Native Chemical Ligation at Phenylalanine

Synthesis of erythro-N-Boc-β-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.