Chiral stationary phases examples

Twenty-eight chiral compounds were separated from their enantiomers by HPLC on a teicoplanin chiral stationary phase. Figure 8-12 shows some of the structures contained in the data set. This is a very complex stationary phase and modeling of the possible interactions with the analytes is impracticable. In such a situation, learning from known examples seemed more appropriate, and the chirality code looked quite appealing for representing such data. 

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. 

Derivatization of a racemic compound with an achiral group may play an important role in the analysis of a chiral compound (Fig. 7-15). In the case of substances with low or no UV-activity, the compounds can be rendered detectable by introducing an UV-absorbing or fluorescent group. If the racemate itself shows selectivity on a chiral stationary phase (CSP), this method can be applied to reduce the limit of detection. Examples have been reported in the literature, especially for the derivatization of amino acids which are difficult to detect using UV detection. Different derivatization strategies can be applied (Fig. 7-16). 

Many racemic mixtures can be separated by ordinary reverse phase columns by adding a suitable chiral reagent to the mobile phase. If the material is adsorbed strongly on the stationary phase then selectivity will reside in the stationary phase, if the reagent is predominantly in the mobile phase then the chiral selectivity will remain in the mobile phase. Examples of some suitable additives are camphor sulphonic acid (10) and quinine (11). Chiral selectivity can also be achieved by bonding chirally selective compounds to silica in much the same way as a reverse phase. A example of this type of chiral stationary phase is afforded by the cyclodextrins. 

A number of specialised stationary phases have been developed for the separation of chiral compounds. They are known as chiral stationary phases (CSPs) and consist of chiral molecules, usually bonded to microparticulate silica. The mechanism by which such CSPs discriminate between enantiomers (their chiral recognition, or enantioselectivity) is a matter of some debate, but it is known that a number of competing interactions can be involved. Columns packed with CSPs have recently become available commercially. They are some three to five times more expensive than conventional hplc columns, and some types can be used only with a restricted range of mobile phases. Some examples of CSPs are given below ... 

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... 

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... 

A few synthetic helical polymers are known to act as chiral selectors.7a,918d l8k i9d i9h ancj are widely used as chiral stationary phases (CSP) in gas or liquid chromatography.73,53 Recently, it has been reported that the preference of one helical sense in isotropic solution can be induced by some interaction between optically inactive polymers and chiral solvents/additives. Examples of this include poly(n-hexyl isocyanate)18d l8k and poly(phenylacetylene)s bearing functional group.19d 19h The polysilane derivatives also show chiral recognition ability in solution at room temperature. Poly(methyl-ft-pinanylsilane) includes two chiral centers per bulky hydrophobic pinanyl side group28 and... 

Complexation with Chiral Metal Complexes. This idea was first suggested by Feibush et al.44 The separation is realized by the dynamic formation of diastereomeric complexes between gaseous chiral molecules and the chiral stationary phase in the coordination sphere of metal complexes. A few typical examples of metal complexes used in chiral stationary phase chromatography are presented in Figure 1-13.45... 

Initially, chiral stationary phases for chiral liquid chromatography were designed for preparative purposes, mostly based on the concept of three-point recognition .47 Pirkle and other scientists48 developed a series of chiral stationary phases that usually contain an aryl-substituted chiral compound connected to silica gel through a spacer. Figure 1-14 depicts the general concept and an actual example of such a chiral stationary phase. 

A large number of chiral molecules have been separated with ligand-exchange chiral stationary phases. A few examples with commercially available columns are given in Table 4. 

Racemic modifications may be resolved. There are very few examples of this approach having been employed successfully. The racemic cylic ether (RS)-36, which contains two CH2OCH2CO2H arms attached to the 3 and 3 positions on the axially chiral binaphthyl units, has been resolved (48-50, 93, 94) to optical purity in both its enantiomers by liquid-liquid chromatography using a chiral stationary phase of either (R)- or (S)-valine adsorbed on diatomaceous eaitii. Very recently, the optical resolution of crown ethers (/ S)-37 and (/ 5)-38, incorporating the elements of planar chirality in the form of a rron -doubly bridged ethylene unit, has been achieved (95) by HPLC on (+)-poly(triphenyl-methyl methacrylate). 

Polarimetric detection of enantiomers eluted from liquid chromatographic columns employing chiral stationary phases has been described 57 58 and interesting applications have been report-ed 59-60, for example, the study of enantiomerizations during chromatography and the evaluation of optica] purity despite incomplete chromatographic enantiomer separation. By this deconvolution method, based on Beer s and Biot s expressions, optical purities rather than enantiomeric purities are determined60. 

Chiral resolution is a matter of great importance. For example, the food and beverage industry is increasingly concerned with enantiomeric separations, because they can affect flavor, fragrance, and nutrition and can be used to monitor fermentation or product adulteration. Separations are carried out mainly on chiral stationary phases (CSPs). 

Conventional gas chromatography (GC) based on the use of chiral stationary phases can handle only a few dozen ee determinations per day. In some instances GC can be modified so that, in optimal situations, about 700 exact ee and E determinations are possible per day [29]. Such meclium-throughputmay suffice in certain applications. The example concerns the lipase-catalyzed kinetic resolution of the chiral alcohol (R)- and (S)-18 with formation of the acylated forms (R)- and (S )-19. Thousands of mutants of the lipase from Pseudomonas aeruginosa were created by error-prone PCR for use as catalysts in the model reaction and were then screened for enantioselectivity [29]. 

Chromatographic separatum of enantiomersThe carbamate, ureido, and amide derivatives obtained without racemization from enantiomeric amines, alcohols, and carboxylic acids, respectively (equations T III), with this isocyanate are stable for months and are suitable for gas chromatographic separation using a polymeric chiral stationary phase (derived, for example, from L-valine-(S)-a-phenylethylamide). This methodology permits separation of chiral a- and /1-hydroxy acids and also N-mclhylnmino acids. 

Silica-base stationary phases have also been employed for enantiomeric separations in CEC [6,72-81]. In the initial work on chiral CEC, commercially available HPLC materials were utilized, including cyclodextrins [6,74,81] and protein-type selectors [73,75,80] such as human serum albumin [75] and ai-acid glycoprotein [73]. Fig. 4.9, for example, depicts the structure of a cyclodextrin-base stationary phase used in CEC and the separation of mephobarbital enantiomers by capillary LC and CEC in a capillary column packed with such a phase. The column operated in the CEC mode affords higher separation efficiency than in the capillary LC mode. Other enantiomeric selectors are also use in CEC, including the silica-linked or silica-coated macrocyclic antibiotics vancomycin [82,83] and teicoplanin [84], cyclodextrin-base polymer coated silicas [72,78], and weak anion-exchage type chiral phases [85]. Relatively high separation efficiency and excellent resolution for a variety of compounds have also been achieved using columns packed with naproxen-derived and Whelk-0 chiral stationary phases linked to 3 pm silica particles [79]. Fig. 4.10 shows the... 

The optical resolution was achieved in most cases on an analytical scale by chromatography [52] (see the example in Figure 5.14) using chiral stationary phases of the Chiralcel OD or Chiralpak AD-H type. The separation factors vary between close to 1 and 2.36. 

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