Preparation of an aspartic acid-containing protected peptide. Alternatives for overcoming a side reaction during HF treatment.

Treatment of the peptide-resin Ac-Asn-Ser(Bzl)-Gly-Asp(OFm)-pMeBHA with HF/anisole cleaves the peptide-resin bond and removes the benzyl group, as expected, but also yields an impurity (ca. 25%) in which the benzyl group is linked to the fluorenylmethyl protecting group. Separation of the desired peptide from the impurity can be accomplished by reversed-phase medium-pressure liquid chromatography. The side reaction does not take place when the gamma-carboxyl of aspartic acid is protected with the 2-(4-acetyl-2-nitrophenyl)ethyl group.