Cellulosic CSP

Microcrystalline cellulose II, like the cellulose I form, does not possess enough mechanical strength to be used as an HPLC-CSP Although its triacetate derivative, CTA-II, can be used as an HPLC support, it is not as useful a CSP as CTA-I (47). Ichida et al. (48) were able to improve on the properties of derivatized cellulose II by depolymerization, followed by coating on macroporous silica gel. In this manner, they created a number of new CSPs, the cellulosic CSPs, based on ester and carbamate derivatives of cellulose (see Table 2). 

The chiral recognition mechanisms that operate on the cellulosic CSPs have been studied by Wainer and co-workers (49,50). In one study, the chiral recognition of amides on the OB CSP was examined (49), The results indicated that the solute/CSP complexes formed between the OB CSP and the amide solutes were based on attractive hydrogen bonding, ti-tt, and dipole-dipole interactions. Chiral recognition within the solute/CSP complex was due to the differential inclusion (or fit) of the solute into a chiral cavity or ravine on the CSP However, studies with aromatic alcohols... 

The type 11 CSPs have been used to resolve a wide variety of pharmacologically active compounds, both with and without derivatization (9-11). Some of the compounds that have been resolved without derivatization are hexobarbital (OA-CSP) (51), glutethimide (OB, OC, OK) (51), warfarin (OB, OC) (51), compounds containing a chiral sulfur atom (OB, OC) (52), verapamil (AD) (53), propranolol (OD) (54), and flurbiprofen (AD) (55), The resolution of the latter two compounds, propranolol, an ot, p-amino-alcohd, and flurbiprofen, a carboxylic acid, is representative of new applications for the cellulosic CSPs that are primarily used with hexane alcohol mobile phases. In the case of propranolol, the mobile phase was modified with N,N-dimethyIoctyl amine, and when flurbiprofen was chromatographed, trifluoroacetic acid was used as the modifier. 

The cellulosic CSPs are most often used with nonpolar mobile phases composed of hexane and a polar modifier such as 2-propanol. These mobile phases are used to maximize the attractive interactions between solute and CSE Since these interactions contain a polar component, an increase in the polarity of the mobile phase, for example, a lager alcohol content, will reduce the retention and vice versa. These phases have also been used with aqueous mobile phases and a new leversed-phase version of the OD CSP is currrently being marketed (56). 

The cellulosic CSPs have been used to stereochemically resolve a vast number of compounds (9-11). However, most of the pharmaceutical applications involve bulk substances and only a relatively small number in vivo or in vitro studies. Some examples of the use of cellulosic CSPs in metabolic and pharmacokinetic studies are the following ... 

Another example of curvature of van t Hoff plots relates to irreversible changes in the conformation of carbamate-derivatized amylose and cellulose CSP, which was observed for the normal-phase separation of the enantiomers of a dihydropyrimidinone acid and methyl ester. The apparent conformational change was thermally induced and depended upon the polar component of the mobile phase. The irreversible change in the conformation of... 

In this context, the enantiomeric pair containing the eutomer of cyclothiazide can be resolved by HPLC on cellulose-derived coated CSPs. Nevertheless, the poor solubility of this compound in solvents compatible with this type of support makes this separation difficult at preparative scale. This operation was achieved with a cellulose carbamate fixed on allylsilica gel using a mixture of toluene/acetone as a mobile phase [59]. 

With regard to the resolution of enantiomers, some applications can be found with modified silica gel supports. Thus, a Pirkle-type CSP was used for the separation of 200 mg of a racemic benzodiazepinone [75]. Also tris-(3,5-dimethylphenyl)carba-mate of cellulose coated on silica [91, 92] was applied successfully to the resolution of the enantiomers of 2-phenoxypropionic acid and to oxprenolol, alprenolol, propranolol among other basic drugs. However, the low efficiency of this technique and the relative high price of the CSPs limits its use to the resolution of milligram range of sample. 

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]. 

