Chiral separations in HPLC

A. M. Krstulovic (Ed.), "Chiral Separations in HPLC , Ellis Norwood, Chichester, OK, 1989. 

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... 

For the majority of the direct chiral separations in HPLC, different kinds of CSPs are used. The CSP can consist of small chiral molecules or polymers [22] and is often immobilized on agarose [23], silica gel [23,24], or polymer particles [23]. During the two last decades, a number of new phases have been introduced. Most commonly used CSPs are listed in Table 17.1. 

Several books are devoted to the chiral separations in HPLC [69,93,104,108,110]. Among several hundreds of CSPs described, ligand-exchange [105], Pirkle-type [113], protein and peptide [114], polysaccharide [115], macrocyclic [93,116], and synthetic polymeric [117-119] CSPs are most widely used. 

There are many parameters which control the enantiomeric resolution by HPLC. The most important of them include parameters of the stationary phase, such as particle size of CSP, pore size of column, and kind of chiral selector, composition, and pH of the mobile phase, flow rate of mobile phase, and temperature. Systematic variation of column temperature should be considered as one way to improve chiral separations in HPLC. From the practical point of view, it is easier to vary column temperatures than mobile phase composition. In addition, variable temperature runs can provide useful information concerning the thermodynamic parameters for the CSP-analyte interactions. The effect of temperature on the resolution... 

Chiral Hplc Columns. There are about 40 commercially available chiral columns which are suitable for analytical and preparative purposes (57). In spite of the large number of commercially available chiral stationary phases, it is difficult and time-consuming to obtain good chiral separation. In order to try a specific resolution meaninghilly, a battery of chiral hplc columns is necessary and this is quite expensive. 

V. A. Davankov, Ligand-exchange phases in Chiral Separations by HPLC, A. M. Krstulovic, Ellis Horwood Ltd., Chichester (1989) Chapter 15. 

Enantiomeric separations have become increasingly important, especially in the pharmaceutical and agricultural industries as optical isomers often possess different biological properties. The analysis and preparation of a pure enantiomer usually involves its resolution from the antipode. Among all the chiral separation techniques, HPLC has proven to be the most convenient, reproducible and widely applicable method. Most of the HPLC methods employ a chiral selector as the chiral stationary phase (CSP). 

The aim of this chapter is to give an overview of chiral separations of pharmaceutical compounds by means of HPLC. Capillary electrophoresis, which is the most popular technique besides HPLC for performing chiral separations at the analytical level, will also be briefly discussed. A second reason to discuss chiral separation in CE in short is the large overlap in the chiral selectors applied in both techniques. 

An overview of the principles of chiral separations in CE and HPLC is given in the first part of this chapter. The second part is dedicated to an overview of chiral separation with CE. The popularity of electrophoretic techniques has grown spectacularly in recent years, mainly due to their exceptional performances in the chiral field. Various selectors can easily... 

Many chiral selectors have been nsed for chiral separations in CE. The most widely used are discussed below. Most of these chiral selectors are also applied in HPLC, as will be discnssed later. 

Resolution using chiral supports in HPLC turned out to be successful for the separation of chiral catenates [135, 136], Unfortunately, this technique seemed to be inappropriate to the resolution of the knot. 

Shibata T, Mori K, Okamoto Y, Polysaccharide phases, in Chiral Separations by HPLC (Krstulovic AM, Ed.) Ellis Horwood, New York, p. 336 (1989). 

Davankov VA, Introduction to chromatographic resolution of enantiomers, in Chiral Separations hy HPLC, Krstulovic A (Ed.), Ellis Horwood, Chichester, p. 175 (1989). 

Chiral separation by HPLC is a practically useful method not only for determining optical purity but also for obtaining optical isomers, and numerous CSPs are presently on the market. In order to achieve the efficient resolution of chiral compounds, we have to choose a suitable chiral column and eluent. The polysaccharide-based CSPs have a high chiral recognition ability and offer a high possibility for the successful resolution of racemates including aliphatic and aromatic compounds with or without functional groups under normal and reversed-phase conditions. 

Lynam and Nicolas have evaluated chiral separations by HPLC versus SFC [14] The enantiomers studied were pharmaceutical synthetic precursors. Repeated injections of trans-stilbene oxide and carbobenzyloxy phe-nylalaninol were made and the chromatographic parameters Rs, N, and a were calculated daily. SFC gave superior enentiomeric resolution of peaks and there was a faster solvent equilibration. The columns were quite stable in both SFC and HPLC systems. 

The idea of two sequential detectors, one conventional the other chiroptical, is the basis of a third strategy for enantiomeric purity determinations using HPLC. It differs from the previous two by not involving a chiral separation. In it the enantiomers co-elute and the total amount is determined from an absorbance measurement. Subsequently a chiroptical... 

---