Macrocyclic Aromatic Oligoamides with Diphenyladamantane Units: Conformational Change from Folded to Open by N-Alkyl Removal

Macrocyclic aromatic amide oligomers of various sizes containing tertiary amides and diphenyladamantane moieties were synthesized by condensation reactions using PhP₃Cl₂ or LiHMDS. The corresponding oligomers with secondary amides were obtained by removal of N-decyloxybenzyl groups from the amide nitrogen. X-ray crystal structure analysis showed that the oligomers with tertiary N-ethyl amides adopted folded conformations, consistent with the <i>cis</i>-preference of tertiary aromatic amides. In contrast, comparison of ¹H NMR spectra and conformational analysis by force field calculations indicated that the oligomers with secondary amides adopted open conformations. These results also demonstrated that the synthetic strategy is effective for the preparation of macrocyclic molecules with large inner cavities.