Impregnated layers enantiomeric separation with

Impregnated layers, 17-18,120-I2I enantiomeric separation with, 122 Indole-3-yIacetic acid (lAA), 784 Indoles, 905-912... 

Bhushan and Ali [25] were the first to propose the use of silica gel G layers coated with acidic L-amino acids for the resolution of racemic alkaloids such as hyoscyamine (or atropine) and colchicine. Preparation of the plates, chromatographic development, and solute detection were performed as reported previously. The slurry of silica gel G was prepared in 100 ml of water containing 0.3 g of L-aspartic acid and chromatograms were developed at 0°C for 3.5 h. Migration distance was 10 cm eluting with n-butanol/chloroform/acetic acid/water (3 6 4 1, v/v/v/v). The extremely low temperature necessary for the enantiomeric separation should be noted as no resolution was observed at 5°C. Based on the results obtained (see Table 5.9), layers impregnated with this chiral selector were suitable for the resolution of the two racemates and particularly of atropine a = 1.86). 

In effects, enantiomeric separations of neutral and basic amino acids, obtained in 2000 on layers impregnated with a neutral amino acid such as (IR,3R,5R)-2-azabicyclo[3.3.0]octan-3-carboxylic acid [35], pointed out that the species involved in the formation of diastereomers could be more different, in contrast to what was hypothesized by Bhushan et al. [35]. In particular, working at pH 8,... 

Impregnation of thin-layer material with different suitable chiral selectors provides an inexpensive wider choice of separation conditions for direct enantiomeric resolution, that is, the analyte can be applied to the plates in the form of racemic or scalemic mixture as such, without resorting to derivatization of the enantiomeric mixture with one or the other chiral reagent that would otherwise involve several experimental stages of synthesis and purification, etc. A general protocol for impregnated TLC successfully apphed in author s laboratory [8] is described in the following text. 

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]. 

Optical isomer separations that are carried out on a chiral layer produced from C-18 modified silica gel impregnated with a Cu(II) salt and an optically active enantiomerically pure hydroxyproline derivative, on a silica layer impregnated with a chiral selector such as brucine,on molecularly imprinted polymers of alpha-agonists,or on cellulose with mobile phases having added chiral selectors such as cyclodextrins have been reported mostly for amino acids and their derivatives. Mixtures of sorbents have been used to prepare layers with special selectivity properties. 

Bhushan and Ali (1987) tested amino acid separations on silica gel layers impregnated with various metal salts. Bhushan and Reddy (1989) reported the separation of phenylthiohydantoin (PTH) amino acids on silica gel with new mobile phases. Laskar and Basak (1988) de.scribed a new ninhydrin-based procedure that produced different colors and good sensitivity for amino acid detection. Bhushan and Reddy (1987) reviewed the TLC of PTH amino acids. Gankina et al. (1989) described a unidimensional multistep silica gel HPTLC method for the separation and identification of PTH and dansylamino acids. Bhushan et al. (1987) developed numerous solvent systems for effective separations of 2,4-dini-trophenyl-(DNP) amino acids. Bhushan (1988) reviewed the TLC resolution of enantiomeric amino acids and their derivatives. Kuhn et al. (1989) reported the amino acid enantiomer separation by TLC on cellulose of d- and L-tryptophan and methyltryptophan. Guenther (1988) determined TLC-separated enantiomers by densitometry. 

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