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.