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  • Author or Editor: Ravi Bhushan x
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Resolution of the enantiomers of racemic atenolol, metoprolol, propranolol, and labetalol, commonly used β-blockers, has been achieved by TLC on silica gel plates using vancomycin as chiral impregnating reagent or as chiral mobile phase additive. With vancomycin as impregnating agent, successful resolution of the enantiomers of atenolol, metoprolol, propranolol, and labetalol was achieved by use of the mobile phases acetonitrile-methanol-water-dichloromethane 7:1:1:1 ( v/v ), acetonitrile-methanol-water 6:1:1 ( v/v ), acetonitrile-methanol-water-dichloromethane-glacial acetic acid 7:1:1:1:0.5 ( v/v ), and acetonitrile-methanol-water 15:1:1 ( v/v ), respectively. With vancomycin as mobile phase additive, successful resolution of the enantiomers of metoprolol, propranolol, and labetalol was achieved by use of the mobile phases acetonitrile-methanol-0.56 mM aqueous vancomycin (pH 5.5) 6:1:1 ( v/v ), acetonitrile-methanol-0.56 mM aqueous vancomycin (pH 5.5) 15:1:2 ( v/v ), and acetonitrile-methanol-0.56 mM aqueous vancomycin (pH 5.5)-dichloromethane 9:1:1.5:1 ( v/v ), respectively. Spots were detected by use of iodine vapor. The detection limits were 1.3, 1.2, 1.5, and 1.4 μg for each enantiomer of atenolol, metoprolol, propranolol, and labetalol, respectively.

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A simple and rapid method has been established for indirect separation of the enantiomers of (R,S)-metoprolol and (R,S)-carvedilol by reversed-phase TLC. Beta blockers derivatized with 1-fluoro-2,4-dinitrophenyl-5-l-alanine amide (Marfey’s reagent, FDNP-l-Ala-NH2) and its six structural variants (FDNP-l-Phe-NH2, FDNP-l-Val-NH2, FDNP-l-Pro-NH2, FDNP-l-Leu-NH2, FDNP-l-Met-NH2, and FDNP-d-Phg-NH2) were spotted on precoated plates. (R,S)- Metoprolol and (R,S)-carvedilol were isolated from pharmaceutical dosage forms and purified. The diastereomers were separated most effectively by use of mobile phases containing acetonitrile and triethylamine-phosphate buffer (50 mM, pH 5.5). The results obtained by use of Marfey’s reagent were compared with those obtained by use of the other variants. The effects of buffer concentration, pH, and concentration of organic modifier were studied.

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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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Resolution of racemic metoprolol, propranolol, carvedilol, bisoprolol, salbutamol, and labetalol, commonly used β-blockers, into their enantiomers has been achieved by TLC on silica gel plates impregnated with optically pure L -Glu and L -Asp. Acetonitrile-methanol-water-dichloromethane and acetonitrile-methanol-water-glacial acetic acid mobile phases in different proportions enabled successful separation. The spots were detected with iodine vapor. The detection limits were 0.23, 0.1, 0.27, 0.25, 0.2, and 0.2 μg for each enantiomer of metoprolol, propranolol, carvedilol, bisoprolol, salbutamol, and labetalol, respectively.

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The scientific development in the area of enantioseparation during the last few decades has centered on the production of new chiral stationary phases (CSPs) and new chiral derivatizing reagents (CDRs) for use in liquid chromatography, and in particular high-performance liquid chromatography (HPLC) only. Both CSPs and CDRs have several limitations which, in general, are ignored. Little attention has been paid to thin-layer chromatography (TLC) despite its many advantages compared to HPLC in pharmaceutical and drug analysis and the areas of natural products chemistry and organic synthesis, particularly enantioselective synthesis in purification of the product prior to establishing enantiomeric ratio by different method(s). TLC provides a rapid, easy, aff ordable, and simple approach in all these situations. The demonstrated capability and effi- ciency of TLC in direct resolution of the racemate clearly establish its superiority, and the methodology should allow its application in the resolution of several other racemates, irrespective of the functional group, in a very short time along with the recovery of native enantiomers (for further use).

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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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A simple and sensitive method for separation and quantitative determination of antidiabetic drugs in pharmaceutical preparations has been established and validated. Commercial formulations of five antidiabetic drugs (metformin, pioglitazone, rosiglitazone, glibenclamide, and gliclazide) were chosen for the studies. The compounds were extracted, isolated, purified, recrystallized, and characterized by measurement of melting point, λ max , and IR. Quantitative determination was performed by HPLC, TLC, and column chromatography supplemented with UV spectrophotometry. Two of the combinations, metformin + pioglitazone and metformin + gliclazide, were separated by open-column chromatography. Detection was by UV spectrophotometry in HPLC and by use of iodine vapor in TLC.

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Direct enantioresolution of (±)-etodolac has been achieved by adopting a new conceptual approach involving both achiral phases in thin-layer chromatography (TLC). Enantiomerically pure l-tryptophan, l-phenyl alanine, l-histidine, and l-arginine were used as chiral inducing reagents (CIR); none of these was impregnated with silica gel (while making TLC plates) or mixed with the mobile phase. The solvent system MeCN—CH2Cl2—MeOH, in different proportions, was found to be successful for enantioresolution. Spots were located in iodine chamber. Effect of concentration of chiral inducing reagent and temperature on enantioresolution was studied.

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Enantioseparation of (RS)-etodolac was achieved by an indirect approach. Extraction of the drug (RS)-etodolac (Etd) was done from commercially available tablets, as its racemic mixture; it was purified, characterized, and was used for enantioseparation studies. Diastereomeric amides of (RS)-Etd were synthesized using a commercial sample of enantiomerically pure (R)-(−)-1-cyclohexylethylamine. Derivatization reactions were carried out under conditions of stirring at room temperature (30°C for 2 h) for (RS)-Etd and under microwave irradiation (MWI). The derivatives (as diastereomeric amides) were separated by thin-layer chromatography (TLC). The successful solvent system for separating the diastereomeric amides of (RS)-Etd was acetonitrile—methanol—dichloromethane—water (6:1:1:0.5, v/v). Spots were located by use of iodine vapor. The diastereomeric amides were recovered by preparative method; these were purified and characterized by recording their m.p., λ max (ultraviolet [UV]), infrared (IR) spectra, specific rotation, and proton nuclear magnetic resonance (1H-NMR) spectra for the verification of the synthesis and for the separation of diastereomeric amides.

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