Polysaccharide-coated chiral separation

Examples with other Pirkle-type CSPs have also been described [139, 140]. In relation to polysaccharides coated onto silica gel, they have shown long-term stability in this operation mode [141, 142], and thus are also potentially good chiral selectors for preparative SFC [21]. In that context, the separation of racemic gliben-clamide analogues (7, Fig. 1-3) on cellulose- and amylose-derived CSPs was described [143]. 

By clicking the appropriate buttons on the form, the user can combine molecular structure queries of sample, CSP and solvent, using operators AND, OR, NOT with data queries in one search. A query for the search of chiral separations of alpha-aromatic acids on any polysaccharide phases coated on silica gel providing an alpha value superior to 1.2 is shown in Eig. 4-4. 

As in analytical chiral LC, Daicel derivatised polysaccharide CSPs are the most frequently used materials in preparative scale chiral separations. Recently CSPs have been prepared in which derivatised polysaccharides have been covalently bonded to the solid support rather than coated on as in the Diacel materials. The rationale for this is that it is advisable to reduce the chance of the chiral selector leeching off the column in trace amounts to contaminate samples of chiral dmgs isolated by production scale LC. However, the extent to which the Daicel coated CSPs are now used in production scale chiral LC would tend to suggest that such a problem, if it exists, is not a very significant risk. 

Cass et al. [66] used a polysaccharide-based column on multimodal elution for the separation of the enantiomers of omeprazole in human plasma. Amylose tris (3,5-dimethylphenylcarbamate) coated onto APS-Hypersil (5 /im particle size and 120 A pore size) was used under normal, reversed-phase, and polar-organic conditions for the enantioseparation of six racemates of different classes. The chiral stationary phase was not altered when going from one mobile phase to another. All compounds were enantioresolved within the elution modes with excellent selectivity factor. The separation of the enantiomers of omeprazole in human plasma in the polar-organic mode of elution is described. 

However, the (undisclosed) proprietary immobilization process appears to modify the enantiomer separation characteristics as compared to the coated versions [145, 146]. Ghanem et al. compared the chiral recognition profile of a coated CHIRALPAK AD CSPs with that of the immobilized version, employing hexane/ 2-propanol containing TEA (0.1%) as mobile phase [145]. They reported superior enantiomer separation for the coated CSP, with some analytes failing to resolve on the immobilized version. These differences in the enantiomer separation capacity of coated and immobilized polysaccharide-type CSPs may complicate attempts at direct method transfer. 

Modern polysaccharide columns are based on cellulose or amylose derivatives coated onto silica. Chiral discrimination and applications have been extensively documented, but the mechanism of chiral discrimination is not yet fully understood. Whereas numerous phases are available within this subset, orthogonality can generally be obtained from a set of three or four columns as a first approach to method development. A typical choice of columns would be to try a set of different amylase (Chiralpak AD and AS) and cellulose (Chiralcel OD or OJ) columns and defer more extensive method development to the subset of samples not separated by these columns. The columns specified are run in the normal-phase mode and, accordingly, mobile phases are typically mixtures of hexane with small amounts of isopropanol or ethanol to control retention. However, selectivity is changed by different polar modifiers. Tailing may be minimized by the addition of 10-50 mM trifluoroacetic acid (TFA) or triethylamine (TEA). Analogue of the columns specified (AD-R, AS-R, OD-R, and OD-J) are available for reversed-phase separation. 

The derivatives are formed by reacting the polysaccharides with the substituted isocyanates in pyridine and the desired material is isolated as the methanol-insoluble fraction. Prior to coating, the silica particles are treated with 3 -aminopropyltriethoxysilane using the standard procedure. The polysaccharides derivatives are then dissolved in tetrahydrofuran and adsorbed on the aminopropylsilica. The stationary phase loading should be about 25% w/w. The derivatized cellulose and amylose have been used extensively in the separation of chiral drugs of all types. 

Examples of the enantioselective stationary phases that have been developed for these purposes include both monomer coated columns [Pirkle columns (2), cyclodextrin columns (3), macrocyclic antibiotic columns (4), etc.] and polymer coated enantioselective stationary phases [polysaccharide (5,6), polyacrylamide (7)]. Although a thorough evaluation of all enantioselective columns is impossible, the commonly used ones do seem to have their limitations. It is fair to say that despite these advances, the chiral resolution of racemic materials remains a major challenge. Resolution of a racemic sample is still a time-consuming, trial-and-error approach, often requiring experimentation with many expensive enantioselective columns. Moreover, typical separation factors achieved are less than 1.5. 

Chiral polymers can be obtained in two different ways. The use of a chiral catalyst during polymerization can lead to helical structures, as observed in polysaccharides. The other synthesis path uses chiral monomers, which are polymerized to give a chiral polymer capable of folding to a supramolecular stmcture [20]. For application in HPLC, all of these polymers must be coated onto silica, since they are imable to withstand the high pressures encountered in HPLC. Currently, chiral stationary phases based on polyacrylates or polymethacrylates play only a minor role. Chirasphere (Merck) is derived from a silica material coated with poly(N-acryloyl-(S)-phenylalanine ethyl ester) and can be used for the separation of P-blockers in the normal-phase mode. The chiral polymethacrylates Chiralpak OP and Chiralpak OT (Daicel) are able to separate aromatic compounds into their enantiomers. 

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