Secondary-Amine Relay Enables Lysine-Directed Multicomponent Functionalization of Peptides.

Multicomponent reactions provide powerful modularity for peptide functionalization, but their application to lysine residues remains limited by the low efficiency of direct primary-amine-based activation pathways under peptide-compatible conditions. Here we report a secondary-amine relay strategy for lysine-directed multicomponent functionalization of peptides. In this approach, salicylaldehyde derivatives, arylboronic acids, and a cyclic secondary amine are combined to generate a transient electrophilic intermediate, which is subsequently intercepted by the ε-amine of lysine-containing peptides. By separating electrophile generation from direct lysine activation, this relay protocol improves the efficiency of lysine modification while retaining the modularity of multicomponent chemistry. Mechanistic experiments, isolated intermediate studies, kinetic comparisons, and density functional theory calculations support a pathway involving secondary-amine-mediated intermediate formation followed by lysine capture. The method enables modification of diverse bioactive peptides, incorporation of drug-derived and fluorescent aldehyde or boronic acid components, and lysine-lysine stapling using bifunctional aldehydes. The reaction is further compatible with automated and parallel solid-phase peptide modification workflows, providing rapid access to libraries of lysine-functionalized peptide conjugates. This study provides a practical secondary-amine relay platform for modular peptide functionalization and expands the utility of multicomponent chemistry in automated peptide synthesis.