COMPARISON OF DIFFERENT YLDIMETHYLSILYL-2,3-DI-O-ETHYL)-BETA-CYCLODEXTRINS AS CHIRAL STATIONARY PHASES IN CAPILLARY GC

Three new beta-cyclodextrin derivatives, yldimethylsilyl-2,3-di-O-ethy l)-beta-cyclodextrin, yldimethylsilyl-2,3-di-O-ethyl)-beta-cyclodextri n, and ptakis(6-O-cyclohexyldimethyl-2,3-di-O-ethyl)-beta cy do dextri n (IPDE-beta-CD, TXDE-beta-CD, and CHDE-beta-CD), were synthesized and the enantioselectivities of these three CD derivatives and 6-O-tertbu tyldimethylsilyl-2,3-di-O-ethyl)-beta-CD (TBDE-beta-CD) were compared for GC separation of a range of chiral test compounds. In particular T XDE-beta-CD showed much higher enantioselectivity than TBDE-beta-CD, E nantioselectivities of TPDE-beta-CD and CHDE-beta-CD are somewhat lowe r than that of TXDE-beta-CD, These observations are indicative of sign ificant effects of subtle changes in the structure of the 6-O-substitu ent on the enantioselectivity of the beta-CD derivatives. The differen ce in enantioselectivities of the 6-O-substituted CD derivatives were explained in terms of relative contributions of the effects of hydroph obicity and steric hindrance of the substituent to the inclusion proce ss, CHDE-beta-CD showed the lowest enantioselectivity among the three derivatives, It is likely that the unfavorable steric hindrance of the bulky cyclohexyl group plays a greater role than the favorable hydrop hobicity effect of the cyclohexyl group in the inclusion process in CH DE-beta-CD, IPDE-beta-CD showed lower selectivity than TXDE-beta-CD an d TBDE-beta-CD, In the case of these CD derivatives having acyclic sub stituents the relative hydrophobicity of the substituent seems to be a dominant factor affecting the inclusion process, Isopropyl groups are less hydrophobic than thexyl and tert-butyl groups.