Chiral Extractions

Walters, R.R., Buist, S.C. (1998). Improved enantioselective method for the determination of the enantiomers of reboxetine in plasma by solid-phase extraction, chiral derivitization, and column-switching high-performance liquid chromatography with fluorescence detection. J. Chromatogr. A 828, 167-176. 

Aboul-Enein, H.Y. and Hefnawy, M.M., Chiral analysis of butaclamol enantiomers in human plasma by HPLC using a macrocychc antibiotic (vancomycin) chiral stationary phase and sohd phase extraction. Chirality, 16, 147, 2004. 

Szekely, E., Simandi, B., Fogassy, E., Kemeny, S., and Kmecz I. Enantioseparation of chiral alcohols by complex formation and supercritical fluid extraction Chirality 2003, 15, 783-786. 

Very recently, Efrima" suggested a combination of SERS and optical-activity measurements. Provided large field gradients are present near the metal surfaces, one should measure very large effects. This may provide an easy way to extract chiral information. 

Fig. 18 TEM and SEM images of extracted chiral hollow particles with differeht morphology, Reprinted with permission from Crudden et al. Copyright 2012 Americah Chemical Society.

Kmecz I, Simandi B, BaUnt J, Szekely E, Fogassy E, Kemeny S. Optical resolution of 6-fluoro-2-methyl-l,2,3,4-tetrahydroquinoline by supercritical fluid extraction. Chirality 2001 13 568-570. 

With relatively simple spectra, it is usually possible to extract the individual coupling constants by inspection, and to pair them by size in order to discover what atoms they coimect. However, the spectra of larger molecules present more of a challenge. The multiplets may overlap or be obscured by the presence of several unequal but similarly sized couplings. Also, if any chiral centres are present, then the two hydrogens in a... 

Traditionally, chiral separations have been considered among the most difficult of all separations. Conventional separation techniques, such as distillation, Hquid—Hquid extraction, or even some forms of chromatography, are usually based on differences in analyte solubiUties or vapor pressures. However, in an achiral environment, enantiomers or optical isomers have identical physical and chemical properties. The general approach, then, is to create a "chiral environment" to achieve the desired chiral separation and requires chiral analyte—chiral selector interactions with more specificity than is obtainable with conventional techniques. 

An hplc assay was developed suitable for the analysis of enantiomers of ketoprofen (KT), a 2-arylpropionic acid nonsteroidal antiinflammatory dmg (NSAID), in plasma and urine (59). Following the addition of racemic fenprofen as internal standard (IS), plasma containing the KT enantiomers and IS was extracted by Hquid-Hquid extraction at an acidic pH. After evaporation of the organic layer, the dmg and IS were reconstituted in the mobile phase and injected onto the hplc column. The enantiomers were separated at ambient temperature on a commercially available 250 x 4.6 mm amylose carbamate-packed chiral column (chiral AD) with hexane—isopropyl alcohol—trifluoroacetic acid (80 19.9 0.1) as the mobile phase pumped at 1.0 mL/min. The enantiomers of KT were quantified by uv detection with the wavelength set at 254 nm. The assay allows direct quantitation of KT enantiomers in clinical studies in human plasma and urine after adrninistration of therapeutic doses. 

Catechin and epicatechin are two flavanols of the catechin family. They are enantiomers. The capillary zone electrophoresis (CE) methods with UV-detection were developed for quantitative determination of this flavanols in green tea extracts. For this purpose following conditions were varied mnning buffers, pH and concentration of chiral additive (P-cyclodextrin was chosen as a chiral selector). Borate buffers improve selectivity of separation because borate can make complexes with ortho-dihydroxy groups on the flavanoid nucleus. 

Figure 3.7 [continued) (b) Chromatograms of (iii) the dichloromethane extract of strawberry fruit yoghurt analysed with an apolar primary column, with the heart-cut regions indicated, and (iv) a non-racemic mixture of y-deca-(Cio) and 7-dodeca-Cj2 lactones isolated by heart-cut transfer, and separated by using a chiral selective modified cyclodextrin column. Reproduced from A. Mosandl, et al J. High Resol. Chromatogr. 1989, 12, 532 (39f. 

The study of biochemical natural products has also been aided through the application of two-dimensional GC. In many studies, it has been observed that volatile organic compounds from plants (for example, in fruits) show species-specific distributions in chiral abundances. Observations have shown that related species produce similar compounds, but at differing ratios, and the study of such distributions yields information on speciation and plant genetics. In particular, the determination of hydroxyl fatty acid adducts produced from bacterial processes has been a successful application. In the reported applications, enantiomeric determination of polyhydroxyl alkanoic acids extracted from intracellular regions has been enabled (45). 

