Chiral separations example

Based on the theory, the separation of enantiomers requires a chiral additive to the CE separation buffer, while diastereomers can also be separated without the chiral selector. The majority of chiral CE separations are based on simple or chemically modified cyclodextrins. However, also other additives such as chiral crown ethers, linear oligo- and polysaccharides, macrocyclic antibiotics, chiral calixarenes, chiral ion-pairing agents, and chiral surfactants can be used. Eew non-chiral separation examples for the separation of diastereomers can be found. 

Chiral separations are concerned with separating molecules that can exist as nonsupetimposable mirror images. Examples of these types of molecules, called enantiomers or optical isomers are illustrated in Figure 1. Although chirahty is often associated with compounds containing a tetrahedral carbon with four different substituents, other atoms, such as phosphoms or sulfur, may also be chiral. In addition, molecules containing a center of asymmetry, such as hexahehcene, tetrasubstituted adamantanes, and substituted aHenes or molecules with hindered rotation, such as some 2,2 disubstituted binaphthyls, may also be chiral. Compounds exhibiting a center of asymmetry are called atropisomers. An extensive review of stereochemistry may be found under Pharmaceuticals, Chiral. 

Chiral separations have become of significant importance because the optical isomer of an active component can be considered an impurity. Optical isomers can have potentially different therapeutic or toxicological activities. The pharmaceutical Hterature is trying to address the issues pertaining to these compounds (155). Frequendy separations can be accompHshed by glc, hplc, or ce. For example, separation of R(+) and 5 (—) pindolol was accompHshed on a reversed-phase ceUulose-based chiral column with duorescence emission (156). The limits of detection were 1.2 ng/mL of R(+) and 4.3 ng/mL of 3 (—) pindolol in semm, and 21 and 76 ng/mL in urine, respectively. 

Examples of entropically driven separations are chiral separations and separations that are dominated by size exclusion. However, it must be emphasized that chromatographic separations can not be exclusively "energetically driven" or "entropically driven" but will always contain both components. It is by the careful adjustment of both "energetic" and "entropic" components of the distribution that very difficult and subtle separations can be accomplished. 

An interesting and practical example of the use of thermodynamic analysis is to explain and predict certain features that arise in the application of chromatography to chiral separations. The separation of enantiomers is achieved by making one or both phases chirally active so that different enantiomers will interact slightly differently with the one or both phases. In practice, it is usual to make the stationary phase comprise one specific isomer so that it offers specific selectivity to one enantiomer of the chiral solute pair. The basis of the selectivity is thought to be spatial, in that one enantiomer can approach the stationary phase closer than the other. If there is no chiral selectivity in the stationary phase, both enantiomers (being chemically identical) will coelute and will provide identical log(Vr ) against 1/T curve. If, however, one... 

From the pioneering studies of Ito et al. [117], CCC has been mainly used for the separation and purification of natural products, where it has found a large number of applications [114, 116, 118, 119]. Moreover, the potential of this technique for preparative purposes can be also applied to chiral separations. The resolution of enantiomers can be simply envisaged by addition of a chiral selector to the stationary liquid phase. The mixture of enantiomers would come into contact with this liquid CSP, and enantiodiscrimination might be achieved. However, as yet few examples have been described in the literature. 

Although some applications for preparative-scale separations have already been reported [132] and the first commercial systems are being developed [137, 138], examples in the field of the resolution of enantiomers are still rare. The first preparative chiral separation published was performed with a CSP derived from (S -N-(3,5-dinitrobenzoyl)tyrosine covalently bonded to y-mercaptopropyl silica gel [21]. A productivity of 510 mg/h with an enantiomeric excess higher than 95% was achieved for 6 (Fig. 1-3). 

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 next section, a few illustrative examples of the use of ChiraLig for the analytical and three-stage preparative chiral separations involving amines and amino acids are presented and discussed. 

Preparative chromatography has been used for chiral separations for years, but examples of multi-kg separations (and hence larger ones) were rare until recently. The development of SMB techniques (both hardware and simulation software) has made major breakthroughs in this field. The ability of SMB as a development tool has allowed the pharmaceutical manufacturer to obtain kilo grams quantities of enantiopure drug substances as well benefit from the economics of large-scale production. 

Transition metal coupling polymerization has also been used to synthesize optically active polymers with stable main-chain chirality such as polymers 33, 34, 35, and 36 by using optically active monomers.29-31 These polymers are useful for chiral separation and asymmetric catalysis. For example, polymers 33 and 34 have been used as polymeric chiral catalysts for asymmetric catalysis. Due... 

This separation is an impressive example of an entropically driven distribution system where the normally random movements of the solute molecules are restricted to different extents depending on the spatial orientation of the substituent groups. For further information the reader is directed to an excellent review of chiral separations by LC (Taylor and Maher (12)) and a monograph on CYCLOBOND materials from ASTEC Inc. (13). 

