Enantiomers, separation techniques

Effusive beam technique, 157-158 Electron bombardment flow radiolysis, 238 Electrospray ionization and ionic clusters, 168 Enantiomers, separation techniques, 154-155 Enantioselectivity of enzymes, 148 Enthalpy-entropy compensation plots, 261 Enthalpy of activation, and quantum tunneling, 67, 70-71... 

Some other optical resolution procedures of rac-BNO (14a) by complexation with various chiral ammonium salts are summarized in the section 6 of this chapter as an example of the progress on novel enantiomer separation technique. 

CMPA-based enantiomer separation techniques appear attractive from the viewpoints of conceptual simplicity and flexibility as they operate with relatively inexpensive achiral stationary phases and easy-to-prepare chiral mobile phases. In practice, the development of CMPA-based analytical assays may pose a considerable challenge for a number of reasons. In the course of the development and optimization of CMPA-based enantiomer separations a set of conditions must be identified that favor CMPA-analyte interactions and simultaneously maximize... 

CDs have a longstanding tradition as versatile host systems in the molecular recognition field, and have seen extensive use in various enantiomer separation techniques [206, 207], including GC, LC, CE and CEC. The widespread applications of CDs for enantiomer separation of chiral drug compounds have been detailed in recent reviews [55, 208]. 

Traditionally, chiral separations have been considered among the most difficult of all separations. Conventional separation techniques, such as distillation, Hquid—Hquid extraction, or even some forms of chromatography, are usually based on differences in analyte solubiUties or vapor pressures. However, in an achiral environment, enantiomers or optical isomers have identical physical and chemical properties. The general approach, then, is to create a "chiral environment" to achieve the desired chiral separation and requires chiral analyte—chiral selector interactions with more specificity than is obtainable with conventional techniques. 

Liquid-liquid extraction is a basic process already applied as a large-scale method. Usually, it does not require highly sophisticated devices, being very attractive for the preparative-scale separation of enantiomers. In this case, a chiral selector must be added to one of the liquid phases. This principle is common to some of the separation techniques described previously, such as CCC, CPC or supported-liquid membranes. In all of these, partition of the enantiomers of a mixture takes place thanks to their different affinity for the chiral additive in a given system of solvents. 

Among the existing separation techniques, some - due to their intrinsic characteristics - are more adapted than others to processing large amounts of material. Such processes, which already exist at industrial level, can be considered in order to perform an enantioselective separation. This is the case for techniques such as distillation and foam flotation, both of which constitute well-known techniques that can be adapted to the separation of enantiomers. The involvement of a chiral selector can be the clue which changes a nonstereoselective process into an enantioselective one. Clearly, this selector must be adapted to the characteristics and limitations of the process itself. 

All enantioselective separation techniques are based on submitting the enantiomeric mixture to be resolved to a chiral environment. This environment is usually created by the presence of a chiral selector able to interact with both enantiomers of the mixture, albeit with different affinities. These differences in the enantiomer-selector association will finally result in the separation that is sought. 

Enantiomeric separations have become increasingly important, especially in the pharmaceutical and agricultural industries as optical isomers often possess different biological properties. The analysis and preparation of a pure enantiomer usually involves its resolution from the antipode. Among all the chiral separation techniques, HPLC has proven to be the most convenient, reproducible and widely applicable method. Most of the HPLC methods employ a chiral selector as the chiral stationary phase (CSP). 

Despite its widespread application [31,32], the kinetic resolution has two major drawbacks (i) the maximum theoretical yield is 50% owing to the consumption of only one enantiomer, (ii) the separation of the product and the remaining starting material may be laborious. The separation is usually carried out by chromatography, which is inefficient on a large scale, and several alternative methods have been developed (Figure 6.2). For example, when a cyclic anhydride is the acyl donor in an esterification reaction, the water-soluble monoester monoacid is separable by extraction with an aqueous alkaline solution [33,34]. Also, fiuorous phase separation techniques have been combined with enzymatic kinetic resolutions [35]. To overcome the 50% yield limitation, one of the enantiomers may, in some cases, be racemized and resubmitted to the resolution procedure. 

In simple experiments, particulate silica-supported CSPs having various cin-chonan carbamate selectors immobilized to the surface were employed in an enantioselective liquid-solid batch extraction process for the enantioselective enrichment of the weak binding enantiomer of amino acid derivatives in the liquid phase (methanol-0.1M ammonium acetate buffer pH 6) and the stronger binding enantiomer in the solid phase [64]. For example, when a CSP with the 6>-9-(tcrt-butylcarbamoyl)-6 -neopentoxy-cinchonidine selector was employed at an about 10-fold molar excess as related to the DNB-Leu selectand which was dissolved as a racemate in the liquid phase specified earlier, an enantiomeric excess of 89% could be measured in the supernatant after a single extraction step (i.e., a single equilibration step). This corresponds to an enantioselectivity factor of 17.7 (a-value in HPLC amounted to 31.7). Such a batch extraction method could serve as enrichment technique in hybrid processes such as in combination with, for example, crystallization. In the presented study, it was however used for screening of the enantiomer separation power of a series of CSPs. 

In this way, we aim to give an overview of what can be used as a separation technique and which conditions will most likely give an (beginning of) enantiomer separation after a first screening. Chiral method development starter kits are also available and evaluated in some papers [2], but we will not focus on this kind of applications. 

CSPs and chiral mobile phase additives have also been used in the separation of amino acid enantiomers. Another technique that should be mentioned is an analysis system employing column-switching. D-and L- amino acids are first isolated as the racemic mixture by reverse-phase HPLC. The isolated fractions are introduced to a second column (a CSP or a mobile phase containing a chiral selector) for separation of enantiomers. Long et al. (2001) applied this technique to the determination of D- and L-Asp in cell culture medium, within cells and in rat blood. 

Although the majority of current pharmacopoeial methods describe the use of specific optical rotation ([a]) for the determination of stereoisomer identity and/or content, separation techniques such as HPLC and CE are now the most frequently applied technologies during drug development and for new pharmaceutical products entering the market. Indeed, Supplementary Chapter K in Volume IV of the 2004 British Pharmacopoeia (BP) deals with this specific issue [22]. It states that in future when a monograph describes an enantiomer it will include both a test for identity ([a]) and a test to control stereoisomeric purity... 

---