The preferred catalysts for the process of the invention are the onium salts and more particularly quaternary ammonium and/or phosphonium salts. [0090] The alkyl ester of the dipeptide is most preferably a methyl ester and may also be an ethyl ester or alkyl of up to about four carbon atoms such as propyl, isopropyl, butyl or isobutyl. [0091] In still other embodiments, the carboxyl groups can be