MECHANISM OF RACEMISATION OF HISTIDINE DERIVATIVES IN PEPTIDE SYNTHESIS

Abstract
Evidence is presented to show that the racemisation of N(α)‐benzyloxycarbonyl‐N(α)‐phenacyl‐L‐histidine which occurs on activation with dicyclohexylcarbodiimide in dimethylformamide takes place by action of the nitrogen as an intramolecular base catalyst in the O‐acylisourea adduct which is in reversible equilibrium with the reactants, rather than by formation of an optically labile heterocyclic intermediate.