Use of derivatized cyclodextrins as chiral selectors for the separation of enantiomers by gas chromatography.

The state-of-the-art of enantioseparations of various chiral compounds on derivatized cyclodextrins, employed as chiral stationary phases (CSPs) by gas chromatography, is reviewed. Heptakis(2,3,6-O-trimethyl)-beta-cyclodextrin and octakis(3-O-butanoyl-2,6-di-O-pentyl)-gamma-cyclodextrin (Lipodex E), either dissolved in a semipolar polysiloxane or bonded to poly(dimethylsiloxane) (Chirasil-Dex), proved to be the most popular chiral selectors. Special applications refer to the use of heptakis (2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin and heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin. Enantioselectivity is generally low with enantioseparation factors in the range of 1.02<alpha<1.20, corresponding to -Delta(D,L)(DeltaG)=RT ln(alpha) of only 0.014-0.14 kcal/mol at 100 degrees C. Therefore, reliable mechanistic studies on chirality recognition are still absent. Concise thermodynamic parameters have been measured in selected systems which involve large enantioselectivies of 1.5<alpha<10. In order to extend the scope of enantioseparations to different classes of chiral compounds in a single gas chromatographic run, mixed chiral stationary phases based on hydrogen-bonding and inclusion-type selectors have been developed. Miniaturized capillary columns (100 cm x 50 microm i.d.) coated with Chirasil-beta-Dex were used in enantioselective gas chromatography (GC) and also utilized in supercritical fluid chromatography (SFC), liquid chromatography (LC) and capillary electrochromatography (CEC) (unified enantioselective approach). Derivatized linear dextrins (acyclodextrins) proved to exhibit chirality recognition which challenges the role of molecular inclusion associated with derivatized cyclodextrins. (Semi)-preparative enantioseparations by GC have been advanced.