Rare ‘head-to-tail’ arrangement of guest molecules in the inclusion complexes of (+)- and (−)-menthol with β-cyclodextrin

The inclusion of (+)- and (−)-menthols in β-cyclodextrin has been studied by X-ray crystallography. The obtained [2:2] complexes show typical for β-cyclodextrin form of head-to-head dimer. In both (+)- and (−)-cases, the menthol molecule in one of cyclodextrins forming dimer exhibits disorder and occupies two major sites. Also both of the diastereoisomeric complexes show unusual 'head to tail' arrangement of guest molecules – two guest molecules are differently oriented inside β-cyclodextrin cavity. Stability constants for both complexes in solution were measured.