Mechanism of racemisation of histidine derivatives in peptide synthesis

Evidence is presented to show that the racemisation of N(alpha)-benzyloxycarbonyl-N(pi)-phenacyl-L-histidine which occurs on activation with dicyclohexylcarbodiimide in dimethylformamide takes place by action of the pi-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.