Native chemical ligation at phenylalanine

Synthesis of erythro-N-Boc-beta-mercapto-L-phenylalanine enables native chemical ligation at phenylalanine. In the form of the S-ethyldisulfanyl derivative, the N-Boc amino acid is used to cap a tetrapeptide generated by Fmoc-SPPS. The native chemical ligation with C-terminal thioesters then occurs successfully and is followed by selective desulfurization of the benzylic C-S bond. The method is applied to the synthesis of LYRAMFRANK, thereby establishing compatibility with the reactive side chains and the ability to ligate other than glycine.