A Thiophilic Metal-Mediated Cysteine S‑Detritylation Strategy: Enhancing Orthogonality in Solid-Phase Peptide...

The selective manipulation of cysteine residues in solid-phase peptide synthesis (SPPS), beyond disulfide bridge formation, remains challenging, and an efficient protocol for the selective removal of the widely used trityl (Trt) protecting group under solid-phase conditions has yet to be established. To overcome this limitation, we developed a thiophilic transition metal-mediated strategy by adapting a three-component reagent system originally conceived for solution-phase synthesis. Systematic screening of copper, zinc and silver salts identified Ag-(CF₃CO₂) and AgNO₃ as the most effective promoters of Trt removal. The optimized protocol enabled reproducible and efficient deprotection, provided preliminary mechanistic insight through control experiments, and proved compatible with common Fmoc-SPPS protecting groups and alternative solvents. Furthermore, in situ thiol release enabled direct on-resin S-alkylation with a selected range of aliphatic substituents, including those bearing aromatic moieties, highlighting the utility of the method for the late-stage functionalization of cysteine-containing resin-bound peptides.