The Mixed Carbonic Anhydride Method of Peptide Synthesis

This chapter describes the mixed carbonic anhydride method of peptide synthesis. Peptide bond formation by the mixed anhydride procedure involves two separate stages. The stages include activation of the carboxyl component, generation of a mixed anhydride of Nα -protected amino acid or peptide, and coupling with the amine component. The activation step is carried out by the addition of carbonic acid monoalkyl ester chloride to a cold solution of the carboxyl component anion in anhydrous organic solvent in the presence of a tertiary base. The activation time is the period between the addition of the alkyl chlorocarbonate and the addition of the amine component, once the mixed anhydride formation is complete. Peptide bond formation usually proceeds rapidly with liberation of carbon dioxide gas and a volatile alcohol as byproduct. This process is not very sensitive to the nature of the solvent, but it is prone to a few side reactions, of which only the formation of a second acylation product by attack of the amine component at the carbonic acid carbonyl is of practical concern.