Development of Helical Aromatic Amide Foldamers with a Diphenylacetylene Backbone

We designed and synthesized aromatic polyamides with a diphenylacetylene backbone, alpha-DPA and beta-DPA, bearing (S)-alpha- and (S)-beta-methyl-substituted triethyleneglycol (TEG) side chains, respectively, and examined their conformations in solution. Both polymers exhibit strong, solvent polarity-dependent circular dichroism spectra, which indicated that they take helical conformations in low-polarity solvents. The spectra were mirror images, depending on the chiral position of the side chains. Thus, the polyamide alpha-DPA bearing (S)-alpha-methyl-substituted TEG groups takes a left-handed helical conformation, while the polyamide beta-DPA with (S)-beta-methyl-substituted TEG groups takes a right-handed helical conformation. The difference in the screw sense of alpha-DPA and beta-DPA would be caused by the steric interaction between the main chain and the side chain, as observed in poly(p-benzamide) possessing (S)-beta-methyl-substituted TEG side chains (beta-PA) because the large cavity of the solvophobically induced helical folding. Detailed conformational analyses of the oligoamides 6-12 with beta-methyl-substituted TEG groups were conducted. Theoretical calculations indicated that the oligoamides with beta-methyl-substituted TEG groups exist in a helical conformation with a cavity of 7 angstrom in diameter. The H-1 NMR spectra of the oligomers revealed interactions with small anions such as chloride and acetate anions and with pyridinium cations.