A Type of Auxiliary for Native Chemical Peptide Ligation beyond Cysteine and Glycine Junctions

Native chemical ligation enables the chemical synthesis of proteins. Previously, thiol-containing auxiliary groups have been used to extend the reaction scope beyond N-terminal cysteine residues. However, the N-benzyl-type auxiliaries used so far result in rather low reaction rates. Herein, a new N^α-auxiliary is presented. Consideration of a radical fragmentation for cleavage led to the design of a new auxiliary group which is selectively removed under mildly basic conditions (pH 8.5) in the presence of TCEP and morpholine. Most importantly and in contrast to previously described auxiliaries, the 2-mercapto-2-phenethyl auxiliary is not limited to Gly-containing sites and ligations succeed at sterically demanding junctions. The auxiliary is introduced in high yield by on-resin reductive amination with commercially available amino acid building blocks. The synthetic utility of the method is demonstrated by the synthesis of two antimicrobial proteins, DCD-1L and opistoporin-2.