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Cu(II) complexes of L-threonine, L-serine, and L-tartaric acid were prepared and used as ligand exchange reagents for enantiomeric resolution of some of the β-blockers (bisoprolol, metoprolol, and propranolol) and a β2-agonist (salbutamol). Impregnated thin-layer plates were prepared by spreading slurry of silica gel prepared in the solutions of each of the three ligand exchange reagents. The spots were detected with iodine. Effect of temperature on enantioresolution was also studied. The detection limits were found in the range 0.19–0.26 μg for each enantiomer. L-Ser proved to be a good ligand using a common mobile phase in each case.

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Enantioresolution of three active pharmaceutical ingredients (APIs), namely, atenolol, betaxolol, and orciprenaline, marketed as racemic mixture, has been achieved in a direct mode using (S)-glutamic acid as chiral additive in thin-layer chromatography. Two different approaches were adopted: (1) (S)-glutamic acid was mixed in the silica gel slurry for making thin-layer plates, or (2) it was added in the mobile phase and plain plates without any chiral additive were used. Both (1) and (2) were capable of separating enantiomers of all the three racemates, but different combinations and proportions of solvents were found successful in the two cases. Good resolution was achieved in both cases, and the results are compared for these two sets of studies among themselves and with other literature reports. Iodine was used to locate the spots of the corresponding enantiomers. The detection limits for each enantiomer were found in the range of 1.4–1.9 μg (per spot).

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The present paper deals with direct enantioresolution of (±)-bupropion using thin-layer chromatography and different Cu(II)-l-amino acid complexes as chiral ligand exchange reagent (LER). Cu(II) acetate and four l-amino acids (viz., l-proline, l-histidine, l-phenylalanine, and l-tryptophan) were used for the preparation of LER. Four different approaches were adopted for impregnating/loading the plate with the LER. In the present work, plate impregnation was achieved (a) by mixing LER with silica gel slurry, (b) by developing plain plates with solutions of the Cu complexes, (c) using a solution of Cu(II) acetate as mobile phase additive for the thin-layer chromatography (TLC) plates impregnated with one of the l-amino acids, and (d) by using the LER as mobile phase additive. Spots were located using iodine vapor. The results obtained with all the approaches have been compared in terms of resolution. Effect of concentration of Cu(II) acetate and chiral selector has also been studied.

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Thin-layer chromatography (TLC) is a simple, cost-effective, and robust analytical technique, very useful in various different application fields like, e.g., fast screening and quality control of pharmaceuticals, phytochemicals, and alimentary products. Its versatility to a large extent owes to the seminal works of the late Professor Edward Soczewiński and, in the first instance, to the rationale implemented to liquid chromatography by the Soczewiński—Wachtmeister and the Snyder—Soczewiński equations. Among the most challenging applications of TLC, one can name the enantioresolution of the racemic and scalemic mixtures and a statement that the chiral TLC in this particular respect outperforms the instrumentally more advanced chromatographic techniques is far from being an exaggeration. In the course of the past ca. 12 years, the authors of this paper have extensively investigated — mostly by means of chiral TLC — the enantioresolution of the low molecular weight carboxylic acids, to discover by sheer luck that all of them spontaneously undergo an oscillatory process, i.e., chiral conversion. Later, they collected abundant experimental evidence that the oscillatory chiral conversion is accompanied by the oscillatory condensation. Herewith, we highlight applications of the chiral TLC to demonstrate the dynamic phenomenon of the spontaneous oscillatory chiral conversion with the low molecular weight carboxylic acids.

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Summary

Direct resolution of enantiomers of (RS)-ketorolac was achieved by thin-layer chromatography on silica gel plates using enantiomerically pure L-tryptophan, L-valine, L-methionine, and L-histidine as chiral additive in the stationary phase. The solvent system (acetonitrile, methanol water, and chloroform) with different ratios was successful in resolving the enantiomers. Spots were detected by use of iodine vapor. The detection limit was 0.4 µg mL−1 for each enantiomer of (RS)-ketorolac. The native enantiomers were isolated and characterized.

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References [1] R. Bhushan , J. Martens , Amino Acids: Chromatographic Separation and Enantioresolution , HNB Publishing, New York

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Direct resolution of enantiomers of (±)-bupropion (BUP) was achieved by thin-layer chromatography on silica gel plates impregnated with optically pure l-Glu as chiral selector. The solvent system acetonitrile-methanol-dichloromethane-water (5.6:1:2.2:1, v/v) was successful in resolving the enantiomers. Spots were detected by use of iodine vapor. The detection limit was 0.2 μg for each enantiomer of BUP. The effects of concentration of chiral selector, temperature, and pH on enantiomeric resolution were examined. The separation of BUP enantiomers was also investigated by high-performance liquid chromatography (HPLC) on a chlorinated methylated cellulose-based stationary phase. Reversed phase HPLC was successful using binary mixture of aqueous ammonium formate and methanol for separation of enantiomeric pair with detection at 230 nm. The factors influencing HPLC separation were also investigated.

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]. R. Bhushan , and J. Martens , Amino Acids: Chromatographic Separation and Enantioresolution , HNB , 2010 , New York . [29]. ICH , Q2B Document: Validation of

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R. Bhushan, J. Martens , Amino Acids: Chromatographic Separation and Enantioresolution. HNB Publishing, New York, 2010. Martens J. Amino Acids

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on relevant literature on separation and determination of enantiomers of Ket using liquid chromatography involving enantioselective bioassay and enantioresolution from pharmaceutical formulations. It does not claim to cover all the references on the

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