Quinine chiral selectors

Silica gel (Kromasil) modified with a basic tert. -butyl carbamoyl quinine chiral selector, 3 pm... 

Overall, the /er/-butyl carbamates of quinine and quinidine evolved as the most effective chiral selectors among this family because of their broad enantioselectivity spectmm and exceptional degree of selectivity for a wide variety of chiral acids. On... 

The postmodification strategy, in which apoly(glycydyl methacrylate-co-ethylene dimethacrylate) monolith was activated with hydrogen sulfide to a thiol-modified monolith and subsequently derivatized with an 0-9-(tert-butylcarbamoyl)quinine selector by radical addition reaction, yielded slightly less efficient capillary columns. However, this procedure has the advantage that only minute amount of chiral selector are needed to end-up with a useful enantioselective capillary column [84]. 

New brush-type phases (donor-acceptor interactions) are appearing all the time. " Examples are stationary phases comprising quinine derivatives and trichloro-dicyanophenyl-L-a-amino acids as chiral selectors. Quinine carbamates, which are suitable for the separation of acidic molecules through an ionic interaction with the basic quinine group, are also commonly used but in general they are classified with the anion-exchange type of chiral selectors (see further) because of their interaction mechanism, even though r-donor, r-acceptor properties occur. (Some separations on Pirkle-type CSPs are shown in Table 2.)... 

Lammerhofer and Lindner reported on the enantiomer separation of derivatized amino acids and profens on a weak-anion-exchange(WAX)-type stationary phase based on chiral quinine carbamate selectors by p-CEC [54,55]. The separations were performed either under aqueous or [54] non-aqueous conditions [55]. The efficiency obtained in the p-CEC mode was about two to three time higher than with LC using an acetonitrile/buffer flow system [54], Very high resolutions and efficiencies were found for non-aqueous p-CEC. For example, the enantiomer separation of Fmoc-leucine was achieved in less than 10 min with a resolution Rs of 6.9 at about 100 000... 

Lammerhofer and Lindner [62] reported on the enantiomer separation of deriva-tized amino acids on an ODS-packed capillary with a chiral quinine-derived selector as buffer additive in two different modes (i) in an electrophoretically dominated mode at high electrolyte concentration and (ii) in an electroosmotically dominated mode at a low electrolyte concentration. Enantiomer separation in the electrophoretically dominated mode (i) leads to high efficieny (about two to three times higher than in LC) but to a moderate enantioselectivity (about the same as in LC). In the electroosmotically dominated mode (ii) a higher enantioselectivity but a lower efficiency (even inferior to LC) occurs. The separations can also been performed in a non-aque-ous buffered mobile phase. Pressurization (8-10 bar) of the flow system on both ends of the separation capillary was applied. 

Recently, Lindner and coworkers developed a series of anion-exchange CSPs based on quinine and quinidine as chiral selectors (Fig. 6.11) [79, 80]. These phases are particularly appropriate for the separation of the enantiomers of chiral acidic compounds. Improvement of the chiral recognition power of these phases by rationally designed structural modifications has led to exceptionally high enantioselectivity which, of course, is of great interest for preparative applications [81]. Screen-... 

Laemmerhofer, M., Lindner,W. Quinine and quinidine derivatives as chiral selectors. I. Brush type chiral stationary phases for high-performance liquid chromatography based on cinchonan carbamates and their application as chiral anion exchangers, J. Chromatogr. A, 1996, 741, 33-48. 

Quinine, quinidine and cinchonidine, which are amino alcohols with high chiral capability, have been used as selectors for the separation of enantiomers of acids containing a hydrogen bonding function [24]. These chiral selectors have high UV absorbance, providing indirect detection possibilities for solutes without inherent UV absorbance (Figure 6). 

FIGURE 9.4 Schematic depiction of (A) ideal and (B) nonideal stereochemical arrangements for diastereomeric complex formation between a chiral selector and a pair of enantiomers. Shown in (C) is an aimotated X-ray crystal stmctnre, which shows the optimal alignment of noncovalent forces to initiate diastereomer complex formation between chloro-ferf-bntylcarbamoyl quinine (top) and 5 -(3,5-dinitrobenzoyl)-leucine (bottom) (Reprinted from Reference [87] with permission from the American Chemical Society 2005). 

Quinine (QN) and quinidine (QD) are natural enantiomers belonging to the cinchona alkaloid family, an important subgroup of naturally occurring polycyclic P-carboline alkaloids. They are widely used as resolving agents for chiral acids via preferentially forming diastereomerie salts with one of the enantiomers [117]. From the reciprocity concept point of view, the logic path is that QN and QD are potential chiral selectors... 

Akasaka K, Gyimesi-Forras K, Lammerhofer M, Fujita T, Watanabe M, Harada N, Lindner W. Investigations of molecular recognition aspects related to the enantiomer separation of 2-methoxy-2-(l-naphthyl)propionic acid using quinine carbamate as chiral selector an NMR and FT-IR spectroscopic as well as X-ray crystallographic study. Chirality 2005 17 544-555. 

Use of the optically pure (-) quinine [22] as chiral selector for the separation of amino acid enantiomers was not as successful as in the case of (-) brucine. Only for methionine, very good results were obtained h/ip for d- and L-methionine were 25 and 50, respectively. 

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