General conditions for efficient on-resin N-formylation of peptides were identified by screening of a number of reagents comprising aliphatic formates (ethyl formate, 2,2,2-trifluoroethyl formate, and cyanomethyl formate), aromatic esters (phenyl formate and p-nitrophenyl formate), and N-formylimidazole and in situ activation of formic acid with the coupling reagent 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. Initially, reaction time and influence of solvent were examined for the formylation of a short model peptide. The most efficient reagents were examined further by using different linkers and solid supports in the synthesis of an array of longer formyl peptide ligands. For p-nitrophenyl formate and N-formylimidazole, lmost complete conversion was reached within 2 h, albeit longer peptides attached to Tentagel resins via different linkers required an extended reaction time. Overall, the commercially available activated ester p-nitrophenyl formate proved to be most convenient and
versatile as high formylation degrees were obtained after 1–3 h at room temperature, while either conventional or microwave-assisted heating allowed reduction of the formylation time to 20 min.