Chiral separation mixtures

Capillary Electrophoresis. Capillary electrophoresis (ce) or capillary 2one electrophoresis (c2e), a relatively recent addition to the arsenal of analytical techniques (20,21), has also been demonstrated as a powerful chiral separation method. Its high resolution capabiUty and lower sample loading relative to hplc makes it ideal for the separation of minute amounts of components in complex biological mixtures (22,23). 

The primary disadvantage of the conjugate addition approach is the necessity of performing two chiral operations (resolution or asymmetric synthesis) ia order to obtain exclusively the stereochemicaHy desired end product. However, the advent of enzymatic resolutions and stereoselective reduciag agents has resulted ia new methods to efficiently produce chiral enones and CO-chain synthons, respectively (see Enzymes, industrial Enzymes in ORGANIC synthesis). Eor example, treatment of the racemic hydroxy enone (70) with commercially available porciae pancreatic Hpase (PPL) ia vinyl acetate gave a separable mixture of (5)-hydroxyenone (71) and (R)-acetate (72) with enantiomeric excess (ee) of 90% or better (204). 

From the pioneering studies of Ito et al. [117], CCC has been mainly used for the separation and purification of natural products, where it has found a large number of applications [114, 116, 118, 119]. Moreover, the potential of this technique for preparative purposes can be also applied to chiral separations. The resolution of enantiomers can be simply envisaged by addition of a chiral selector to the stationary liquid phase. The mixture of enantiomers would come into contact with this liquid CSP, and enantiodiscrimination might be achieved. However, as yet few examples have been described in the literature. 

However, it was not until the beginning of 1994 that a rapid (<1.5 h) total resolution of two pairs of racemic amino acid derivatives with a CPC device was published [124]. The chiral selector was A-dodecanoyl-L-proline-3,5-dimethylanilide (1) and the system of solvents used was constituted by a mixture of heptane/ethyl acetate/methanol/water (3 1 3 1). Although the amounts of sample resolved were small (2 ml of a 10 inM solution of the amino acid derivatives), this separation demonstrated the feasibility and the potential of the technique for chiral separations. Thus, a number of publications appeared subsequently. Firstly, the same chiral selector was utilized for the resolution of 1 g of ( )-A-(3,5-dinitrobenzoyl)leucine with a modified system of solvents, where the substitution of water by an acidified solution... 

In general, a liquid membrane for chiral separation contains an enantiospecific carrier which selectively forms a complex with one of the enantiomers of a racemic mixture at the feed side, and transports it across the membrane, where it is released into the receptor phase (Fig. 5-1). 

Lin et al. [95] used capillary electrophoresis with dual cyclodextrin systems for the enantiomer separation of miconazole. A cyclodextrin-modified micellar capillary electrophoretic method was developed using mixture of /i-cyclodextrins and mono-3-0-phenylcarbamoyl-/j-cyclodextrin as chiral additives for the chiral separation of miconazole with the dual cyclodextrins systems. The enantiomers were resolved using a running buffer of 50 mmol/L borate pH 9.5 containing 15 mmol/L jS-cyclodextrin and 15 mmol/L mono-3-<9-phcnylcarbamoyl-/j-cyclodextrin containing 50 mmol/L sodium dodecyl sulfate and 1 mol/L urea. A study of the respective influence of the /i-cyclodcxtrin and the mono-3-(9-phenylcarbamoyl-/i-cyclodextrin concentration was performed to determine the optical conditions with respect to the resolution. Good repeatability of the method was obtained. 

As yet, the number of applications is limited but is likely to grow as instrumentation, mostly based on existing CE systems, and columns are improved and the theory of CEC develops. Current examples include mixtures of polyaromatic hydrocarbons, peptides, proteins, DNA fragments, pharmaceuticals and dyes. Chiral separations are possible using chiral stationary phases or by the addition of cyclodextrins to the buffer (p. 179). In theory, the very high efficiencies attainable in CEC mean high peak capacities and therefore the possibility of separating complex mixtures of hundreds of... 

Not only chiral separations have been achieved with Mi-stationary phases. It has also been demonstrated that the MIP could distinguish between ortho- and para-isomers of carbohydrate derivatives. For example, a polymer imprinted with o-aminophenyl tetraacetyl P-D-galactoside was used to analyze a mixture of p-and o-aminophenyl tetraacetyl P-D-galactoside. As expected, the imprinted ortho analyte eluted after the non-imprinted para component see Fig. 5. Although baseline separation was not obtained, a separation factor of a = 1.51 was observed [19]. 

Geiser et al. [50,51] illustrated the screening of different chiral stationary phases and the separation of highly polar amine hydrochlorides using EEL methanol/C02 mixtures and the columns, Chiralpak-AD-H, Chiralpak-AS. This method is advantageous because no acid or base additive was required to achieve base line separation of the racemates and conversion to free base form for enantiomer separation was not required. Preparative-scale separations of the amine-hydrochloride were accomplished using similar mobile phase conditions [51], Furthermore, this is believed to be the first chiral separation of highly polar solutes without the addition of acid or base additive to effect the separation. 

In general, charged CDs have shown superior discrimination abilities, especially the highly-sulfated (HS-CDs) ones. Furthermore, the separation mechanism is altered by the introduction of electrostatic interactions. Finally, the use of chiral selectors carrying a charge opposite to that of the analytes can greatly improve the mobility difference between the two enantiomers. The use of mixtures of CDs in chiral separation is also possible. ... 

The main drawback in chiral separation methods using derivatized CDs is that these selectors are mainly available as complex mixtures that contain a large number of isomers differing in their degree of substitution, which may result in batch-to-batch selectivity differences.The use of pure single enantiomers or very reproducible mixtures is thus required to obtain reproducible and robust methods. 

P. A. (2000). Generic approach to chiral separations chiral capillary electrophoresis with ternary cyclodextrin mixtures. /. Microcol. Sep. 12, 568 — 576. 

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