Comparison of different heptakis(6‐O‐alkyldimethylsilyl‐2‐3‐di‐O‐ethyl)‐β‐cyclodextrins as chiral stationary phases in capillary GC

Abstract

SummaryThree new P-cyclodextrin derivatives, heptakis(6-0-isopropyldimethylsilyl-2,3-di-O-ethyl)-P-cyclodextrin, heptakis(6-0-thexyldimethylsilyl-2,3-di-O-ethyl)-P-cyclodextrin, and heptakis(6-0-cyclohexyldimethyl-2,3-di-U-ethyl)-P-c y clodex t r i n (IPDE -P-CD, TXDE-P-CD, and CHDE-P-CD), were synthesized and the enantioselectivities of these three CD derivatives and heptakis(6-0-tertbutyldimethylsilyl-2,3-di-O-ethyl)-~-cyclodextrin (TBDE-P-CD) were compared for GC separation of a range of chiral test compounds. In particular TXDE-P-CD showed much higher enantioselectivity than TBDE-0-CD. Enantioselectivities of IPDE-P-CD and CHDE-P-CD are somewhat lower than that of TXDE-P-CD. These observations are indicative of significant effects of subtle changes in the structure of the 6-0-substituent on the enantioselectivity of the 0-CD derivatives. The difference in enantioselectivities of the 6-0-substituted CD derivatives were explained in terms of relative contributions of the effects of hydrophobicity and steric hindrance of the substituent to the inclusion process. CHDE-P-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 hydrophobicity effect of the cyclohexyl group in the inclusion process in CHDE-P-CD. IPDE-P-CD showed lower selectivity than TXDE-P-CD and TBDE-0-CD. In the case of these CD derivatives having acyclic substituents 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.

Cyclodextrin derivatives in gas chromatographic separation of racemates with different volatility. Part XV: 6-O-t-butyldimethylsilyl- versus 6-O-t-hexyldimethylsilyl-? and -? derivatives

“…group column f . This is in disagreement with the w x results published by Park et al 12 , who reported better enantioselectivity with ETTHDMS-␤-CDs than with ETTBDMS-␤-CDs. On the other hand, Ž .…”