Effect of tertiary amine on the carbodiimide‐mediated peptide synthesis

The effect of tertiary amine (DIEA) on reaction rate and product purity of a carbodiimide/HOBt‐mediated peptide synthesis was studied. It was found that very rapid activation can be achieved using carbodiimide/HOBt in non‐polar solvents, such as DCM. Although the HOBt is poorly soluble in DCM, the activation proceeds within 2 min, probably forming the HOBt‐ester. By such a preactivation followed by a coupling in the presence of DIEA the rate of coupling is comparable with other rapid methods using BOP or TBTU, and no racemization was found in a model coupling (< 0.1%). For comparison, syntheses of neurotensin by means of different coupling reagents (BOP, TBTU, OPfp‐esters) and the DIEA‐catalyzed coupling after carbodiimide/HOBt‐activation under comparable conditions have shown that these procedures are of the same value in view of coupling efficiency and product purity.