Deprotection Strategies for Thioimidates during Fmoc Solid-Phase Peptide Synthesis: A Safe Route to Thioamides

Thioamides are important biophysical probes of peptide folding but are prone to alpha-C epimerization during Fmoc solid-phase peptide synthesis. The stereochemical integrity of thioamide-containing peptides can be dramatically improved by protecting the thioamide as a thioimidate during synthesis. A drawback of this approach, however, is that once synthesis of the peptide is complete, regeneration of the thioamide requires the toxic, corrosive, and flammable gas H2S. This work examines several approaches to supplant H2S as a deprotection reagent in favor of a safer and more convenient alternative. Ultimately, a new application of the 4-azidobenzyl protecting group to thioamides was found to provide the most suitable means of both protection of alpha-C stereochemistry and conversion back to thioamide.