The properties of two aspartic acid β-esters (Asp(OR), R = cycloheptyl (Chp) and cyclooctyl (Coc)) were examined. These two protecting groups were found to be stable to trifluoroacetic acid (0°C, 2h), but were cleaved by HF or 1 M Trifluoromethanesulfonic acid-thioanisole in trifluoro-acetic acid (0°C, 60 min). Using a model peptide, Z(OMe)-Ala-Asp(OR)-Gly-OBzl, the behavior of the Asp(OR) residue with base or acid was examined. These esters were less susceptible to succinimide formation than the Bzl group in Et 3 N treatment. In acid deprotection, succinimide formation from these peptides was less than 7% in both cases. © 1986, The Pharmaceutical Society of Japan. All rights reserved.