Efficient method of circumventing insolubility problems with fully protected peptide carboxylates via <i>in situ</i> direct thioesterification reactions

Abstract

A straightforward and convenient protocol is presented for the direct thioesterification of fully protected peptide C-terminal carboxylates synthesized by Fmoc strategy. This methodology specifically serves to overcome the frequent insolubility problem of these fully protected carboxolate isolates during the thioesterification process by carrying out the reaction as an in situ procedure on the freshly cleaved 1% TFA/DCM solution of carboxylate. The direct thioesterification of a number of insolubility prone peptide systems is explored and compared with some control systems for ease of conversion to the corresponding thioesters. It is shown that although the fully protected carboxylates are indeed insoluble to varying degrees in the thioesterification reactions carried out using the classical approach, full dissolution is maintained and complete conversion is evident using the in situ methodology. This protocol serves to remove a frequent stumbling block in the preparation of peptide thioesters via the direct approach, allowing for facile entry into previously difficult systems traditionally unapproachable through this method.