Biomimetic synthesis of cyclic peptides using novel thioester surrogates

ABSTRACT Acyl shifts involving N‐S and S‐S rearrangements are reactions central to the breaking of a peptide bond and forming of thioester intermediates in an intein‐catalyzed protein splicing that ultimately leads to the formation of a new peptide bond by an uncatalyzed S‐N acyl shift reaction. To mimic these three acyl shift reactions in forming thioesters and the subsequent peptide ligation, here we describe the development of two 9‐fluorenylmethoxycarbonyl (Fmoc)‐compatible thioester surrogates that can undergo uncatalyzed N‐S, S‐S, and S‐N acyl shifts for preparing thioesters and cyclic peptides. These surrogates were incorporated as a C‐terminal amido moiety of a target peptide using Fmoc chemistry by solid‐phase synthesis, and then transformed into a thioester or thiolactones via two acyl shift reactions with or without the presence of an external thiol under acidic conditions. The proposed intein‐mimetic thioester surrogates were prepared using readily available starting materials including N‐methyl cysteine or 2‐thioethylbutylamide. A key functional moiety shared in their design is the thioethylamido (TEA) moiety, which is essential to effect a proximity‐driven N‐S acyl shift under a favorable five‐member ring transition in the breaking of a peptide bond. Thus, the tandem series of acyl shifts effected by a TEA moiety in a thioester surrogate together with a thioethylamino moiety of an N‐terminal Cys residue in a linear peptide precursor are chemical mimics of an intein, as they mediate both excision and ligation reactions in forming cyclic peptides including cyclic conotoxin and sunflower trypsin inhibitor described herein. © 2013 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 100: 492–501, 2013.