Twice tied tight: Enforcing conformational order in bicyclic peptoid oligomers

Head-to-tail cyclization of N-substituted glycine peptoid oligomers is known to enforce their conformational ordering. We now describe the installation of an additional covalent constraint by bridging the side chains of a peptoid octamer macrocycle via Cu(I)-catalyzed azide–alkyne cycloaddition. Under dilute conditions, intramolecular side chain tethering is favored over formation of intermolecular crosslinked macrocycles. Both the bicyclic octamer and the hexadecamer intermolecular reaction product are characterized by crystallographic studies, providing X-ray diffraction structures of the largest peptoid compounds to date.