Diastereoisomer-selective inclusion complexation of cinchona alkaloids with a modified beta-cyclodextrin: Fluorescent behavior enhanced by chiral-tether binding

The molecular 1:1 complexation of cinchona alkaloids by mono(6-deoxy-6-{[ (R)-1-(hydroxymethyl)propyl]amino})-beta-cyclodextrin (1) in aqueous solution has been investigated by 2D-NMR, fluorescence titration, and fluorescence-lifetime experiments. Generally, with 1 as the host, in contrast to P-cyclodextrin proper, strong binding of quinine (2; K-a = 84200 m(-1)) and quinidine (3; K-a = 27300 m(-1)) at pH 6.8 was observed, as monitored by an increase in fluorescence intensity, with a fair degree of diastereoisomer discrimination (ca. 3:1). To rationalize these results, two possible cooperative complexation modes, including specific H-bonding interactions to the chiral tether of the cyclodextrin portion, are proposed.