A peptide comprising a free terminal alpha amine is treated with an aryl sulfonamide activating agent, resulting in an activated amide. The resulting activated amide is deprotonated with a base and modified by the addition of a substituent group. The aryl sulfonamide activating group is cleaved using a nucleophilic substitution reaction. The method is particularly useful for the modification of peptides at specific N-alpha positions, and is compatible with conventional solid phase peptide synthesis, including those that utilize Fmoc protecting groups.