Chiral separation strategies

Matthijs, N., Maftouh, M., Vander Heyden, Y. Chiral separation strategy in polar organic solvent chromatography and performance comparison with normal-phase liquid and supercritical-fluid chromatography. J. Sep. Sci. 2006, 29, 1353-1362. 

Ates H, Mangelings D, Vander Heyden Y. Fast generic chiral separation strategies using electrophoretic and liquid chromatographic techniques. J. Pharm. Biomed Anal. 2008 48 288-294. 

A variety of strategies have been devised to obtain chiral separations. Although the focus of this article is on chromatographicaHy based chiral separations, other methods include crystallisation and stereospecific ensymatic-catalysed synthesis or degradation. In crystallisation methods, racemic chiral ions are typically resolved by the addition of an optically pure counterion, thus forming diastereomeric complexes. 

In considering the applicability of preparative classical electrophoretic methods to chiral separations, it should be noted that practitioners in the art of classical electrophoresis have been particularly inventive in designing novel separation strategies. For instance, pH, ionic strength and density gradients have all been used. Isoelectric focusing and isotachophoresis are well-established separation modes in classical electrophoresis and are also being implemented in CE separations [7, 8]. These trends are also reflected in the preparative electrophoretic approaches discussed here. 

Chiral method development is often referred to as one of the most difficult fields in terms of development time. Interaction with a chiral selector is required to achieve separation but the enantioselectivity of a given selector for a given chiral molecule is a priori unknown. For some compounds, it can take several days to find suitable separation conditions when using sequential approaches. Therefore, industry most often defines generic separation strategies, which are often kept internally or are... 

The short-end injection was also used in a paper by Perrin et al. [28]. They saw a very high chiral recognition capability of highly sulfated cyclodextrins (HS-CD). Using a test set of 27 amino acid derivatives, the application of HS-a-CD, HS-fl-CD, and HS-y-CD in a 5% w/v concentration allowed the separation of 26 compounds, of which 22 had a Rs > 2. From their experiments, a screening and optimization scheme was derived (Figure 3.3), and based on this scheme, a separation strategy was defined... 

Most screening approaches and separation strategies for chiral separations were developed in CE. The CDs and their derivatives are then undoubtedly the most used and preferred chiral selectors to enable chiral separations. HS-CDs seem very popular in this field, and exhibit a very broad enantioselectivity, making them suitable for generic screening approaches. 

Mangelings, D., Discry, J., Maftouh, M., Massart, D.L., Vander Heyden, Y. Strategy for the chiral separation of nonacidic pharmaceuticals using capillary electrochromatography. Electrophoresis 2005, 26, 3930-3941. 

Separation strategies are interesting for their ability to evalnate the enantioselectivity of a given system towards a chiral molecnle. They are of practical use mainly when the system (chiral selector) nsed shows broad enantiorecognition abilities. For this reason, in the separation of pharmaceuticals, strategies using CDs as chiral selector are most interesting. Several factors (see below) may affect the separation of enantiomers which makes the development of a separation far from evident. 

FIGURE 9 Chiral separation of acebutolol using (a) the NPLC and (b) the RPLC strategies. Experimental conditions. NPLC stationary phase Chiralpak AD-H, mobile-phase composition hexane/EtOH/DEA (90/10/0.1), temperature 20°C and flow rate 1.0 mL/min RPLC Chiralcel AD-RH, 20mM borate buffer pH 9.O/CH3CN (60/40 v/v), room temperature, flow rate = 0.5 mL/min. 

The molecular imprinting strategy can be applied for the recognition of different kinds of templates from small organic molecules to biomacromolecules as proteins. Some examples of separations investigated with MIP monoliths in CEC and LC are shown in Table 2. The influence of the imprinted monolithic phase preparation procedure and of the separation conditions on the selectivity and chromatographic efficiency have been widely studied [154, 157, 161, 166, 167, 192]. The performance of imprinted monoliths as chromatographic stationary phase has also been compared to that of the traditional bulk polymer packed column [149, 160]. It was shown that the monolithic phases yielded faster analyses and improved chiral separations. 

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