Side reactions in peptide synthesis. IX. Suppression of the formation of aminosuccinyl peptides with additives.

The base-catalyzed ring closure of beta-benzylaspartyl peptides was efficiently suppressed by the addition of phenols (with electron-withdrawing substituents) to the reaction mixtures. From a series of compounds tested, 2,4-dinitrophenol and pentachlorophenol were the most effective. No direct relationship was found between the acidity of the additives and their ability to suppress the formation of aminosuccinyl peptides. The applicability of 2,4-dinitrophenol and pentachlorophenol in practical syntheses was also examined.