Chirality preparative enantioseparation

A selection of the most successful CSPs, chiral particles and chiral additive techniques used for analytical and preparative enantioseparation by LC is discussed in the following sections with respect to molecular recognition and experimental application. As additional sources of background information recent books and review articles2-16, which contain numerous relevant references and examine the most important aspects of the field of liquid chromatographic enantioseparation, should be consulted. 

In this chapter we will focus on the moleciilcir recognition niechanisms of the diverse chiral SOs and CSPs in combination with their spectra of applicability, but also aspects concerning the separation systems as well as on issues that are of interest for practical applications. This will include a discussion of structure resolution relationships as support for the selection of certain CSPs for a given separation problem, operation modes and mobile phase composition, stability, the ability to reverse the elution order to elute each of the enantiomers as the first peak, and loadability which is of primary importance for preparative enantioseparations. 

The early systematic preparative enantioseparations of drugs performed on cross-linked chiral polyacrylamides in low-pressure LC mode clearly showed that even this non-opti-mal technique, from the viewpoints of performance and costs, may be useful for comparative biomedical studies of enantiomers of chiral drugs. 

Table 6. Selected examples of preparative enantioseparation of chiral drugs, drug candidates and drug intermediates using closed-loop recycling chromatography (CLRC)...

Some examples of preparative enantioseparation of chiral drugs are summarized in Table 9. Overall, SMB chromatography is a powerful tool for the production of enantiomerically pure compounds on a large scale within a short phase of development. The availability of software which allows scaling-up of an analytical enantioseparation to SMB technology is a crucial advantage. The characteristics of SMB chromatography are such as to provide pure... 

R.-M. Nicoud, G. Fuchs, P Adam, M. Bailly, E. Kusters, F. D. Antia, R. Reuille and E. Sclimid, Prepar ative scale enantiosepar ation of a cliiral epoxide compar ison of liquid cliromatography and simulated moving bed adsorption technology . Chirality 5 267-271 (1993). 

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

Inspired by the separation ability of cyclic selectors such as cyclodextrins and crown ethers, Malouk s group studied the synthesis of chiral cyclophanes and their intercalation by cation exchange into a lamellar solid acid, a-zirconium phosphate aiming at the preparation of separation media based on solid inorganic-organic conjugates for simple single-plate batch enantioseparations [77-80]. 

Nieoud R. M., Euehs G., Adam P, Bailly M., Kusters E., Antia E, Reuille R., Sehmid E. (1993) Preparative Seale Enantioseparation of a Chiral Epoxide Comparison of Liquid Chromatography and Simulated Moving Bed Adsorption Teehnology, Chirality 5 267-271. 

Chiral SFC can be performed in open tubular [41,42], and packed column [43,44] modes. Packed column SFC can be further categorized into analytical, semipreparative, and preparative SFC [7, 8], Packed column SFC is more suitable for fast separations than open tubular column SFC, since a packed column generally provides low mass transfer resistance and high selectivity [45, 46], Packed column SFC also provides high sample loading capacity [27,47], which can increase sensitivity. Only packed column SFC is suitable for preparative-scale enantioseparation. This chapter will focus on chiral separation using packed column SFC in the analytical scale. 

The importance of tailoring surface chemistry was first demonstrated by three different monolithic capillary columns that were prepared by directly incorporating of the chiral monomer 2-hydroxyethyl methacrylate ([Pg.239]

Preparative chromatography is a proven technology for the separation of specialty chemicals mainly in food and pharmaceutical industries, particularly the enantioseparation of chiral compounds on chiral stationary phases. The potential of preparative chromatographic systems were further increased by the development of continuous chromatographic processes like the simulated moving bed (SMB) process. Compared to the batch column chromatography, the SMB process offers better performance in terms of productivity and solvent consumption [2]. 

The chiral SO is immobilized onto a chromatographic support (most often silica) forming a chiral stationary phase (CSP). which is operated with an achiral mobile phase. Nowadays this is the preferred mtxle for preparative and analytical LC enantioseparations, and will be discussed thoroughly in the following sections. 

Recently, a chiral monomer (a polymerizable L-valine derivative) instead of the commonly used achiral functional monomer has been employed for the preparation of a MIP-type CSP [189], In another attempt, a chiral monomer, (S)-(—)-N-methacryloyl-1-(1-naphthyl)ethylamine, and a racemic template were used for imprinting. This resulted in a MIP-type CSP, which showed enhanced enantioseparation capability for the racemate of the template (or = 1.40) compared to the corresponding MIP-type CSP which was obtained with the enantiomeric template (a = 1.18) (190]. 

The use of a convective macroporous polymer as an alternative support material instead of silica for the preparation of protein-based CSPs has successfully been demonstrated by Hofstetter et al. [221]. Enantioseparation was performed using a polymeric flow-through-type chromatographic support (POROS-EP, 20 pm polymer particles with epoxy functionalities) and covalently bound BSA as chiral SO. Using flow rates of up to 10 ml/min, rapid enantiomer separation of acidic compounds, including a variety of amino acid derivatives and drugs, could be achieved within a few minutes at medium efficiencies, typical for protein chiral stationary phases (Fig. 9.13). 

In another approach, reactive monodisperse porous poly(chloromethylstyrene-co-styrene-co-divinylbenzene) beads have been employed for the preparation of chiral HPLC packings. Thus, reactive chloromethyl groups were derivatized to yield amino functionalized beads onto which both rt-basic and rt-acidic type chiral. selectors, (/ )- -(l-naphthyl)ethylamine and (/ )-A -(3.5-dinitrobenzoyl)phenylglycine, respectively, were attached. The resulting chiral particles were chromatographically tested for the enantioseparation of model SAs. Despite the presence of strongly competitive it-TT-binding sites of the styrenic support these chirally modified beads afforded baseline separations for 2,2,2-trifluoro-l-(9-anthryl) ethanol and Af-(3.5-dinitro-benzoyl) leucine enantiomers, respectively [369. ... 

---