Late-Stage Sidechain-to-Backbone Macrocyclization of N -Amino Peptides.

Cysteine-containing N -amino peptides undergo chemoselective reactions with haloaldehydes to afford ethylene-bridged cyclic peptides. This bis-alkylation strategy provides macrocycles harboring a novel covalent H-bond surrogate. Mimicry of a native sidechain-to-backbone ( sb ) H-bond is demonstrated in the context of a model loop-helix peptide. The described method is amenable to the synthesis of diverse ring sizes from crude unprotected linear substrates under aqueous conditions.