Polysaccharide-based CSPs

Although about 20 polysaccharide-based CSPs have been commercialized and much work on enantioresolution has been carried out on these phases, it remains very difficult to predict the best CSP for the chiral resolution of a particular compound. It has been observed that most of the resolved racemic compounds contain aromatic rings or groups such as carbonyl, sulfinyl, nitro, amino, and benzoyl. However, some reports have been published on the chiral resolution of nonaromatic racemates on polysaccharide CSPs [61]. As in the case of other CSPs, polysaccharide-based CSPs do not require a certain combination of functional groups. However, only one group can afford a satisfactory separation. Presumably some chiral space (e.g., a concavity or ravine existing on a polysaccharide derivative) could enable such a separation [62],... 

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]. 

These polysaccharide-based stationary phases appear to be the most useful in organic, bio-organic and pharmaceutical analysis. Of the above-mentioned derivatives three of them, namely cellulose tris-(3,5-dimethylphenylcarbamate), amylose tris-(3,5-dimethylphenylcarbamate) and cellulose tris-(4-methylbenzoate), have very complementary properties and numerous publications have demonstrated that they have been able to achieve the chiral resolution of more than 80% of the drugs currently available on the market. " These CSPs are known under the commercial names, Chiralcel OD-H , Chiralpak AD and Chiralcel OJ , respectively (Figure 4). Their very broad enantiorecognition range is also the... 

The use of polysaccharide-based CSPs instead of protein-based CSPs often increases the peak efficiency and facilifafes faster separafions. Papini ef al. [159] recently developed a method for the enantioseparation of lorazepam and on a Chiralpak OD-R column and an enzymatic hydrolysis was used to determine the amount of the glucoronide metabolite of lorazepam present. The separation was performed in 7 min with an LOQ of 1 and 10 ng/mL for lorazepam in plasma and urine, respectively. Another relatively fast separation for chiral analysis was published by Lausecker and Eischer [188]. They developed a method for determination of the drug candidate R483 within... 

It has also been reported from circular dichroism (CD) studies [36] that polysaccharide-based CSPs can induce chirality in enantiomeric guests such as (4Z,15Z)-bilirubin-Ixoc (BR) (Fig. 5). Although not optically active, BR has two enantiomeric helical conformations maintained by six intramolecular hydrogen bonds between two carboxylic acid moieties and two pyrromethenone — NH— protons. These (R)- and (5)-helical conformers are in dynamic equilibrium in an achiral solution [37], but some optically active compounds can enantioselectively bind to BR to induce CD spectra in solution [38-40]. A significant induced CD... 

FIGURE 8 Chemical structures and chemical and trade names of most of the commonly used polysaccharide-based CSPs. 

CSPs (CTA-I and CTA-II) have inverse selectivity for Troger s base and trans-1,2-diphenyloxirane racemates. These characteristics of CTA CSPs are responsible for good chiral resolution of small cychc carbonyl compounds [42]. In 2001 Aboul-Enein and Ah [63] observed the reversed order of elution of nebivolol on a Chiralpak AD column when ethanol and 2-propanol were used separately as the mobile phases. Table 1 presents selectivity data for the polysaccharide-based CSPs. Okamoto et al. [42] observed that the introduction of a methyl group at the para position of cellulose tribenzoate results in a dramatic shift of the structural selectivity toward aromatic compounds with larger skeletons, and its selectivity was rather similar to that of cellulose tricinnamate. 

TABLE 1 Various Polysaccharide-Based Commercial CSPs... 

Analytical methods can be transferred easily to the preparative scale in polysaccharide-based CSPs, and these compounds have been used in preparative chromatography. Cellulose triacetate, which has good loading capacity, has been used for preparative separation at the industrial scale [70,85]. Moreover, the low cost of cellulose triacetate synthesis makes CTA ideal for preparative chromatography. 

TABLE 2 The Mobile Phases Most Commonly Used with Polysaccharide-Based CSPs... 

Chiral resolution on polysaccharide-based CSPs is sensitive, and therefore, the optimization of HPLC conditions on these phases is very important. The most important factors that control enantiomeric resolution are the composition, pH, and flow rate of the mobile phase and parameters, including temperature and solute structure. The optimization of these parameters on polysaccharide-based CSPs is discussed next. 

SCHEME 1 Protocol for the development and optimization of mobile phases on polysaccharide-based CSPs in the normal phase mode. This brief outline gives the procedure for developing a resolution on polysaccharide-based CSPs in the normal phase mode. However, other mobile phases may be used. 

Resolution is optimized by adjusting the buffer pH and the amount of organic modifiers. The most commonly used buffers are perchlorate, acetate, and phosphate. The protocol of the selection and optimization of the mobile phase for the enantiomeric resolution of drugs on polysaccharide-based CSPs in reversed-phase mode is presented in Scheme 2. Table 4 correlates the effects of separation conditions for neutral, acidic, and basic drugs on polysaccharide-based CSPs. From Table 4, it may be concluded that a simple mixture of water and an organic modifier will produce chiral separation of a neutral molecule because there is no... 

Resolution was effected by changing the polarity of the mobile phases. It is very interesting to note that the change in resolution with respect to mobile phase compositions varied from compound to compound. Resolution on polysaccharide-based CSPs in the reversed-phase mode was improved by adding cations and anions. For propranolol enantiomers on Chiralcel OD-R with sodium perchlorate salt-acetonitrile (60 40, v/v) as the mobile phase, in the presence of cations, the order of retention was Na+ > Li+ > K+ > NH4+ > N(C2H5)4+, while in the presence of anions this order was C104 > SCN- > I- > N03 > Br > Cl > AcO [90],... 

Chiral resolution can be controlled by regulating flow rate on polysaccharide-based CSPs. However, there are only a few studies dealing with the optimization of chiral resolution by adjusting flow rates. Aboul-Enein and Ali [93] have optimized the chiral resolution of some antifungal agents (Fig. 18) on Chiralpak... 

Chiral resolution on polysaccharide-based CSPs is due to the different types of bonding between racemates and CSP, as discussed later in this chapter. Therefore, different racemate structures provide bondings of different types, which in turn means that different patterns of chiral recognition will be observed. The effects of... 

For applications and development of chiral resolution methods, it is essential to have knowledge of the chiral resolution mechanism on polysaccharide-based CSPs. At the molecular level, this mechanism is still unclear because of difficulties associated with spectroscopic studies, as discussed earlier. Nevertheless, some experimental efforts have been made, and chiral resolution reportedly has been achieved through different types of bonding on the chiral grooves of polysaccharide-based CSPs. 

FIGURE 27 Graphical representation of the mechanism of chiral resolution on a polysaccharide-based CSP. 

Attempts have also been made to compile the results of chiral resolutions by SFC using polysaccharide-based CSPs, as summarized in Table 11. To show the nature of the SFC chromatograms, Figure 28 represents the SFC chiral resolution of ibuprofen on a Chiralpak AD CSP. Stringham et al. [144] resolved the enantiomers of four intermediates encountered in the process of developing synthetic antiviral drugs on a Chiralcel OD CSP. In another report, Blackwell... 

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