An Approach to Helical Tubular Self-Aggregation Using <i>C</i>₂-Symmetric Self-Complementary Hydrogen-Bonding Cavity Molecules

In an approach to helical self-aggregation, C2-symmetric cavity compounds based on the fusion of the bicyclo[3.3.1]nonane and indole framework and incorporating two 2-pyridone hydrogen-bonding motifs, compounds (−)-4 (pyrrole N-butyl) and (−)-5 (pyrrole N-decyl), have been synthesized. The 2-pyridone AD−DA hydrogen-bonding motif failed to operate in the solid state as demonstrated by X-ray diffraction analysis of (−)-4. Instead, the hydrogen-bonded (D−A) chains ···OC−N−H···OC−N−H···OC−N−H···, interconnecting columnar stacks, comprise helices of the right-handed (P) chirality motif. In solution, the aggregation of (−)-5 was studied by NMR, electronic, and CD spectroscopies, and VPO measurements. These investigations strongly suggest that (−)-5 associates to oligomers in CHCl3 and CH2Cl2 using the 2-pyridone motif, fitting the equal K model, and that π-stacking can be ruled out as a mode of aggregation. We conclude that the so formed aggregates of (−)-5 have a helical structure, based on the fact that only helical tubular structures can result when enantiomerically pure 5 uses its 2-pyridone AD−DA hydrogen-bonding motifs for aggregation.