Suitability of Preparative CSP

Before starting to screen for a suitable CSP some general considerations have to be taken into account to optimize the separation in the best direction right from the beginning. The required purity and amount and time frame should be know n alongw ith some limitations w ith regard to solvents or other process conditions, for example, temperature range. In addition, it should be known if only one or both enantiomers have to be delivered in purified form. 

Over 100 chiral stationary phases are now commercially available (Section 3.1.5). 

Not all of them are designed and suited for preparative purposes. Certainly, every CSP might be used to isolate some mg amounts of an enantiomer in case of urgent need, but to develop a production process the CSP should fulfill some critical requirements. The main parameters are  

As solubility is often an issue for pharmaceutical compounds, the chosen CSP should have a good stability against solvents with different polarities so that the whole spectrum of retention adjustment can be used. For production processes the availability of the CSP in bulk quantities at a reasonable price should also be taken into account. Different CSP groups are characterized according to these parameters in Table 3.13 (Section 3.1.5.3). 

---

The main limitation of these CSPs is their limited pressure stability, which makes them not very suitable for HPLC application. However, they have proved to be an excellent tool for the preparative separation of drugs by low-pressure HPLC. To make these CSPs accessible to HPLC, silica gel-based phases were developed. " This type of phase is available from Merck (Darmstadt, Germany) under the name Chiraspher. Polymer phases of different types have been developed by Okamoto s group. > They are prepared by the asymmetric polymerization of triphenylmethyl-methacrylate monomers. The original character of these polymers is that they do not possess any chiral centre and therefore their chirality is only due to their helicity. However, clear mechanisms have not been proposed... 

This successful development has been made possible thanks to the concomitant development of both new efficient chiral stationary phases and powerful separation techniques. In particular, the design of numerous CSPs has provided new tools suitable for preparative separations. 

CSP Coverage of analytes Compatibility with different separation modes Suitability for preparative-scale separations... 

The multiplicity of recognition sites determines one of the characteristics of polymeric CSPs. These materials are provided with a significant loading capacity, which is what makes them suitable for preparative applications. 

In the next step, the best candidate from the series 2-oxo-4-(9-phenanthryl)-dihy-dropyrimidine 22 was prepared and isolated in enantiomerically pure form, then attached to a macroporous polymer support. To attach the isolated selector to the amino functionalized macroporous polymethacrylate support, a suitable reactive handle had to be introduced into the dihydropyrimidine. We chose to functionalize the methyl group at the C6 carbon atom by a simple bromination to afford (-)-22. Coupling of this compound to the amino functionalized support then gave the desired chiral stationary phase CSP 12 (Scheme 3-6) containing 0.20 mmol g of the selector. 

On the other hand, the direct chromatographic approach involves the use of the chiral selector either in the mobile phase, a so-called chiral mobile phase additive (CMPA), or in the stationary phase [i.e., the chiral stationary phase (CSP)]. In the latter case, the chiral selector is chemically bonded or coated or allowed to absorb onto a suitable solid support. Of course chiral selectors still can be used as CMPAs, but the approach is a very expensive one owing to the high amount of chiral selector required for the preparation of the mobile phase, and the large amount of costly chiral selector that is wasted (since there is very little chance of recovering this compound). Moreover, this approach is not successftd in the preparative separation of the enantiomers. 

As a result of certain drawbacks attached to coated silica gel CSPs, they could not be used for preparative separations. However, the literature provides some reports on the preparative chiral separations on the bonded CSPs. The bonded CSPs are suitable for use at a preparative scale but the yield is very poor due to poor loading capacities. Davankov et al. [73] packed a column with 300 g of the L-hydroxyproline-type resin and used the chiral separation at a preparative scale. 

The CSPs based on chiral crown ethers were prepared by immobilizing them on some suitable solid supports. Blasius et al. [33-35] synthesized a variety of achiral crown ethers based on ion exchangers by condensation, substitution, and polymerization reactions and were used in achiral liquid chromatography. Later, crown ethers were adsorbed on silica gel and were used to separate cations and anions [36-39]. Shinbo et al. [40] adsorbed hydrophobic CCE on silica gel and the developed CSP was used for the chiral resolution of amino acids. Kimura et al. [41-43] immobilized poly- and bis-CCEs on silica gel. Later, Iwachido et al. [44] allowed benzo-15-crown-5, benzo-18-crown-6 and benzo-21-crown-7 CCEs to react on silica gel. Of course, these types of CCE-based phases were used in liquid chromatography, but the column efficiency was very poor due to the limited choice of mobile phases. Therefore, an improvement in immobilization was realized and new methods of immobilization were developed. In this direction, CCEs were immobilized to silica gel by covalent bonds. 

In the initial discovery phase of drug research, time is the most important factor where a successful process must be rapidly identified, have a short run time, and have general applicability. As the phase of the project changes to full development, the process needs to be established and cost becomes a crucial factor. Thus, on scale up of an LC method to the preparative level (100 mg and above), a number of additional important aspects become relevant. The selection of a suitable CSP from the plethora available depends on the following factors CSP availability,loading capacity and selectivity, throughput, and mobile phase. 

---