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Fmoc-based synthesis of peptide thioacids for azide ligations via 2-cyanoethyl thioesters.

Rapid and efficient preparation of peptide thioacids from 2-cyanoethyl peptide thioesters has been accomplished. S-2-Cyanoethyl peptide thioesters were obtained cleanly without the need for purification from resin-bound tert-butyl peptide thioesters using 3-mercaptopropionitrile as a nucleophile. Elimination of the 2-cyanoethyl group proceeded rapidly (t(1/2) < 8 min) under mild conditions and furnished peptide thioacids up to the size of a 16-mer. Peptide thioacids could be isolated or formed in situ and reacted smoothly with electron-deficient azides yielding an amide as the ligation product.