The diisopropylcarbodiimide/1-hydroxy-7-azabenzotriazole system: Segment coupling and stepwise peptide assembly

For a group of model peptide segments, coupling reactions carried out via solution or solid phase techniques have demonstrated the advantages of the system DIC/HOAt over DIC/HOBt and in addition for systems involving other selected carbodiimides and substituted HOBt derivatives bearing electron-withdrawing substituents. Very little, if any, loss of configuration occurred in DCM regardless of the additive used, although the relative order of efficiency was similar in solvents such as DMF in which more extensive epimerization resulted. In application of DIC/HOAt to stepwise peptide assembly by solid phase techniques, it was found that the hindered pyridine base collidine enhanced the step involving preactivation of the carboxylic acid residue in contrast to the normal situation in which bases such as DIEA, NMM, or non-hindered pyridine bases inhibit this step. These results led to development of a stepwise procedure for peptide assembly in which collidine is added to enhance activation and subsequently DIEA is added to enhance coupling, (C) 1999 Elsevier Science Ltd. All rights reserved.