Fast and Selective Labeling of Serine and Threonine in HFIP for Facile Editing of Peptide Alcohol Side Chains

Selective labeling of alcohol side chains in peptides and proteins remains a major challenge for biomolecule modification. Herein, we report a rapid, efficient, and highly selective method for labeling both serine and threonine residues using fluorosulfuryl isocyanate (FSI) reagent in hexafluoroisopropanol (HFIP) solvent under mild conditions. The labeling reaction proceeds to completion within 1 min at room temperature, with excellent tolerance for all proteinogenic amino acid side chains except cysteine. The resulting fluorosulfuryl carbamate products can be further diversified through sulfur(VI) fluoride exchange (SuFEx) with amines to generate sulfamide-linked conjugates, or through intramolecular macrocyclization with lysine side chains to yield stapled peptides via Ser/Thr-Lys pairs. Notably, the fluorosulfuryl carbamate tag can be selectively eliminated under mildly basic aqueous conditions to generate dehydroamino acid residues, which serve as versatile electrophilic handles for one-pot conjugation with thiols, amines, or phosphines. Together, this platform offers a convenient strategy for site-selective editing of peptide alcohol side chains into structurally and functionally diverse deoxygenative analogs.