Indirect chiral separations

Since dorzolamide possesses two chiral centers, an indirect chiral separation has been developed [18]. The procedure employs the chemical derivatization of the secondary amino group of the inhibitor, formation of diastereomeric urea derivatives (each with three chiral centers in the molecule), and their separation under non-ehiral HPLC conditions. Using... 

Although these chiral derivatization reagents have demonstrated their utility for indirect chiral separation, the arguments against them include length of time involved for derivatization, possibilty of racemization, and presence of optically active contaminants, variability of the formation rate of the diasteromers [184]. 

Wan H, Andersson PE, Engstrom P, Blomberg LG (1995) Direct and indirect chiral separation of amino acids by capillary electrophoresis. J Chromatogr A 704 179-193 Varesio E, Gauvrit JY, Longeray R, Lanteri P, Veuthey J-L (1997).Central composite design in the chiral analysis of amphetamines by capillary electrophoresis. Electrophoresis 18 931-936... 

The indirect and direct approaches are discussed below. Indirect chiral separations are less common and, thus, only are described briefly. In the direct approach section, different CSs are discussed such as cyclodex-trins, crown ethers, linear oUgo- and polysaccharides, macrocyclic antibiotics, proteins, Ugand exchange type selectors, chiral ion-pairing reagents, and chiral surfactants. In every section, a theoretical explanation of the... 

Indirect chiral separations are especially suitable for compounds containing an amino group connected to an asymmetric carbon atom, such as AAs. Furthermore, the... 

Indirect chiral separation was performed derivatizing metoprolol, oxprenolol, and propranolol with 5-(- -)-benoxaprofen chloride, pindolol with 2,3,4,6-tetra-0-acetyl-/S-D-glucopyranosyl-isothiocyanate and propranolol, alprenolol, pindolol, oxprenolol, and bunitrolol with f -(—)-l-(l-naphthyl)ethyl isocyanate. The formed derivatives were further resolved to antipodes using achiral stationary phases. 

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... 

A. Principles of Chiral Separation in LC Direct and Indirect Approaches... 

A general comment about detection in chiral separations, both direct and indirect, is that in general all detectors used for non-chiral separations can also be used. The most frequently applied detector is thus the UV detector. 

The separation mechanism is quite different from other chromatographic techniques and a broader spectrum of possible impurities can be detected at the same system (e.g., inorganic small cations, anions by indirect detection, chiral separations by adding a chiral selector, proteins and peptides by adding a polymer to the separation buffer, etc.)... 

N. R. Srinivas and L. N. Lgwemezie, Chiral separation by HPLC.l. Review of indirect separation of enantiomers as diastero-meric derivatives using ultraviolet, fluorescence and electrochemical detection, Biomed. Chromatogr., 6 163 (1992). 

W. Lindner, Indirect separation of enantiomers by hquid chromatography, in M. Zief and L. J. Crane (eds.), Chromatographic Chiral Separation, Marcel Dekker, New York, 1988, pp. 91-130. 

Initial studies on the GLC separation of enantiomeric alcohols as esters of various chiral adds used packed columns (1,4). A major advance in GLC separations was the advent of capillary columns, and most of the recently published indirect GLC separations used capillary columns. 

There are several characteristics of diastereomeric chiral separations (also known as indirect enantiomeric separations) that are worth mentioning. Achiral phases that are cheaper, more rugged, and widely commercially available are used. The elution order can be controlled by choice of the chirality of the derivatizing agent. This feature is useful for the analysis of trace levels of enantiomers. The separation can be designed such that the minor enantiomer is eluted first, allowing for more accurate quantitation. 

Current chiral separation methods using liquid chromatographic techniques can be divided into two categories a direct method based on diastereomer formation on CSPs or in mobile phases, and an indirect... 

Chiral separation by CE can be achieved by a direct or an indirect method. Direct separation is the more common approach. The chiral selector is dissolved in the running buffer, where it interacts selectively with the enantiomers to form reversible and transient diastereoisomeric or inclusion complexes of differing effective mobility. In indirect chiral CE separation, the enantiomers form covalent diastereomeric derivatives with a chiral reagent. A chiral selector is unnecessary... 

As mentioned above, the different solubility of diastereomers offers a challenge for their separation by diastereomeric crystallization. The indirect HPLC separation of enantiomers relies on their different interaction with a (achiral) stationary phase. The interaction of the enantiomeric pair (R, S) with one (let us assume it to have S configuration) enantiomer of a chiral derivatizing reagent may be expressed as follows ... 

Methods for the chiral separation of amino acids can be divided into two categories one is an indirect method based on the formation of diastereomers by the reaction of amino acids with a chiral derivatiza-tion reagent and separation of the diastereomeric derivatives on an achiral stationary phase. The other is a direct method based on the formation of diastereomers on a stationary phase or in a mobile phase, where the former uses a chiral stationary phase (CSP) and the latter adds a chiral selector in a mobile phase and uses an achiral stationary phase. 

Derivatization requires that the species of interest must contain a functional group that can be chemically modified. There should be no enantioselectivity of the rate of the derivatization. There are several disadvantages to an indirect chromatographic chiral separation. The derivatization procedure may be complex and time-consuming and there is always a possibility of racemization during the derivatization procedure. In the case of preparative chromatography of the diastereomeric species, they have to be... 

---