A novel thiazolium type peptide coupling reagent for hindered amino acids

Abstract

A highly efficient coupling reagent, 2-bromo-3-ethyl-4-methyl thiazolium tetrafluoroborate (BEMT), was designed, synthesized and successfully applied to the synthesis of oligopeptides containing N-alkyl or α- C-dialkyl amino acids. Its efficiency was evaluated by HPLC and 1H NMR methods, and demonstrated by synthesis of a number of N-methyl-rich peptide segments with good yields and negligible racemization. The mechanism of coupling was studied by HPLC, 1H NMR and IR monitoring; it is proposed that labile (acyloxy)thiazolium salts and N-protected amino acid bromides were the major active intermediates with concomitant formation of N-ethyl-4-methyl thiazolidones and a small amount of oxazolones and N-protected amino acid anhydrides.