X-ray structures of 1:1 complexes of (L)-menthol with β-cyclodextrin and permethylated β-cyclodextrin

Abstract

Inclusion complexes of (L)-menthol with β-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB) have been crystallised and their structures determined by single crystal X-ray diffraction methods. The menthol-β-cyclodextrin complex crystallises in the monoclinic system, space group P2₁, Z = 4, D_x = 1.380 g cm⁻³, a = 15.342(3) Å, b = 32.54(2) Å, c = 15.324(3) Å and β = 102.44(2)°. The structure was solved by isomorphous replacement and was refined to a final R-value of 0.108 for 5469 observed reflections. This is a dimeric β-cyclodextrin complex which packs in screw channel mode and the guest is oriented differently in the two crystallographically independent host molecules. The menthol-TRIMEB complex crystallises in the orthorhombic system, space group P2₁2₁2₁, Z = 4, D_x = 1.256 g cm⁻³, a = 11.060(3) Å, b = 26.138(6) Å and c = 29.669(6) Å. The structure was solved by direct methods and refined to a final R-value of 0.105 for 4387 observed reflections. In contrast to what was found for the menthol-β-cyclodextrin complex, the hydroxyl group of the menthol molecule is located within the cavity of the TRIMEB molecule, and although the crystal packing is different from that observed in other TRIMEB complexes, the host adopts a similar conformation.