Native Chemical Ligation Strategy to Overcome Side Reactions during Fmoc-Based Synthesis of C-Terminal Cysteine-Containing Peptides

The Fmoc-based solid phase synthesis of C-terminal cysteine-containing peptides is problematic, due to side reactions provoked by the pronounced acidity of the C alpha proton of cysteine esters. We herein describe a general strategy consisting of the postsynthetic introduction of the C-terminal Cys through a key chemoselective native chemical ligation reaction with N-Hnb-Cys peptide crypto-thioesters. This method was successfully applied to the demanding peptide sequences of two natural products of biological interest, giving remarkably high overall yields compared to that of a state of the art strategy.