Epimerization-free block synthesis of peptides from thioacids and amines with the Sanger and Mukaiyama reagents.

Highly activated thioesters formed by the rapid reaction of C-terminal thioacids derived from protected amino acids and peptides with the Sanger reagent and other electron-deficient aryl halides in the presence of a free amine immediately form a peptide bond with the amine. This essentially epimerization-free method was used for the 4+4 block synthesis of a hindered octapeptide (see scheme; Boc, Pbf, and Trt are protecting groups).