CSPs, polysaccharide-based structures

Polysaccharide-based CSPs incorporate derivatives of cellulose and amylose adsorbed on silica gel. The selectivity of these CSPs depends upon the nature of the substituents introduced during the derivatization process. The secondary structure of the modified polysaccharide is believed to play a role in selectivity, but the chiral recognition mechanisms have not been fully elucidated [55]. 

FIGURE 8 Chemical structures and chemical and trade names of most of the commonly used polysaccharide-based CSPs. 

CSPs (CTA-I and CTA-II) have inverse selectivity for Troger s base and trans-1,2-diphenyloxirane racemates. These characteristics of CTA CSPs are responsible for good chiral resolution of small cychc carbonyl compounds [42]. In 2001 Aboul-Enein and Ah [63] observed the reversed order of elution of nebivolol on a Chiralpak AD column when ethanol and 2-propanol were used separately as the mobile phases. Table 1 presents selectivity data for the polysaccharide-based CSPs. Okamoto et al. [42] observed that the introduction of a methyl group at the para position of cellulose tribenzoate results in a dramatic shift of the structural selectivity toward aromatic compounds with larger skeletons, and its selectivity was rather similar to that of cellulose tricinnamate. 

Chiral resolution on polysaccharide-based CSPs is sensitive, and therefore, the optimization of HPLC conditions on these phases is very important. The most important factors that control enantiomeric resolution are the composition, pH, and flow rate of the mobile phase and parameters, including temperature and solute structure. The optimization of these parameters on polysaccharide-based CSPs is discussed next. 

Chiral resolution on polysaccharide-based CSPs is due to the different types of bonding between racemates and CSP, as discussed later in this chapter. Therefore, different racemate structures provide bondings of different types, which in turn means that different patterns of chiral recognition will be observed. The effects of... 

Figure 5. Structures and suppliers of commercially available polysaccharide-based CSPs. CTA-I is also available from Merck and Macherey-Nagel.

In a few cases, the option of preparing tailor-made CSPs for a particular racemic structure has been applied. For example, we prepared on an empirical basis a particular polysaccharide-based CSP for the separation of the enantiomers of the enantiomers of the LTD4 antagonist iralukast and of the antimalaria agent benflumethol [87]. These two racemic drugs were only poorly resolved on the commercially available polysacharide-based phases whereas an excellent separation was obtained on the carbamate derivative of cellulose obtained from cellulose and 3-chloro-4-methylphe-nylisocyanate. The prepared CSP was also used to perform pharmacokinetic studies. 

Many researchers have documented the effect of the mobile phase on the enantioselectivity of different racemates on polysaccharide-based CSPs. However, up to the present time, no comprehensive study aimed at identifying the association between the stmctural features present on solute and appropriate mobile phase conditions has yet been proposed. Piras et al. [33] have studied the characteristic features of about 2363 racemic molecules separated on a Chiralcel OD CSP. The mobile phase used for these racemates was compared with their structures, which are available from CHIRBASE (http //www.chirbase.u-3mrs.fr/chirbase/). The data setup was submitted to data-mining programs for molecular pattern recognition and mobile phase predictions for new cases. Some of the substmctural... 

In this chapter, the development and chiral recognition mechanism of poly-saccharide-based CSPs for the efficient chromatographic separation of enantiomers have been outlined. The recognition abilities of native polysaccharides are not sufficient for use as CSPs, but their abilities can be substantially improved by the proper modifications of their structures. At present, more than 10 kinds of polysaccharide-based CSPs are commercially available and practically used around the world as... 

Different classifications for the chiral CSPs have been described. They are based on the chemical structure of the chiral selectors and on the chiral recognition mechanism involved. In this chapter we will use a classification based mainly on the chemical structure of the selectors. The selectors are classified in three groups (i) CSPs with low-molecular-weight selectors, such as Pirkle type CSPs, ionic and ligand exchange CSPs, (ii) CSPs with macrocyclic selectors, such as CDs, crown-ethers and macrocyclic antibiotics, and (iii) CSPs with macromolecular selectors, such as polysaccharides, synthetic polymers, molecular imprinted polymers and proteins. These different types of CSPs, frequently used for the analysis of chiral pharmaceuticals, are discussed in more detail later. 

According to their chemical structures, CSPs can be divided into three different groups. A multitude of chiral stationary phases is derived from (modified) natural or synthetic polymers, e.g., the polysaccharides, proteins or polyacrylamides. A second type of selectors is based on large chiral ring systems, such as cyclo-dextrins, macTocycKc antibiotics, and crown ethers. The last group comprises moleailes of small and medium size, such as amino acids and their derivatives, alkaloids, and fuUy synthetic selectors. 

Derivatized polysaccharide CSPs are operational, quite well, in all three HPLC separation modes as well as under super- or sub-critical fluid conditions [153, 160, 166, 170, 176-178]. A previous study based on a collection of more than 100 pharmaceutically important compounds with diverse structures clearly showed that polysaccharide CSPs generally had much higher success rate in resolving enantiomers under normal-phase and SFC conditions, followed by RP and polar organic modes (Fig. 17) [167]. This study also revealed that amylose tris(3,5-dimethylphenylcarbamate) AD phase was more effective than the other studied polysaccharide CSPs in polar organic mode and SFC, whereas cellulose tris(3,5-dimethylphenylcarbamate) phase is more applicable in reversed-phase mode. This observation is consistent with two other studies [159, 160]. 

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