Enantiomeric resolution chromatography

Figure 10.3 Gas cliromatograms of a cold-pressed lemon oil obtained (a) with an SE-52 column in the stand-by position and (b) with the same column showing the five heart-cuts (c) shows the GC-GC chiral chromatogram of the ti ansfeired components. The asterisks in (b) indicate electric spikes coming from the valve switcliing. The conditions were as follows SE-52 pre-column, 30 m, 0.32 mm i.d., 0.40 - 0.45 p.m film tliickness cairier gas He, 90 KPa (stand-by position) and 170 KPa (cut position) oven temperature, 45 °C (6 min)-240 °C at 2 °C/min diethyl-tert-butyl-/3-cyclodextrin column, 25 m X 0.25 mm i.d., 0.25 p.m film thickness cairier gas He, 110 KPa (stand-by position) and 5 KPa (cut position) oven temperature, 45 °C (6 min), rising to 90 °C (10 min) at 2 °C/min, and then to 230 °C at 2 °C/min. Reprinted from Journal of High Resolution Chromatography, 22, L. Mondello et al, Multidimensional capillary GC-GC for the analysis of real complex samples. Part IV. Enantiomeric distribution of monoterpene hydrocarbons and monoterpene alcohols of lemon oils , pp. 350-356, 1999, with permission from Wiley-VCH.

In another study, the authors reported a comparative study of the enantiomeric resolution of miconazole and the other two chiral drugs by high performance liquid chromatography on various cellulose chiral columns in the normal phase mode [79], The chiral resolution of the three drugs on the columns containing different cellulose derivatives namely Chiralcel OD, OJ, OB, OK, OC, and OE in normal phase mode was described. The mobile phase used was hexane-isopropanol-diethylamine (425 74 1). The flow rates of the mobile phase used were 0.5, 1, and 1.5 mL/min. The values of the separation factor (a) of the resolved enantiomers of econazole, miconazole, and sulconazole on chiral phases were ranged from 1.07 to 2.5 while the values of resolution factors (Rs) varied from 0.17 to 3.9. The chiral recognition mechanisms between the analytes and the chiral selectors are discussed. 

His current area of research interest is enantiomeric resolution of compounds of pharmaceutical importance using liquid chromatography. So far, he has guided 28 doctoral and 50 Masters theses, published more than 230 research papers in international refereed journals including invited chapters in books and encyclopedia. Besides, he authored two books. 

AM Rizzi, S Cladrowa-Runge, H Jonsson, S Osla. Enantiomeric resolution of derivatized DL-amino acids by high-performance liquid chromatography using /3-cyclodextrin chiral stationary phase a comparison between derivatization labels. J Chromatogr A 710 287-295, 1995. 

TABLE 11 The Enantiomeric Resolution of Racemic Compounds on Cyclodextrin-Based CSPs by Means of Subcritical and Supercritical Fluid Chromatography... 

Lienne et al. [170] resolved the enantiomers of albendazole sulfoxides on a column derived from the 6S )-/V-(3,5-dinitrobcnzoyl)tyrosinc chiral selector. The developed method was applied for the enantiomeric resolution of albendazole sulfoxides in plasma samples. Witherow et al. [171] immersed a commercially available thin-layer plate (thin-layer chromatography) into a solution of N-(3,5-dinitrobenzoyl)-L-leucine solution. The developed plate was used for the chiral resolution of 2,2,2-trifluoro-(9-anthryl)ethanol and l,l -binaphthol enantiomers. 

Konig, W.A., Kruger, A., Icheln, D. and Runge, T. (1992) Enantiomeric composition of the chiral constituents in essential oils. Part 1 monoterpene hydrocarbons, journal of High Resolution Chromatography 15, 184-189. 

In gel inclusion chromatography (GIC), the insoluble, swelling cyclodextrin polymers are utilized (24-26). For routine analytical purposes this method is too slow and time consuming, but some highly effective preparative separations including enantiomeric resolutions have been published. This approach seems to be very promising for semi-micro or laboratory scale preparative separations. 

Bhushan, R. Thuku, G.T. Direct enantiomeric resolution of some 2-arylpropionic acids using (-) brucine-impregnated thin layer chromatography. Biomed. Chromatogr. 1999, 13, 276-278. 

