Reinvestigation of a Critical Reductive Amination Step Leads to an Optimized Protocol for the Synthesis of N-2-Hydroxybenzylcysteine Peptide Crypto-Thioesters

An in-depth study of the Fmoc-based solid phase peptide synthesis of N-Hnb-Cys crypto-thioester peptides, advantageous building blocks for the native chemical ligation-based synthesis of proteins, led to the identification of epimerized and imidazolidinone side products formed during a key reductive amination step. The understanding of the underlying reaction mechanisms was crucial for the developement of an automatable optimized synthetic protocol.