O-to-O acyl transfer for epimerization-free peptide C-terminal salicylaldehyde ester synthesis

Peptide salicylaldehyde esters are the requisite coupling partner in Ser/Thr ligation reactions towards chemical protein synthesis. In general, it would be cost-effective and efficient to use side-chain-protected peptide acids, after Fmoc solid-phase peptide synthesis, for direct C-terminal derivatization; however, this has yet to be achieved, due to an intrinsic epimerization pathway. Here we report the development of 2-(dichloromethyl)phenol as a reagent that can directly form peptide salicylaldehyde esters in an epimerization-free manner. Mechanistic studies reveal that the 2-(dichloromethyl)phenol reagent serves as a source of highly reactive quinone methide species that can be trapped by the peptide C-terminal carboxylate to give α-chloroesters, followed by an O_benzylic-to-O_phenolic acyl transfer and chloride extrusion process. The peptide salicylaldehyde ester reaction products have been applied in the convergent total chemical synthesis of linker histone H1.2 using sequential Ser/Thr ligation reactions.