Chiral molecular recognition in a tripeptide benzylviologen cyclophane host

A cationic chiral cyclophane was synthesized and studied as a host for chiral and racemic π-donor molecules. The cyclophane host has a rigid binding cavity flanked by (S)-(valine-leucine-alanine) and N,N'-dibenzyl- 4,4'-bipyridinium subunits, which allow for hydrogen-bonding and π-stacking interactions with included aromatic guest molecules. ¹H NMR binding titrations were performed with several different pharmaceutically interesting guest molecules including β-blockers, NSAIDs, and amino acids and amino acid derivatives. The host-guest complexation constants were generally small for neutral and cationic guests (0-39 M^-1 at 20 °C in water/acetone mixtures. However, a (R)/(S) enantioselectivity ratio of 13 ± 5 was found for DOPA, a strongly π-donating cationic guest. Two-dimensional NOESY ¹H NMR spectra confirm that (R)-DOPA binds inside the cavity of the host and that there is no measurable interaction of the cavity with (S)-DOPA under the same conditions.