Eluents, chiral recognition

The D-enantiomer of such bidentate compounds was generally retained more than L-enantiomer. Mixtures of methanol/acetonitrile/water or dichloromethane/methanol were often used as eluents. Chiral recognition based on CLEC was also involved in the enantiomer separation of amino acids and (3-adrenergic blocking agents on silica gel plates coated with the copper(II) complex of enantiomeric amino acids (L-proline, L-arginine, and lR,3R,5R-2-azobicyclo [3.3.0]octan-3-carboxylic acid). 

Macaudiere and co-workers performed a comparison of LC and SFC on a polymer based-CSP (Chiralpak OT) [64]. The chromatographic behavior of this CSP seemed to be quite different in SFC than in LC, although satisfactory separations were achieved with both techniques. The chiral recognition mechanisms may be altered by the nature (hexane-based or CO,-based) of the eluent. 

Polysaccharide-based CSPs also exhibit a chiral recognition for alcohols and a large number of resolutions have been reported. Chiral alcohols can usually be directly resolved with hexane containing a small amount of an alcohol as the eluent. For aliphatic alcohols, which cannot be directly resolved, their resolution is often efficiently attained as phenylcarbamate or benzoate derivatives on OD (Figure 17).85 For example, 2-butanol and 2-pentanol are completely resolved with a very high selectivity on OD as their phenylcarbamates. The derivatization of alcohols to phenylcarbamates and benzoates can be easily achieved by the reaction with phenyl isocyanates and benzoyl chlorides, respectively. In most cases, the phenylcarbamates are better resolved than the benzoates. For chiral compounds bearing phenolic hydroxy groups, the addition of a small amount of an acid to an eluent is recommended to depress its dissociation. 

Chiral separation by HPLC is a practically useful method not only for determining optical purity but also for obtaining optical isomers, and numerous CSPs are presently on the market. In order to achieve the efficient resolution of chiral compounds, we have to choose a suitable chiral column and eluent. The polysaccharide-based CSPs have a high chiral recognition ability and offer a high possibility for the successful resolution of racemates including aliphatic and aromatic compounds with or without functional groups under normal and reversed-phase conditions. 

The mechanism of chiral recognition of the enantiomers in series of clathrochelates was studied in research reported in Ref. 311, in which [Sb2(cZ-tart)2]2- anion was employed as a chiral eluent. It was established that in the majority of cases, A-enantiomer is eluted first, and that bulky alkyl capping groups increase the degree of resolution. 

When cyclodextrins are used in normal-phase mode, chiral recognition is based on the interaction of the outer functional groups through hydrogen bonds, dipole-dipole interactions, and Jt-Jt interactions. The normal-phase mode is used for analytes that show a low solubility in aqueous eluents here, mostly alkane/alcohol mixtures are used. 

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