Figure 3.8 Second-dimension chiral cyclodextrin capillary column separation of a non-racemic pair of nonachlorobomane compounds extracted from dolphin blubber, shown with expanded attenuation in the inset. The primary separation (not shown) was performed on an apolar primary capillary column. Reproduced from H.-J. de Geus et al. J. High Resol. Chromatogr. 1998, 21, 39 (59).

Determination of chiral-y-lactones from raw flavour extract of strawberries and other-fruit-containing foods and beverages... 

Determination of chiral a-pinene, /3-pinene and limonene in essential oils and plant extracts... 

In order to reduce or eliminate off-line sample preparation, multidimensional chromatographic techniques have been employed in these difficult analyses. LC-GC has been employed in numerous applications that involve the analysis of poisonous compounds or metabolites from biological matrices such as fats and tissues, while GC-GC has been employed for complex samples, such as arson propellants and for samples in which special selectivity, such as chiral recognition, is required. Other techniques include on-line sample preparation methods, such as supercritical fluid extraction (SFE)-GC and LC-GC-GC. In many of these applications, the chromatographic method is coupled to mass spectrometry or another spectrometiic detector for final confirmation of the analyte identity, as required by many courts of law. 

Most of the chiral membrane-assisted applications can be considered as a modality of liquid-liquid extraction, and will be discussed in the next section. However, it is worth mentioning here a device developed by Keurentjes et al., in which two miscible chiral liquids with opposing enantiomers of the chiral selector flow counter-currently through a column, separated by a nonmiscible liquid membrane [179]. In this case the selector molecules are located out of the liquid membrane and both enantiomers are needed. The system allows recovery of the two enantiomers of the racemic mixture to be separated. Thus, using dihexyltartrate and poly(lactic acid), the authors described the resolution of different drugs, such as norephedrine, salbu-tamol, terbutaline, ibuprofen or propranolol. 

Liquid-liquid extraction is a basic process already applied as a large-scale method. Usually, it does not require highly sophisticated devices, being very attractive for the preparative-scale separation of enantiomers. In this case, a chiral selector must be added to one of the liquid phases. This principle is common to some of the separation techniques described previously, such as CCC, CPC or supported-liquid membranes. In all of these, partition of the enantiomers of a mixture takes place thanks to their different affinity for the chiral additive in a given system of solvents. 

The instrumentation which until now has been used in chiral extraction experiments is very diverse, ranging from the simple extraction funnel [123, 180], the U-or W-tubes [171, 181], to more sophisticated devices, such as hollow-fiber extraction apparatus [175] or other membrane-assisted systems. Most of these experiments... 

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

It is worth noting that the extractive process can be performed continuously. Thus, the separation of ( )-mandelic acid into its enantiomers was achieved with a liquid particle extractor described by Abe et al. [190-192] using A-docecyl-L-proline as chiral selector. 

In supported liquid membranes, a chiral liquid is immobilized in the pores of a membrane by capillary and interfacial tension forces. The immobilized film can keep apart two miscible liquids that do not wet the porous membrane. Vaidya et al. [10] reported the effects of membrane type (structure and wettability) on the stability of solvents in the pores of the membrane. Examples of chiral separation by a supported liquid membrane are extraction of chiral ammonium cations by a supported (micro-porous polypropylene film) membrane [11] and the enantiomeric separation of propranolol (2) and bupranolol (3) by a nitrate membrane with a A/ -hexadecyl-L-hydroxy proline carrier [12]. 

In the classical set-up of bulk liquid membranes, the membrane phase is a well-mixed bulk phase instead of an immobilized phase within a pore or film. The principle comprises enantioselective extraction from the feed phase to the carrier phase, and subsequently the carrier releases the enantiomer into the receiving phase. As formation and dissociation of the chiral complex occur at different locations, suitable conditions for absorption and desorption can be established. In order to allow for effective mass transport between the different liquid phases involved, hollow fiber... 

Nonselective membranes can assist enantioselective processes, providing essential nonchiral separation characteristics and thus making a chiral separation based on enantioselectivity outside the membrane technically and economically feasible. For this purpose several configurations can be applied (i) liquid-liquid extraction based on hollow-fiber membrane fractionation (ii) liquid- membrane fractionation and (iii) micellar-enhanced ultrafiltration (MEUF). 

As described above, the application of classical liquid- liquid extractions often results in extreme flow ratios. To avoid this, a completely symmetrical system has been developed at Akzo Nobel in the early 1990s [64, 65]. In this system, a supported liquid-membrane separates two miscible chiral liquids containing opposite chiral selectors (Fig. 5-13). When the two liquids flow countercurrently, any desired degree of separation can be achieved. As a result of the system being symmetrical, the racemic mixture to be separated must be added in the middle. Due to the fact that enantioselectivity usually is more pronounced in a nonaqueous environment, organic liquids are used as the chiral liquids and the membrane liquid is aqueous. In this case the chiral selector molecules are lipophilic in order to avoid transport across the liquid membrane. 

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