Conformational Properties of Aromatic Oligoamides Bearing Pyrrole Rings

N-Alkylbenzanilides
generally exist in cis conformation
both in the crystalline state and in various solvents, and this cis
conformational preference can be utilized to construct dynamic helical
oligoamides. Here, we synthesized the pyrrole-containing amides 2–5 and their oligomers 6–8 and examined their conformations in the crystalline
state and in solution. All the N-methylated amides
showed cis conformational preference in solution, but the ratio of
the cis isomer was decreased when the amide bond was attached at the
4-position of the pyrrole ring, probably because the destabilization
of the trans conformer due to electronic repulsion between the pyrrole
π electrons and the amide carbonyl lone-pair electrons is reduced
due to the small torsion angle between the 5-membered N-pyrrole and the amide bond. In the crystalline state, N-methylated amides showed cis structure, except for compound 5, and cis conformational preference was observed for the
pyrrole amides. The CD spectra of oligoamides 15–18 bearing chiral N-substituents were consistent
with the presence of dynamic and well-defined chiral foldamers, which
were structurally distinct from N-alkylated poly­(p-benzamide)­s 1.