Molecular Recognition Study on a Supramolecular System. 10. 1 Inclusion Complexation of Modified β-Cyclodextrins with Amino Acids: Enhanced Enantioselectivity for l / d -Leucine

The novel β-cyclodextrin (β-CD) derivatives bearing a m -toluidinyl or (9-fluorenyl)alkylamino moiety have been synthesized by a convenient method in 45% and 66% yields, respectively. The stability constants ( K s ) and Gibbs free energy changes (−Δ G ) for inclusion complexation of mono-[6-( m -toluidinyl)-6-deoxy]-β-cyclodextrin 1 and mono-[6-[(9-fluorenyl)alkylamino]-6-deoxy]-β-cyclodextrin 2 with various l / d -amino acids have been examined by the fluorescence spectrum method in buffered aqueous solution (pH = 7.20) at 20−23 °C. The modified β-cyclodextrins, possessing a toluidinyl or fluorenyl moiety as fluorescent probe, can recognize not only the size and shape but also the chirality of l / d -amino acids, giving fairly good enantioselectivity up to 33 for l / d -leucine. β-Cyclodextrin derivative 1 gave the highest K s for l -leucine and the lowest for d -leucine among the amino acid series, eventually showing the highest enantioselectivity for l / d -amino acids. The molecular recognition ability and enantioselectivity for amino acids of the modified β-cyclodextrins 1 and 2 are discussed from the viewpoint of the size/shape−fit relationship between the host cavity and the guest molecules.