Synthesis and use of a pseudo-cysteine for native chemical ligation

The process of native chemical ligation (NCL) is well described in the literature. An N‐terminal cysteine‐containing peptide reacts with a C‐terminal thioester‐containing peptide to yield a native amide bond after transesterification and acyl transfer. An N‐terminal cysteine is required as both the N‐terminal amino function and the sidechain thiol participate in the ligation reaction. In certain circumstances it is desirable, or even imperative, that the N‐terminal region of a peptidic reaction partner remain unmodified, for instance for the retention of biological activity after ligation. This work discusses the synthesis of a pseudo‐N‐terminal cysteine building block for incorporation into peptides using standard methods of solid phase synthesis. Upon deprotection, this building block affords a de facto N‐terminal cysteine positioned on an amino acid sidechain, which is capable of undergoing native chemical ligation with a thioester. The syntheses of several peptides and structures containing this motif are detailed, their reactions discussed, and further applications of this technology proposed. Copyright © 2003 European Peptide Society and John Wiley & Sons, Ltd.