(4H)-oxazolone as reagent for activation and coupling of N-substituted aspartic acid

Reaction of the title oxazolone with N-benzyloxycarbonyl-L-aspartic acid in dichloromethane followed by addition of phenylalanine methyl ester.HCl and N-methylmorpholine gave a 90% yield of a mixture of alpha- and beta-isomers of Z-aspartylphenylalanine methyl esters in a 7:3 ratio. Reaction of the oxazolone with N-acetyl-L-aspartic acid anhydride gave a 75% yield of crystalline N-acetyl-L-aspartic acid anhydride. tert-Butoxycarbonylaspartic and Z-glutamic acids also underwent activation to give the anhydrides.