The purpose of this study is only intended to illustrate and evaluate the decision tree approach for CSP prediction using as attributes the 166 molecular keys publicly available in ISIS. This assay was carried out a CHIRBASE file of 3000 molecular structures corresponding to a list of samples resolved with an a value superior to 1.8. For each solute, we have picked in CHIRBASE the traded CSP providing the highest enantioselectivity. This procedure leads to a total selection of 18 CSPs commercially available under the following names Chiralpak AD [28], Chiral-AGP [40], Chiralpak AS [28], Resolvosil BSA-7 [41], Chiral-CBH [40], CTA-I (microcrystalline cellulose triacetate) [42], Chirobiotic T [43], Crownpak CR(-i-) [28], Cyclobond I [43], DNB-Leucine covalent [29], DNB-Phenylglycine covalent [29], Chiralcel OB [28], Chiralcel OD [28], Chiralcel OJ [28], Chiralpak OT(-i-) [28], Ultron-ES-OVM [44], Whelk-0 1 [29], (/ ,/ )-(3-Gem 1 [29]. 

A precursor in the synthesis of a promising calcium sensitizing agent from E. Merck [33], a chiral thiadiazin-2-one EMD 53986, 3,6-Dihydro-5-[l,2,3,4-tetrahy-dro-6-quinolyl]-6-methyl-2H-l,3,4-thiadiazin-2-one [26]. The study was performed using Celluspher , a CSP prepared from cellulose tri(p-methylbenzoate) according to a patent from Ciba-Geigy [34]. The spherical particles had a mean particle diameter of 20 3 pm and the mobile phase was pure methanol. 

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]. 

The selectivity of another cellulose-based CSP, Chiralcel OJ, has also been examined in SFC [60]. Separations of racemic drugs such as benoxaprofen, temazepam, and mephobarbital were obtained. Acetonitrile proved to be a better modifier than methanol for some of the compounds investigated. The four optical isomers of a calcium channel blocker were resolved by Siret et al. on the Chiralcel OJ CSP [30]. In LC, two CSPs were required to perform the same separation. 

Derivatized amylose is the basis for the Chiralpak AD CSP. This CSP has been utilized for the resolution of ibuprofen and flurbiprofen, as well as other members of the family of nonsteroidal inflammatory drugs (NSAIDs) [39, 61]. Ibuprofen was not resolved on the Chiralpak AD CSP in LC. Pressure-related effects on stereoselectivity were observed by Bargmann-Leyder et al. on a Chiralpak AD CSP [58]. No corresponding effect of pressure on selectivity was observed with a Chiralcel OD CSP. The authors speculated that the helical conformation of the amylose-based CSP is more flexible than that of the cellulose-based CSP. 

Starting from a collection of samples remarkably well resolved (alpha > 6) on Chiralcel OD (Cellulose tris(3,5-dimethylphenylcarbamate) coated on aminopropyl silica), a putative three-point enantiophore for binding to CSP was derived (Fig. 4-10). This enantiophore query was used to search (CFS 3D search) within a list comprising 4203 compounds tested on Chiralcel OD. From this search domain of CHIRBASE 3D, 191 structures were found to match the enantiophore. 

Following these announcements, the first wave of publications addressing the use of SMB for the manufacture of pharmaceutical products of interest was published. The separation of a chiral hetrazepine [26], WEB 2170 6-(2-chlorophenyl)-8-9-di-hydro-l-methyl-8-[(morpholinyl)-carbonyl]-4H,7H-cyclopenta[4,5]-thieno[3,2-f][l,2,4]triazolo[4,3-a][l,4]diazepine. WEB 2170 is a chiral hetrazepine from Boehringer-Ingelheim. The enantioseparation of WEB 2170 was performed using cellulose triacetate (CTA) from Merck (Darmstadt) as the CSP and with pure methanol as eluent. 

The chiral recognition ability of a CSP is quantitatively evaluated from the results of chromatographic separation of enantiomers. Figure 3.4 shows a chromatogram of the resolution of benzoin (19) on cellulose tris(3,5-dimethylphenylcarbamate). The (+)-isomer elutes first followed by the (—)-isomer complete baseline separation is achieved. The results of the separation can be expressed by three parameters—capacity factors (k1), separation factor (a), and resolution factor (Rs)—defined as follows ... 

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