Chiral Discrimination by Modified Cyclodextrins

Interest in the naturally occurring cyclodextrins and their chemically modified derivatives stems from the fact that they act as host molecules to form inclusion complexes with a wide variety of guests. The cyclodextrins each exist as a single enantiomer with the consequence that when they act as host molecules, interaction with a racemic guest may lead to the formation of diastereoisomeric complexes of different thermodynamic stability. However, the extent of chiral discrimination displayed by the naturally occurring cyclodextrins is typically quite modest with efficient resolution of racemates only resulting from repeated interactions with a cyclodextrin. The low enantioselectivity may be attributed to the inherent symmetry. In addition,inclusion complex formation often occurs principally as a result of interaction of the hydrophobic annulus of the cyclodextrin with an achiral hydrophobic portion of a guest, and there is little interaction between the chiral centres of the cyclodextrin and those of the guest. It follows that increased chiral discrimination can be expected with modified cyclodextrins where throught the modification,the degree of asymmetry of the cyclodextrin has been increased and there is the possibility of greater interaction between chiral portions of the cyclodextrins and those of the guests.