The base catalyzed ring closure in n-tert-butoxycarbonyl-Asp-X .beta.-naphthylamides was examined in a series of 2-peptide derivatives in which position X was occupied by the neutral and acidic amino acid residues that occur in proteins. Bulkiness and functional groups in the side chain of X have a major effect on the rate of cyclization, e.g., acidic groups slow down the formation of aminosuccinyl derivatives. Rate-enhancing effect can be observed in serine and threonine, while the side reaction is unexpectedly slow when X is methionine.