Comparison of the enantioselectivity of beta-cyclodextrin vs. heptakis-2,3-O-dimethyl-beta-cyclodextrin LC stationary phases

The enantioselectivity of native beta-cyclodextrin and its 2,3-methylated analogue were examined in both the ''polar-organic'' mode and the reversed phase mode. By comparing these two chiral selectors, the function of the secondary 2,3-hydroxyl groups at the mouth of the cyclodextrin can be examined (in regard to selectivity and retention). In the ''polar-organic'' mode, compounds that are easily resolved on the native beta-cyclodextrin chiral stationary phase (CSP) cannot be resolved on the methylated CSP. These results, as well as the retention data, lend support to the previously proposed ''noninclusion'' mechanism. In the reversed phase mode, many more compounds could be resolved on the native beta-cyclodextrin CSP than on the methylated analogue. However, a few compounds were resolved only on the 2,3-methylated cyclodextrin CSP. In these cases, steric interactions at the mouth of the cyclodextrin cavity may be more important to chiral recognition than hydrogen bonding.