Pharmaceuticals, separation enantiomeric compounds

Analytical and preparative separation of enantiomers is of basic importance. In general, one of the two antipodes is pharmaceutically active the other one may be either inactive or toxic. There are three basic methods for the separation of enantiomeric compounds ... 

Many hydroxylated pharmaceutical compounds are effectively separated and quantitated on NP support materials using alkane-based solvents. Polar modifiers are added to the alkane to cause more rapid elution and enhance selectivity. Hexane and heptane are the mainstay solvents for use in the enantiomeric separations that utilize Pirkle-type support materials. Often alcohols are used as the modifier in such separations. The major disadvantage to the use of alkane mobile phases is the often-limited solubility of the analyte therein. It should be noted, however, that the separation of enantiomeric compounds still utilizes these solvents, especially if per-analysis derivatization is done. 

Health authorities worldwide have fixed purity requirements for active pharmaceutical ingredients (APIs). When applied to chiral drugs, this implies that, if one enantiomer is chosen to be developed and marketed as an API, the counterpart isomer will be considered an impurity. The rule affects new chemical entities (NCEs) and chiral dmgs previously commercialized as a racemic mixture chiral switches). Therefore, techniques to perform the analytical control of the enantiomeric composition, at any of the drug development steps, together with processes to produce enantiomeric compounds with the desired enantiomeric purity, are essential in this domain. Liquid chromatography using chiral stationary phases (CSPs) is applied at two levels, analysis and production of enantiomerically pure compounds. At present, it can be considered the most universal technique for enantiomer separation. 

Fanah, S., Catarcini, P., and Presutti, C., Enantiomeric separation of acidic compounds of pharmaceutical interest by capillary electrochromatography employing glycopeptide antibiotic stationary phases, J. Chromatogr. A, 994, 227, 2003. 

Note that not all enantioseparations in SFC are better than in HPLC [34], Bernal et al. [62] described the enantiomeric separation of several pharmaceutical-related compounds on a polysaccharide-based column using HPLC and SFC. They showed that most of the separations obtained by SFC are better, in terms of resolution and analysis time, than the separations obtained by HPLC. However, one compound could not be resolved using SFC, but LC provided baseline resolution. 

Chiral separation of drng molecules and of their precursors, in the case of synthesis of enantiomerically pure drugs, is one of the important application areas of HPLC in pharmaceutical analysis. Besides HPLC, capillary electrophoresis (CE) is another technique of choice for chiral separations. Chapter 18 provides an overview of the different modes (e.g., direct and indirect ones) of obtaining a chiral separation in HPLC and CE. The direct approaches, i.e., those where the compound of interest is not derivatized prior to separation, are discussed in more detail since they are cnrrently the most frequently used techniques. These approaches require the use of the so-called chiral selectors to enable enantioselective recognition and enantiomeric separation. Many different molecnles have been nsed as chiral selectors, both in HPLC and CE. They can be classified into three different groups, based on their... 

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. 

Only a little research on the separation and synthesis of chiral compounds has been published so far. Because enzymes have extremely high selectivity, and owing to the great importance of enantioselective synthesis or enantiomeric resolution in the pharmaceutical industry, the most intense research in this area can be expected, along with minimizing the use of substances and maximizing their effect. 

Resolution Methods. Chiral pharmaceuticals of high enantiomeric purity may be produced by resolution methodologies, asymmetric synthesis, or the use of commercially available optically pure starting materials. Resolution refers to the separation of a racemic mixture. Classical resolutions involve the construction of a diastcrcomcr by reaction of the racemic substrate with an enantiomerically pure compound. The two diastereomers formed possess different physical properties and may be separated by crystallization, chromatography, or distillation. A disadvantage of the use of resolutions is that the best yield obtainable is. 50%, which is rarely approached. However, the yield may he improved by repeated raccmization of the undcsired enantiomer and subsequent resolution of the racemate. Resolutions are commonly used in industrial preparations of homochiral compounds. 

A kinetic resolution is a chemical reaction in which one enantiomer of a racemate reacts faster than the other. Most kinetic resolutions of pharmaceutical compounds are catalyzed processes. Catalysts used in a kinetic resolution must be chiral. Binding of a chiral catalyst with a racemic material can form two different diastereomeric complexes. Since the complexes are diastereomers, they have different properties different rates of formation, stabilities, and rates of reaction. The products form from the diastereomeric substrate-catalyst complexes at different rates. Therefore, a chiral catalyst is theoretically able to separate enantiomers by reacting with one enantiomer faster than the other. The catalysts used in kinetic resolutions are often enzymes. Enzymes are constructed from chiral amino acids and often differentiate between enantiomeric substrates. 

A more detailed discussion of the stationary phase types and mechanism of interaction and separation theory in relation to chiral compounds is given in Chapter 22. A large number of chiral stationary phases are currently available to meet the needs of the pharmaceutical industry for determination of the enantiomeric purity of active pharmaceutical ingredients, raw materials, and metabolites. As a consequence, there are a multitude of options in terms of columns, separation mode, and separation conditions to explore in achieving an enantioseparation. 

The product class of enantiomerically pure amines is of considerable importance in both pharmaceutical and agrochemical applications. For instance, enantiopure aryl-alkyl amines are utilized for the synthesis of intermediates for pharmaceutically active compounds such as amphetamines and antihistamines. Several chemical as well as biotransformation methods for the asymmetric synthesis/dynamic kinetic resolution [29] or separation of enantiomers of chiral amines have been described. These are illustrated in Scheme 4.5 for (S)-a-methylbenzylamine [30]. 

---