Enantiomer separations thin-layer plates

In contrast, CSPs have achieved great repute in the chiral separation of enantiomers by chromatography and, today, are the tools of the choice of almost all analytical, biochemical, pharmaceutical, and pharmacological institutions and industries. The most important and useful CSPs are available in the form of open and tubular columns. However, some chiral capillaries and thin layer plates are also available for use in capillary electrophoresis and thin-layer chromatography. The chiral columns and capillaries are packed with several chiral selectors such as polysaccharides, cyclodextrins, antibiotics, Pirkle type, ligand exchangers, and crown ethers. 

Alkaloids, for example, quinine, are widely applied as catalytic reagents in chiral organic syntheses and also in the production of HPLC stationary phases for chiral separations. Cinchona alkaloids also induce the enantioselection of chiral fluoro-compounds analyzed by FNMR spectroscopy [32]. (-)-Brucine and (-)-berberine mixed with silica gel were used for preparation of thin-layer plates to resolve racemic amino acids. Other optically active pure enantiomers of natural compounds are applied for impregnation of TLC plates (-l-)-tartaric acid, L-aspartic acid [33]. (—)-Menthol is used for preparation of diastereomeric derivatives in indirect enantiomeric separation of, for example, 2- and 3-hydroxy acids [34]. 

Practically every type of separation that has been done by the column technique can also be carried out by thin-layer chromatography. Several papers and reviews were published on the various aspects of the technique. In addition to the books on chromatography [17,26-301, an overview of ion-exchange application of TLC was presented by Devenyi and Kalasz 311. Recent results on the separation of enantiomers have been reviewed by Mack, Hauck and Herbert (32.33) (enantiomer. separation on an RP-18 plate, impregnated with copper salt and proline derivative as chiral selectors) and Lepri, Coas and Desideri, using a microcrystalline triacetylcellulose stationary phase, or modified beta-cyclodextrins in the mobile phase 134.35). 

D-enantiomers of amino acids have been frequently reported in various tissues of diverse organisms. A simple and rapid method of separating optical isomers of amino acids on a reverse-phase thin layer chromatography plate is described. Amino acids, derivatized with l-fluoro-2,4-dinitrophenyl-5-L-alanine amide, were spotted onto a reverse-phase thin layer chromatography plate. Acetonitrile in triethylamine-phosphate buffer was used as the developer. 

Acid catalyzed hydrolysis followed by the identification of D-amino acids in the hydrolysate is equally useful. To make this possible the amino acids in the mixture are acylated with an enantiomerically pure amino acid, for instance with the N-carboxyanhydride of L-leucine. In the resulting mixture of dipeptides any racemized residue is revealed by the formation of two dipeptides that are diastereoisomers of each other, for instance L-leucyl-L-phenylalanine and L-leucyl-D-phenylalanine. Since these are compounds with different physical properties they are separable and appear as a doublet on recordings of an amino acid analzyer. In recent years the conversion to diastereoisomers became unnecessary because the availability of chiral supports now permits separation of enantiomers by high pressure liquid chromatography (HPLC) and also by thin layer chromatography on plates covered with a chiral layer. 

Wall, P.E., Preparation and Application of TLC Plates for Enantiomer Separation, Proc. Int. Symp. Instrumental Thin-Layer Chromatography/Planar Chromatography, Brighton, Sussex, U.K., 1989, pp. 237-243. 

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