Polysaccharide CSPs

Neutral MP = IPA-hexane, pH has no effect on the resolution MP = water-ACN, pH has no effect on the resolution 

Basic MP = IPA-hexane-DEA, IPA-hexane-TFA with pH near 2.0, ion-pair separation MP = pH 7 buffer-ACN, typical buffer is 0.5 M NaC104 

mobile phase IPA, isopropanol ACN, acetonitrile TFA, triflaoroacetic acid DEA, diethylamine. 

Many researchers have documented the effect of the mobile phase on the enantioselectivity of different racemates on polysaccharide-based CSPs. However, up to the present time, no comprehensive study aimed at identifying the association between the stmctural features present on solute and appropriate mobile phase conditions has yet been proposed. Piras et al. [33] have studied the characteristic features of about 2363 racemic molecules separated on a Chiralcel OD CSP. The mobile phase used for these racemates was compared with their structures, which are available from CHIRBASE (http //www.chirbase.u-3mrs.fr/chirbase/). The data setup was submitted to data-mining programs for molecular pattern recognition and mobile phase predictions for new cases. Some of the substmctural 

Temperature also contributes to the chiral resolution of racemic drugs on polysaccharide CSPs. Even then, only a few studies are available that deal with the influence of temperature on chiral resolution. The capacity and the separation factors may be related to temperature using the following equations  

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Fig. 4-4. The query menu form search of chiral separations of alpha-aromatic acids on any polysaccharide CSPs with a > 1.2.

Kennedy et al. [57] have described the use of an intelligent chiral resolution system using a rapid screening of conditions on polysaccharide CSP, aiming to transfer the separation to preparative LC afterward. For the analytical part of the strategy, lO-pm particles were used for the preparative section, the particles had a 20-ttm diameter. The screening performs 11 experiments on analytical columns, which are displayed in Table 3.3. Chiralpak AD, Chiralcel OD, Chiralcel OJ, and Chiralpak AS are the considered stationary phases, and they are analyzed either in POSC or NPLC mode. The... 

Another remarkable fact is that the number of published papers largely depends on the maturity of the technique and its frequency of use in the industry. For example, CE and HPLC are widely accepted techniques and take the majority of papers for their account, while CEC, a rather immature technique, and SFC, a less frequently applied one, are much less studied. Therefore, CE and HPLC can be recommended as the two techniques to be tested in a first instance, using either CD or polysaccharide CSP, respectively. 

Successful enantioseparations using polysaccharide CSPs in combination with pure polar organic mobile phases have been achieved... 

FIGURE 8 Separation of fenoprofen and mianserin enantiomers on three different polysaccharide CSPs. Mobile phase 50 mM phosphate buffer pH 2.0 with lOOmM KPF /ACN (60/40), except for mianserin on Chiralcel OJ-R 20mM borate buffer pH 9.0/ACN (60/40). 

Figure 3.1 Chemical structures of the current most successfully employed derivatised polysaccharide CSPs. (a) CHIRALPAK AD Amylose tris (3,5-dimethylphenylcarbamate) coated onto a silica support, (b) CHIRALPAK AS Amylose tris [(S)-a-methylbenzylcarbamate] coated onto a silica support, (c) CHIRALCEL OD Cellulose tris (3,5-dimethylphenylcarbamate) coated onto a silica support, (d) CHIRALCEL OJ Cellulose tris (4-methylbenzoate) coated onto a silica support.

In 2001, Franco et al. [52] reviewed an alternative method using a cross-linking technique to improve the durability of polysaccharide CSPs. Kimata et al. 

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],... 

Gaffney [89] reported the reverse order of elution of 2-phenoxypropanoic acid on Chiralcel OB CSP when different alcohols were used. In 2001 Aboul-Enein and Ali [63] observed the reverse order of elution of nebivolol enantiomers on a Chiralpak AD chiral stationary phase when ethanol and 2-propanol were used separately as the mobile phases. However, the best resolution was obtained when ethanol served as the mobile phase. The inversion of the elution may be due to the different conformation of the polysaccharide CSPs [63]. The pattern of conversion of order of elution using different ratios of ethanol and 2-propanol is shown in Figure 15. 

The countercurrent movement of a stationary phase is cumbersome in practice but it can be circumvented by an array of short columns connected by multi-position valves connected with eluent, feed, extract and raffinate, a method referred to as simulated moving bed chromatography (SMB) (Schulte and Strube, 2001). In SMB chromatography, the continuous countercurrent flow of the fluid and of the solid adsorbent is simulated by periodically switching the different inlets and outlets in the multi-column unit. Enantioselective SMB-LC has first been demonstrated for racemic 1-phenylethanol resolved on the polysaccharide CSP Chiralcel OD. In this pioneering work the principle of the method and the set-up has been depicted in a lucid educational fashion (cf. Figures 22 23) (Negawa and Shoji, 1992). 

The Daicel polysaccharide CSPs that became available in 1985 have been largely used in the field of atropisomeric heterocycles both at the analytical or semipreparative scale. Cellulose tris (p-methylbenzoate), cellulose tris(3,5-dimethylphenylcarbamate), amylose tris(3,5-dimethylphe-nyl-carbamate), and amylose tris[(S)alpha-phenethyl]carbamate) were... 

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. 

The reversal of enantiomeric elution order for the polysaccharide CSP was first reported by Okamoto et al. in 1991. They found that the reversal of the elution order of the enantiomers on a modified cellulose column was associated with changes in the mobile phase modifiers during the investigation of the direct chromatographic enantioseparation of pyriproxyfen, an insect growth regulator. If one can find such phenomena, although very rare in HPLC, it will be important to understand the reasons for this behavior and to anticipate when such inversions of elution order are likely to occur. 

Chiral resolution on polysaccharide CSPs occurs due to the different types of bondings between racemates and CSP, which will be discussed later in this chapter. Therefore, different racemate stmctures provide different types of bondings and, therefore, the different patterns of chiral recognition observed on different CSPs. Recently, Ali and Aboul-Enein [28] studied the effect of various CSPs on the chiral resolution of o,p-DDT and o,p-DDD on Chiralpak AD-R, Chiralcel OD-R and Chiralcel OJ-R. The results of this research are given in Table 7.4, which shows that the best resolution of these pesticides was obtained on Chiralpak AD-R under the normal phase mode. The chiral resolution can also be controlled by the flow rate on polysaccharide CSPs. However, there are only few studies that deal with the optimization of chiral resolution by adjusting flow rates. Yashima et al. [40] recently studied the effect of the pore size of silica gel, the... 

Scheme 7.2 A protocol for the development and optimization of mobile phases on polysaccharide CSPs under reversed phase mode. Note that this is only a brief outline of the procedure that should be followed in developing chiral resolution on polysaccharide CSPs imder reversed phase mode. Other mobile phases may also be used.

Cordyceps sinensis, known as caterpillar fungus, containing polysaccharides (CSP-1) were capable of reducing the plasma glucose level in both streptozocin and alloxan-induced diabetic mice through the stimulation of pancreatic releases of insulin (Li et al. 2006). 

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