Crystal structures of 6-deoxy-6-monosubstituted β-cyclodextrins. Substituent-regulated one-dimensional arrays of macrocycles

Abstract

Crystal structures of four 6-monosubstituted β-cyclodextrins, 6-deoxy-6-(1-propyl)amino-β-cyclodextrin (1), 6-deoxy-6-[(R)-1-cyclohexylethyl]amino-β-cyclodextrin (2), 6-deoxy-6-[(R)-1-phenylethyl]amino-β-cyclodextrin (3), and 6-deoxy-6-[(1R,2S)-2-hydroxyindan-1-yl]amino-β-cyclodextrin (4) were determined by X-ray analysis. In each crystal, the substituent group is inserted into the adjacent β-cyclodextrin ring from the secondary hydroxy side. This donor–acceptor type self-association through intermolecular inclusion generates a one-dimensional polymeric chain. Two types of arrangement of the β-cyclodextrin rings, a helical form and a linear form, were observed. Compounds 1 and 2 form a helically extended chain along a crystallographic twofold screw axis. In contrast the β-cyclodextrin rings are linearly stacked in the crystals of 3 and 4. The crystal packing of 3 is similar to that of a channel-type structure, while the arrangement of the β-cyclodextrin ring of 4 belongs to the cage-type. These crystal structures suggest that the self-assembly of the 6-monosubstituted β-cyclodextrins is regulated by the physical and chemical properties of the substituent group included in the adjacent β-cyclodextrin ring. A β-cyclodextrin ring with a linear and/or flexible substituent group tends to produce twofold helical packing. The inclusion of a planar and rigid group imposes restrictions on the relative orientation of the macrocycle and this causes variation in the one-dimensional arrangement depending on the shape, size, and orientation of the substituent group.