Chapter 5 - The Carbodiimide Method

Abstract

This chapter discusses the mechanism of the carbodiimide method. The carbodiimide method was introduced to peptide synthesis with the use of dicyclohexylcarbodiimide (DCC) to affect dehydration and peptide bond formation. The procedure consisting of adding DCC to a solution of the N-protected amino acid and amino acid or peptide ester won wide acceptance because of its simplicity, speed, and compatibility with water. It was soon used to synthesize oxytocin because of the ease with which they are obtained through DCC activation. Water-soluble carbodiimides were used to further expand the method by facilitating the work-up of some peptides. Dicyclohexylcarbodiimide has become the most widely used activating agent in peptide synthesis. Virtually every solid phase synthesis and many solution syntheses employ DCC, either directly or through the use of active esters. Addition of amines to a DCC coupling mixture alters the reaction mechanism. DCC reacts 30-fold slower with acids in the presence of triethylamine, and consequently more N-acylurea is formed. As both triethylamine and benzylamine produce the same amount of N-acylurea from the reaction with acetic acid and DCC, benzylamine reacts directly with the O-acylisourea rather than with an intermediate anhydride.