As in the case of chromatography, a chiral selector is also required in CE for enantiomeric resolution. Generally, suitable chiral compounds are used in the background electrolyte (BGE) as additives and hence are called chiral selectors or chiral BGE additives. There are only a few publications available that deal with the chiral resolution on a capillary coated with the chiral selector in CE. The analysis of the chiral pollutants discussed in this chapter is restricted only to using chiral selectors in the BGE. The most commonly used chiral BGE additives are cyclo-dextrins, macrocyclic glycopeptide antibiotics, proteins, crown ethers, ligand exchangers, and alkaloids.A list of these chiral BGE additives is presented in Table 1. 

It is well known that a chiral environment is essential for the enantiomeric resolution of racemates. In CE, this situation is provided by the chiral compounds used in the BGE and is known as the chiral selector or chiral BGE additive. Basically, the chiral recognition mechanisms in CE are similar to those in chromatography using a chiral mobile-phase additive mode, except that the resolution occurred through different migration velocities of the diastereoisomeric complexes in CE. The chiral resolution occurred through diastereomeric complex formation between the enantiomers of the pollutants and the chiral selector. The formation of diastereomeric complexes depends on the type and nature of the chiral selectors used and the nature of the pollutants. 

In ligand-exchange chromatography (LEC), the separation of analytes is due to the exchange of ligands from the mobile phase with other ligands coordinated to metal ions immobilized on a stationary phase. LEC has been used successfully for the resolution of free amino acids, amino acid derivatives, and for enantiomeric resolution of racemic mixtures [3]. 

Thin layer chromatography is also used for direct enantiomeric resolution of D,L-arginine, D,L-histidine, d,l-lysine, D,L-valine, and D,L-leucine on silica gel plates impregnated with optically pure (IR, 3R, 5R)-2-azabicy-clo[3,3,0]octan-3-carboxylic acid, which serves as a chiral selector in the pharmaceutical industry. To successfully resolve D,L-amino acids, various combinations of aceto-nitrile-methanol-water were proposed. The spot was detected by ninhydrin (0.2% in acetone). 

Nagata, Y. lida, T. Sakai, M. Enantiomeric resolution of amino acids by thin-layer chromatography. J. Mol. Catal., B Enzym. 2001, 12, 105-108. 

The iV-substituted carbamoyl chloride is considered to be the intermediate of this reaction [626]. W-methyl-2-amino acids [1152] and N-benzyloxy-2-amino acids [22] react simiiarly to give the corresponding oxazolidine-2,5-diones. Phosgene has been used as a reagent for the enantiomeric resolution of iV-methyl-2-amino acids by gas chromatography [1152],... 

Y Walbroehl, J Wagner. Enantiomeric resolution of primary amines by capillary electrophoresis and high-performance liquid chromatography. J Chromatogr A 680 253-261, 1994. 

As Table 2-3 shows, imprinted polymers have been mainly used as separation media (mostly in chromatography). Of special interest is the enantiomeric resolution of race-mates. Further applications are as immunosorbents and chemosensors. The cavities in the imprinted polymers have also been used as microreactors for selective reactions and, more interestingly, as the active sites of catalytically active polymers. In 1998 nearly 100 papers appeared in the literature on molecular imprinting, together with one book [114] another book is imminent [115]. 

Aboul-Enein H.Y., Ah 1., Macrocyclic antibiotics as effective chiral selectors for enantiomeric resolution by liquid chromatography and capillary electrophoresis. Chromatographia, 52, 679-691... 

For achiral molecules that, when coupling with a chiral molecule (mostly alkaloids, such as brucine, cinchonidine, and sparteine), form a pair of diastereomers with different physical properties that can be separated by the methods of fractional crystallization, column chromatography, etc., resulting in the enantiomeric resolution. No actual mechanism is necessary for this reaction. 

D.W. Armstrong and J. Zukowski, Direct Enantiomeric Resolution of Monoterpene Hydrocarbons Via Reversed-Phase High-Performance Liquid Chromatography with an a-cyclodetrin Bonded Stationary Phase, J. Chromatogr. A, 666(1994)445. 

Cassabianca, H. Graff, J.B. Faugier, V. Fleig, F. Grenier, C. (1998). Enantiomeric distribution studies of linalool and linalyl acetate, a powerful tool for authenticity control of essential oils. Journal of High Resolution Chromatography, Vol.21, No.2, (February 1998), 107-112, ISSN 0935-6304... 

Aboul-Enein, H.Y. El-Awady, M.I. Heard, C.H. Enantiomeric resolution of some 2-arylpropionic acids using L-(-)serine impregnated silica as stationary phase by thin layer chromatography. J. Pharm. Biomed. Anal. 2003, 32,1055-1059. 

---