Separation of chiral compounds

Analytically, the inclusion phenomenon has been used in chromatography both for the separation of ions and molecules, in Hquid and gas phase (1,79,170,171). Peralkylated cyclodextrins enjoy high popularity as the active component of hplc and gc stationary phases efficient in the optical separation of chiral compounds (57,172). Chromatographic isotope separations have also been shown to occur with the help of Werner clathrates and crown complexes (79,173). 

A number of specialised stationary phases have been developed for the separation of chiral compounds. They are known as chiral stationary phases (CSPs) and consist of chiral molecules, usually bonded to microparticulate silica. The mechanism by which such CSPs discriminate between enantiomers (their chiral recognition, or enantioselectivity) is a matter of some debate, but it is known that a number of competing interactions can be involved. Columns packed with CSPs have recently become available commercially. They are some three to five times more expensive than conventional hplc columns, and some types can be used only with a restricted range of mobile phases. Some examples of CSPs are given below ... 

CE has been applied extensively for the separation of chiral compounds in chemical and pharmaceutical analysis.First chiral separations were reported by Gozel et al. who separated the enantiomers of some dansylated amino acids by using diastereomeric complex formation with Cu " -aspartame. Later, Tran et al. demonstrated that such a separation was also possible by derivatization of amino acids with L-Marfey s reagent. Nishi et al. were able to separate some chiral pharmaceutical compounds by using bile salts as chiral selectors and as micellar surfactants. However, it was not until Fanali first showed the utilization of cyclodextrins as chiral selectors that a boom in the number of applications was noted. Cyclodextrins are added to the buffer electrolyte and a chiral recognition may... 

One direct approach to the separation of chiral compounds is called molecular imprint polymers (MIPs) that involves the formation of a three-dimensional cavity with the shape and electronic features that are complementary to the imprinted or target molecule. 

S Fanali, G Caponecchi, Z Aturki. Enantiomeric resolution by capillary zone electrophoresis use of pepsin for separation of chiral compounds of pharmaceutical interest. J Microcolumn Sep 9 9—14, 1997. 

GC separations of chiral compounds made easier by new (3-cyclodextrin capillary columns, Supelco, 14. 

Figure 1 Optically active reagents for the derivatization and separation of chiral compounds.

Chiral Gompounds. As CD s are composed of D-glucose they are chiral. CD complexation represents therefore a potential tool for separation of other chiral compounds into enantiomers. In fact various interesting separations of chiral compounds into enantiomers have been achieved using p -CD silica stationary phases (3,9, 12-16). ... 

CE is playing a major role in the separation of chiral compounds, a field that is gaining increasing attention in pharmaceutical sciences as well as in forensic toxicology (Lurie, 1994 Novotny et al., 1994 Ward, 1994). The chirally active selectors used in CE include optically active complexes such as Cu(II)-l-histidine, Cu(II)-aspartame, cyclodextrins, modified CDs, bile salts, crown ethers, and proteins (bovine serum albumin, aracid glycoprotein, etc.). 

The separation of chiral compounds will be discussed in Chapter 22. However, the separation of diastereomers can be accomplished using achiral stationary phases. Another alternative is the use of chiral columns for the separation of diastereomers in either the reversed-phase or normal-phase mode. The use of achiral bonded phases without chiral additives, such as phenyl and alkyl bonded phases for the separation of diastereomeric pharmaceutical compounds, is acceptable. Different selectivities can be obtained by employing stationary phases containing varying functionalities (phenyl, polar embedded moieties). The effect of aqueous mobile-phase pH, temperature, and type of organic eluent (acetonitrile versus methanol) can also play a dramatic role on the separation selectivity of diastereomeric compounds. 

The alternative to using racemic mixtures is to find a way to produce the substance as a pure isomer or a way to separate the isomers from the racemic mixtures. Both of these options are difficult and thus expensive. However, it is becoming increasingly clear that many pharmaceuticals must be administered as pure isomers to produce the desired results with no side effects. Therefore, a great deal of effort is now being directed toward the synthesis and separation of chiral compounds. ... 

The selection of a stationary phase is extremely important in GC since it is the major controllable variable of selectivity in the separation process. Stationary phases can be non-polar, polar, or of intermediate polarity. Cyclodextrins, cyclic oligosaccharides composed of varying numbers of glucopyranose units, were found recently to be extremely useful for the separation of chiral compounds. Three types of derivatives, 5-hydroxypropyl (hydrophilic), dialkyl (hydrophobic), and trifiuoroacetyl (intermediate) have been used, each of these phases having a selected area of specificity. 

Several other stationary phases made from different proportions of typical phases (methyl, phenyl, cyanopropyl), or from special compounds such as polytrifluropropyl-siloxane, or different columns such as PLOT (porous layer open tubular), columns coated with a modified graphitized carbon or with a silicone based polymer with chiral groups incorporated into the polymeric chain, columns coated with derivatized cyclodextrins (for the separation of chiral compounds), etc. are also utilized. 

Separation of Chiral Compounds by CE and MEKC with Cyclodextrins... 

To influence the equilibrium constant of association reactions between analytes and additives (e.g., for the adjustment of the degree of com-plexation an important example is the separation of chiral compounds by the use of cy-clodextrins). 

Marina, M.L. Crego, A.L. Capillary electrophoresis A good alternative for the separation of chiral compounds of environmental interest. J. Liq. Chromatogr. Relat. Technol. 1997, 20, 1337-1365. 

Screening of an impressive series of polymers derived from different bulky methacrylate esters, e.g., 42 (Chart 8), and using a variety of chiral ligands has revealed the scope of the process of forming helical poly(methacrylate ester)s and their applicability in, for example, the separation of chiral compounds.151 These polymers were prepared not only by anionic polymerization, but also by cationic, free-radical, and Ziegler—Natta techniques. Recently, Nakano and Okamoto reported the use of a co-balt(II)—salophen complex (43) in the polymerization of methacrylate ester 41.155 The free-radical polymerization in the presence of this optically active metal complex resulted in the formation of an almost completely isotactic polymer with an excess of one helical sense. 

Analysis of Amino Acids. Differentiation of the l- and D-forms of amino acids is essential because they differ in their biological and physiological properties. Although chromatographic columns that effect separations of chiral compounds could be used for analysis of solutions of these acids, a combination of a reactor containing a stereoselective immobilized enzyme and a chromatographic system provides the necessary selectivity for such analyses. 

Catalytic distillation can also be used for selective separations such as the separation of piperidine from n-amylamine, separation of isobutylene in a C4 stream, and removal of acetic acid from dilute aqueous streams. The application of CD for separations will not be reviewed in this article. The potential use of RD for the separation of chiral compounds is very noteworthy although no corresponding CD process was reported. ... 

Modern TLC can be conducted in both the normal-phase and the reversed-phase formats and can be extended to the separation of chiral compounds by modifying the stationary phase or mobile phase with chiral selectors. Using automated systems, performance equal to that achieved by HPLC is, in some cases, possible. Applications for quantitative analysis, including examples of stability-indicating and validated methods for pharmaceuticals, have been reviewed. 

K Verleysen, P Sandra. Separation of chiral compounds by capillary electrophoresis. Electrophoresis 19 2798-2833, 1998. 

GC-MS Gas chromatography-mass spectrometry is the most versatile method. It can be used for pyrolysates (see above), but also for the detection of PAHs in extracts. In combination with commercially available derivatization protocols, amino acids can be analyzed as well. If chiral columns are used, enantiomeric separation of chiral compounds is possible. Usually, either quadrupole or ion trap mass spectrometers are used as detectors, but Time-of-flight (ToF) MS can also be used. 

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