Role of hydrogen bond and spacial fitting in the chiral recognition by cyclodextrins. Crystal structures of hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin inclusion complexes with (R)- and (S)-1-phenylethanol

Abstract

Crystal structures of 1 : 1 inclusion complexes of hexakis(2,3,6-tri-O-methyl)-α-cyclodextrin with (R)- and (S)-1-phenylethanol have been determined by X-ray analysis and refined to R-values of 0.077 and 0.073, respectively. In both isomorphous crystals, host molecules are arranged in a ‘head-to-tail’ mode which forms a column structure along the a-axis and guest molecules are aligned within the column. The phenyl group is inserted into the host molecule from the O(2), O(3) side and is in van der Waals contact with the inside wall of the host cavity. The hydroxyl group of the guests is hydrogen-bonded to an O(2) oxygen atom of the host. Compared with the disposition of the (S)-isomer in the host cavity, the (R)-isomer is rotated by 12° around an axis normal to the phenyl group. The geometry of the host–guest interaction indicates that the host molecule recognizes the phenyl group by the spacial fitting to the hydrophobic cavity and the hydroxy group by the hydrogen-bond formation.