Enantioselective separation method

Many times an analyte must be derivatized to improve detection. When this derivatization takes place is incredibly important, especially in regards to chiral separations. Papers cited in this chapter employ both precolumn and postcolumn derivatization. Since postcolumn derivatization takes place after the enantiomeric separation it does not change the way the analyte separates on the chiral stationary phase. This prevents the need for development of a new chiral separation method for the derivatized analyte. A chiral analyte that has been derivatized before the enantiomeric separation may not interact with the chiral stationary phase in the same manner as the underivatized analyte. This change in interactions can cause a decrease or increase in the enantioselectivity. A decrease in enantioselectivity can result when precolumn derivatization modifies the same functional groups that contribute to enantioselectivity. For example, chiral crown ethers can no longer separate amino acids that have a derivatized amine group because the protonated primary amine is... 

Substrate 2 has also been used as a test substrate HPLC separation methods exist for 2, while ee-value determination of 1 is more difficult [6, 17]. Reflecting the general recent interest in the hydrogenation of unfunctionalized olefins, the past few years have seen the publication of a number of results for this substrate [15, 18-26]. The highest enantioselectivities were achieved using catalysts 12b [22] and 14a [26],... 

Obviously, the monolithic material may serve its purpose only if provided with a suitable surface chemistry, which depends on the desired application. For example, hydrophobic moieties are required for reversed phase chromatography, ionizable groups must be present for separation in the ion-exchange mode, and chiral functionalities are the prerequisite for enantioselective separations. Several methods can be used to prepare monolithic columns with a wide variety of surface chemistries. 

Conversion of summarized values (e.g. mean ER cr to the equivalent EF tr) can lead to substantial discrepancies, and should be avoided [109]. In addition, the conventions used in describing ERs and EFs may differ between studies and analytical methods. For example, enantioselective separations on different stationary phases may result in reversal of elution order and lead to different values if elution orders are not known. In this discussion, EFs are used to the extent possible, and both EF and ER are defined using using Equations (4.1) and (4.3), respectively, for those analytes for which the elution order is known unless otherwise indicated. Otherwise, these metrics are defined using Equations (4.2) and (4.4) on the specified column. 

In this context, enantioselective analysis and in particular chromatographic separation methods have gained a vital interest and central importance for the following reasons. 

Tesarova and Bosakova [58] proposed an HPLC method for the enantio-selective separation of some phenothiazine and benzodiazepine derivatives on six different chiral stationary phases (CSPs). These selected CSPs, with respect to the structure of the separated compounds, were either based on b-CD chiral selectors (underivatized (J>-CD and hydroxypropyl ether (3-CD) or on macrocyclic antibiotics (vancomycin, teicoplanin, teicoplanin aglycon and ristocetin A). Measurements were carried out in a reversed-phase separation mode. The influence of mobile phase composition on retention and enantio-selective separation was studied. Enantioselective separation of phenothiazine derivatives, including levopromazine (LPZ), promethazine and thioridazine, was relatively difficult to achieve, but it was at least partly successful with both types of CSPs used in this work (CD-based and glycopeptide-based CSP), except for levomepromazine for which only the [CCD-based CSP was suitable. 

Electrophoretic methods are widely used alternatives for the analytical determination of the enantiomeric purity of chiral compounds [194]. Due to the high elTi-ciency of capillary electrophoresis, separations can be achieved even when very low selectivities are observed. At a preparative scale, these methods are well established for the purification of proteins and cells [195] but there is very little published on enantioselective separations. Only recently, some interest in chiral preparative applications has been manifested. Separation of the enantiomers ofterbu-taline [196] and piperoxan [197] have been reported by classical gel electrophoresis using sulfated cyclodextrin as a chiral additive, while the separation of the enantiomers of methadone could be successfully achieved by using free-fluid isotachophoresis [198] and by applying a process called interval-flow electrophoresis [199]. 

Another important problem is the enantiopurity of chiral pharmaceuticals and many other compounds with biological activity. Therefore, there is a gi eat interest in developing methods that can help in stereoisomer separation and improve the stereoselectivity of separation methods. In this case, a specific chiral environment should be created to ensure enantioselectivity of separation. 

In this section, one-pot preparations of optically active compounds by a combination of solid-state reaction and enantioselective inclusion complexation in a water suspension medium are described. In order to establish the suspension procedure as a general enantiomeric separation method, enantiomeric separations of various compounds by complexation in hexane and water suspension media were studied. Furthermore, by combining enantioselective inclusion complexation with a chiral host in the solid state with distillation, a fascinating enantiomeric separation method by fractional distillation was established. 

A chiral CE separation method was developed for ephe-drine, norephedrine, synephrine, and epinephrine using a 20-mM phosphate buffer with pH 3.0 containing 0.4% w/v S-P-CD as CS. Peng et al. performed the separation in 50-pm i.d. capillaries with total and effective lengths of 60 and 50 cm, respectively. The effect of the temperature (20-60°C) on the CD enantioselectivity was investigated. An increase in temperature decreased the enantioselectivity toward ephedrine and norephedrine but increased that toward synephedrine and epinephrine. This observation suggests that the enantioseparation of ephedrine and norephedrine was an enthalpy-driven process, while for synephrine and epinephrine, it was entropy-driven. 

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