Studies on the Development of Selectively Modified Cyclodextrin Stationary Phases for HPLC

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The method to isolate selectively methylated beta-and gamma-cyclodextrin (CD) derivatives could be established. Their hydroxyl groups at 2-position (2-MM-) 3-position (3-MM-), both 2- and 3-positions (2,3-DM-) and 2-, 3-and 6-positions (2,3,6-TM-) were methylated. The mono-, di- and trimethylated CD derivatives are donared by prefixing the unmodified CD with MM-, DM- and TM-, respectively.After monoallyloxyethylation, the unmodified and methylated CDs were treated with dimethoxymethylsilane, followed by coupling on the silica gel surface. Selectively methylated CD-bonded stationary phases for HPLC without an unreacted spacer arm could be thus obtained. This preparation method of the CD stationary phases brought about a ready estimation of the precise amount of CD immobilized and easy immobilization of the gamma-CD derivatives as well as the beta-CD ones.The chiral recognition abilities of the resulting gamma-CD stationary phases were evaluated using dansylamino acids as model solutes. Unmodified gamma-CD and 3-MM-gamma-CD stationary phases could resolve most of their enantiomers, while 2-MM-gamma-CD and 2,3,6-TM-gamma-CD stationary phases could not at all. These results positively suggest an important contribution of the 2-hydroxyl group to the chiral recognition for these solutes. It is found that the selectively methylated gamma-CD stationary phases exhibit the high enantioselectivities as expected. Similar investigation is now in progress on the selectively methylated beta-CD stationary phases.