Synthesis and helical properties of aromatic multilayered oligoureas

Several aromatic multilayered oligoureas with different chain lengths and different numbers of chiral N-substituents were synthesized, and their helical conformation and induced handedness were examined by means of CD spectroscopy. Introduction of one chiral N-substituent is enough to induce handedness, and all the oligoureas examined exist predominantly as helical structures with all-S axial chirality. The hexaureas 6 and 7 had similar CD intensity to the tetraureas 4 and 5, and had larger CD intensity than diurea 8. The results indicate that the effect of a chiral N-substituent at the central benzene ring in inducing well-ordered handedness at the terminal positions of hexaureas 6 and 7 is relatively weak, even though these compounds mainly take the form of aromatic multilayered foldamers.