CYCLODEXTRIN-INDUCED CONFORMATIONAL ENANTIOMERISM OF DINAPHTHYLMETHANES

Pamoic acid (4,4'-methylenebis[3-hydroxy-2-naphthalenecarboxylic acid]) included in the gamma-cyclodextrin (gamma-CDx) cavity shows an extremely strong, bisignate Cotton effect on its circular dichroism (CD) spectrum, suggesting that (R)-helix pamoic acid is selectively bound to gamma-CDx. 2,2'-Dihydroxy-1,1'-dinaphthylmethane also exhibits the (+) to (-) bisignate CD signals in aqueous gamma-CDx solution but the CD intensities are much weaker than those of pamoic acid-gamma-CDx complex. The CD intensities are correlated with the stabilities of the inclusion complexes. A larger binding constant for the pamoic acid-gamma-CDx complex (K = 4100 dm3 mol-1) may be ascribed to a hydrogen-bonding interaction between a carboxylate anion of the guest and a secondary hydroxyl group of the host. The H-1 NMR spectroscopic measurements suggest a plausible structure of the pamoic acid-gamma-CDx complex where a naphthalene moiety of pamoic acid is situated inside of the gamma-CDx cavity and another naphthalene ring is located at the rim of the primary hydroxyl group side of the gamma-CDx cavity. Although the hydrogen-bonding interaction is not essential for conformational enantiomerism of dinaphthylmethane derivatives, it seems to enhance the chiral recognition by cyclodextrins.