Development of Helical Aromatic Amide Foldamers with a Diphenylacetylene Backbone

We designed and synthesized aromatic
polyamides with a diphenylacetylene
backbone, α-DPA and β-DPA, bearing
(S)-α- and (S)-β-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 α-DPA bearing (S)-α-methyl-substituted
TEG groups takes a left-handed helical conformation, while the polyamide β-DPA with (S)-β-methyl-substituted
TEG groups takes a right-handed helical conformation. The difference
in the screw sense of α-DPA and β-DPA would be caused by the steric interaction between the main chain
and the side chain, as observed in poly­(p-benzamide)
possessing (S)-β-methyl-substituted TEG side
chains (β-PA) because the large cavity of the helical
structure of DPA would disturb the solvophobically induced
helical folding. Detailed conformational analyses of the oligoamides 6–12 with β-methyl-substituted TEG groups were
conducted. Theoretical calculations indicated that the oligoamides
with β-methyl-substituted TEG groups exist in a helical conformation
with a cavity of 7 Å in diameter. The 1H NMR spectra
of the oligomers revealed interactions with small anions such as chloride
and acetate anions and with pyridinium cations.