The enantioselectivity of modified cyclodextrins: Studies on interaction mechanisms

Abstract

SummaryThe separation of enantiomers of rather simple molecules, such as alkanes and alkanoic acid esters substituted at the 2-position, has been investigated on alkylated 0-cyclodextrin stationary phases. By observing the effect of varying the substrate and the cyclodextrin alkylating agent it has been possible to propose a mechanism for the separation.The most important factors seem to be hydrophobic interactions and the ability of the cyclodextrin to change its conformation to accept the guest molecule.

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

“…It is, however, noteworthy that racemic alcohols with the phenyl and hydroxyl groups located on adjacent carbons (2-phenyl-1-propanol, 1 -phenyl-2-propanol, and l-phenyl-2-pentanol) were not resolved on TBDE-P-CD. We do not have a Contrary to the speculation that derivatization at 6-0-position with bulky alkyl groups would result in an adverse effect on the enantioselectivity [7], the separation of chiral test compounds on the 6-0-alkyldimethylsilylated CDs studied in this work have shown promising chiral selectivities. The 6-0-alkyldimethylsilylated CDs also have the additional advantages as the chiral GC stationary phase.…”

“…It has been speculated that rigid bulky groups in position 6 of the glucose units of CDs may hinder conformational changes of CD induced by the guest molecule in the inclusion process, thus reducing the enantioselectivity [7]. Although there have been many studies on the effect of 2-0-and 3-0-substituents of 6-0-tert-butyldimethylsi1yl-2,3-di-O-alkylated CDs on chiral selectivities [11, 121, a systematic study on the influence of the 6-0-substituent of 6-0-alkyldimethylsilylated CDs having fixed 2,3-di-O-alkyl substituents on the chiral selectivity has not been reported.…”

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

“…ing 6-O-acetyl AC -CD is not enantioselective at all, probably because of the reduced CD flexibility due to the dipole-dipole interactions between adjacent acyl groups andror to the shielding effect of the acyl group against intrusion of the guest molecules into w x the host. Venema et al 4 hypothesized that the introduction of a rigid bulky group in position C-6 reduces enantioselectivity, since it may inhibit changes in the CD conformation induced by the host molecule. On the other hand, in a study on the enantiomeric separation of ␥-and ␦-lactones, w x Mosandl et al 5 showed that 2,3-AL-and 2,3-ACCDs substituted at C-6 with the tert-butyldimethyl-Ž .…”

tert‐Butyldimethylsilyl‐substituted cyclodextrin derivatives as versatile chiral stationary phases in capillary GC

“…It is well known that the chiral selectivity of modified cyclodextrins is influenced by different substitution patterns at the primary hydroxyl rim of the cyclodextrin cavity. Whereas Venema et al [30] suggested that bulky substituents at the narrow end of the cyclodextrin cavity could hinder conformational changes induced by the host molecule and reduce enantioselectivity, the examples above demonstrate that cyclodextrin derivatives with bulky TBDMS groups at C-6 can be rather versatile chiral stationary phases.…”