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 (Ks) and Gibbs free energy changes (−ΔG) 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 Ks 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 enantioselectivit...