Brush-type CSPs

The brush-type of CSP was introduced by Pirkle who was one of the pioneers of modern enantioselective liquid chromatography [55]. The most frequently used 7i-acceptor phases are derived from the amino acids phenylglycine (DNBPG) (Fig. 6.8) or leucine (DNBLeu) covalently or ionically bonded to 3-aminopropyl silica gel [56, 57]. These CSPs are commercially available for analytical or preparative separation of enantiomers. Further CSPs based on amino acid or amine chiral selectors such as valine, phenylalanine, tyrosine [58] and l,2-tr s-diaminocyclohexane (DACH-DNB phase) [59] and 1,2-traus-diphenylethylene diamine (ULMO phase) [60] were also developed (Fig. 6.8). These CSPs have been applied for the preparative separation of the enantiomers of a few racemic compounds, but the number of reported preparative applications has remained very limited over the last 10 years. 

6 Isolation and Production of Optically Pure Drugs by Enantioselective Chromatography 

The application of the reciprocality concept has led to the design of various phases of the Ji-donor/acceptor type [61, 62]. One successful phase is the Whelk-O 1 CSP developed by Pirkle and Welch [63-65]. 

These chemically modified silica gels are stable at high pressures and exhibit good chromatographic performances. Usually, these CSPs were used under normal-phase conditions, but chiral resolutions under reversed-phase mode [66] or supercritical fluid chromatography [67] have also been performed. 

Cyclodextrins are cyclic olygosaccharides that can form inclusion complexes in their highly hydrophobic chiral cavity with a large variety of molecules (Fig. 6.9). 

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The majority of the original chiral selectors for brush-type CSPs were derived from natural chiral compounds. Selectors prepared from amino acids, such as phenyl... 

A new brush-type CSP, the Whelk-0 1, was used by Blum et al. for the analytical and preparative-scale separations of racemic pharmaceutical compounds, including verapamil and ketoprofen. A comparison of LC and SFC revealed the superiority of SFC in terms of efficiency and speed of method development [50]. The Whelk-0 1 selector and its homologues have also been incorporated into polysiloxanes. The resulting polymers were coated on silica and thermally immobilized. Higher efficiencies were observed when these CSPs were used with sub- and supercritical fluids as eluents, and a greater number of compounds were resolved in SFC compared to LC. Compounds such as flurbiprofen, warfarin, and benzoin were enantioresolved with a modified CO, eluent [37]. 

Methanol remains the most widely used modifier because it produces highly efficient separations, but it does not always produce the highest selectivity [8]. Recent studies have provided insight into the role of the modifier in enantioselectivity in SFC [69]. Blackwell and Stringham examined a series of phenylalanine analogues on a brush-type CSP and developed a model that allowed prediction of selectivity based on the bulk solvation parameters of various modifiers [70]. Careful choice of modifiers can be used to mask or enhance particular molecular interactions and ultimately provide control of selectivity [71]. 

An understanding of the recognition of chirality at a molecular level has become of interest in many fields of chemistry and biology. In the past decade, many attempts to clarify the mechanism of chiral recognition on CSPs for liquid chromatography have been made by means of chromatography, NMR spectroscopy,199 202 X-ray analysis, and computational methods.203 - 206 The successful studies have been mostly carried out for the small-molecule CSPs, especially cyclodextrin-based CSPs and Pirkle-type (brush-type) CSPs. In contrast, only a few mechanistic studies on chiral discrimination at the molecular... 

Figure 13. Schematic view of brush-type CSPs showing the chiral selector substituents oriented towards the liquid phase. Solvent molecules and the respective solvation are not shown. Stereoselective [SO-SA] interactions, attractive or repulsive, are located invariably within the heterogeneously structured chiral stationary phase.

Silica with monomolecular ligands (Brush-type CSP)... 

In a similar manner other brush-type CSPs are used (see Table 22.1) dinitro-benzoylleucine, naphthylalanine, naphthylleucine, chrysanthemoyl-phenylglycine, naphthylethylurea, to name but a few. They all have their specificities for certain classes of compounds, although this is sometimes difficult to predict. To gain some idea of their individual abilities it is best to consult the literature or the excellent brochures provided by the manufacturers. Brush-Type CSPs are robust and allow high sample loads. 

Silica with monomolecular ligands (Brush-type CSP) 3-[N-(3,5-dinitrobenzoyl) phenylglycin-amido]propyl-silica... 

The DNBPG phase has a number of features which are typical for almost all of the brush-type CSPs. It has two amide groups which are rigid (planar) therefore the whole chiral moiety will prefer a limited (i.e. not an unlimited) number of conformations which is important for chiral recognition. The amide... 

Brush -type CSPs where small molecules, usually with Ti-active groups, are bonded to silica. 

Brush-type CSPs are robust and allow high sample loads. As an example, Figure 2 shows the separation of oxazepam acetate and its hydrolysis product on dinitrobenzoylleucine. 

Chiral stationary phases can exist in different forms [10] (see Fig. 8). Some selectors can be used as particulate phase materials, such as polymeric cellulose triacetate. Polymeric cellulose and amylose derivatives are often coated onto silica carrier particles so that only 20% of the CSP consists of the chiral selector. This combination of stationary phase and chiral polymer combines good chromatographic properties (due to the homogeneous particle size distribution) with a high density of chiral adsorption sites in the polysaccharide derivatives. Another approach is selected for the so-called brush-type CSPs. In these, the chiral selector is covalently bound to the surface of the silica particles. These phases show high chemical inertness and allow the use of a multitude of different mobile phases. 

Very often, analysis times are shortened by using SFC. CSPs compatible with SFC are cellulose and amylose derivatives, brush-type CSPs, as well as macrocyclic phases. The selectivity of the CSPs is in many cases similar to that found on these phases in the normal-phase mode. 

Among brush-type CSPs for HPLC, the Whelk-0(R) CSP, first designed for naproxen enantioseparation, has shown an outstanding applicability for a broad diversity of racemic compounds [72]. Nowadays it has become one of the CSPs of reference included in most screening processes addressed to the search of adequate conditions for enantioselective separation in drug discovery [73, 74]. A CS containing the Whelk-0( skeleton, and a lipophilic chain aimed to enhance solubility... 

FIGURE 54.8. Structures of some brush-type CSPs. 

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