New isocysteine building blocks and chemoselective peptide ligation.

Boc-, Fmoc- and Cbz-protected isocysteine building blocks were prepared by a concise three-step procedure starting from thiomalic acid. The use of Boc/Trt-protected isocysteine provided convenient access to isocysteinyl peptides that allow the chemoselective ligation of unprotected peptide fragments in water. The pH-dependency of the isocysteine-mediated ligation was compared with that of cysteine-mediated native chemical ligation.