Many times an analyte must be derivatized to improve detection. When this derivatization takes place is incredibly important, especially in regards to chiral separations. Papers cited in this chapter employ both precolumn and postcolumn derivatization. Since postcolumn derivatization takes place after the enantiomeric separation it does not change the way the analyte separates on the chiral stationary phase. This prevents the need for development of a new chiral separation method for the derivatized analyte. A chiral analyte that has been derivatized before the enantiomeric separation may not interact with the chiral stationary phase in the same manner as the underivatized analyte. This change in interactions can cause a decrease or increase in the enantioselectivity. A decrease in enantioselectivity can result when precolumn derivatization modifies the same functional groups that contribute to enantioselectivity. For example, chiral crown ethers can no longer separate amino acids that have a derivatized amine group because the protonated primary amine is... 

As yet, the number of applications is limited but is likely to grow as instrumentation, mostly based on existing CE systems, and columns are improved and the theory of CEC develops. Current examples include mixtures of polyaromatic hydrocarbons, peptides, proteins, DNA fragments, pharmaceuticals and dyes. Chiral separations are possible using chiral stationary phases or by the addition of cyclodextrins to the buffer (p. 179). In theory, the very high efficiencies attainable in CEC mean high peak capacities and therefore the possibility of separating complex mixtures of hundreds of... 

Not only chiral separations have been achieved with Mi-stationary phases. It has also been demonstrated that the MIP could distinguish between ortho- and para-isomers of carbohydrate derivatives. For example, a polymer imprinted with o-aminophenyl tetraacetyl P-D-galactoside was used to analyze a mixture of p-and o-aminophenyl tetraacetyl P-D-galactoside. As expected, the imprinted ortho analyte eluted after the non-imprinted para component see Fig. 5. Although baseline separation was not obtained, a separation factor of a = 1.51 was observed [19]. 

In a recent study, chiral separations for pyrethroic acids, which are the chiral building blocks of synthetic pyrethroids and the primary metabolites of the acid part of these potent ester insecticides, have been developed [62], For example, a polar-organic mobile phase allowed the complete baseline resolution of all four stereoisomers of chrysanthemic acid (2,2-dimethyl-3-(2-methylprop-l-enyl)-cyclopropanecarboxylic acid) on a 0-9-(tcrt-butylcarbamoyl)quinine-based CSP(acjj = 1.20, oLtrans = 1-35, critical Rs = 3.03) (Figure 1,32a). This chiral acid is the precursor of pyrethroids like allethrin, phenothrin, resmethrin, and tetramethrin but not excreted as metabolite. The primary acid metabolite of these pyrethroids is chrysanthemum dicarboxylic acid (3-[(l )-2-carboxyprop-l-enyl]-2,2-dimethylcyclopropanecarboxylic acid) the stereoisomers of which could also be resolved with a reversed-phase eluent (acetonitrile— 30-mM ammonium acetate buffer 90 10, v/v pHa = 6.0) and employing an O-9-(2,6-diisopropylphenylcarbamoyl)quinine-based CSP ads = 1-09, atrans = 1-50,... 

It is evident that the chromatographic term is the only source for enantioselecti vity because the retention factors may differ for the distinct enantiomers, while electrophoretic mobilities are identical for enantiomeric species. In other words, electrophoretic mobilities, like Veo, are nonselective contributions in view of generating chiral separations, but may positively contribute to the selectivity between distinct compounds (such as, for example, chemical impurities) but also of diastereomeric species. 

Williams and Wainer (2002) use examples of two chiral separations to demonstrate their utility in research. In one example, the difference between enantiomers in the competitive displacement of cyclosporine from immobilized P-glycoprotein was studied. In the other, the pharmacokinetic profiles of (+) and (—)-ketamine and (-h) and (—)-norketamine were determined (Williams and Wainer, 2002). 

Numerous applications of chiral separations using CDs can be found in the literature. Some examples, either reviewing chiral separations in CE or presenting the separation of several compounds, can be found in References 97,101,107,121-124. Several studies related to the chiral separation of amino-acid derivatives by CE and micellar electrokinetic capillary chromatography with different types of CDs have also been reported 102,103,114,125-128... 

TABLE 4 Examples of Chiral Separations in Chiral Ligand-Exchange Chromatography... 

The use of protein immobilised to the surface of a silica gel or to another support has been a very successful approach for the chiral separation of various pharmaceuticals. The AGP stationary phase has been shown to have the broadest enantiorecognition abilities while the BSA stationary phase is especially useful for aromatic compounds. " Table 9 shows some examples of separations that were obtained on the protein-type of CSPs. 

TABLE 9 Examples of the Chiral Separation of Drugs Using Proteins as Chiral Selectors... 

TABLE 10 Examples of Chiral Separations of Drugs Using Chiral MIPs as Stationary Phase... 

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