Enantioselective chiral stationary phases

Separation of enantiomers by physical or chemical methods requires the use of a chiral material, reagent, or catalyst. Both natural materials, such as polysaccharides and proteins, and solids that have been synthetically modified to incorporate chiral structures have been developed for use in separation of enantiomers by HPLC. The use of a chiral stationary phase makes the interactions between the two enantiomers with the adsorbent nonidentical and thus establishes a different rate of elution through the column. The interactions typically include hydrogen bonding, dipolar interactions, and n-n interactions. These attractive interactions may be disturbed by steric repulsions, and frequently the basis of enantioselectivity is a better steric fit for one of the two enantiomers. ... 

Recently, multidimensional GC has been employed in enantioselective analysis by placing a chiral stationary phase such as a cyclodextrin in the second column. Typically, switching valves are used to heart-cut the appropriate portion of the separation from a non-chiral column into a chiral column. Heil et al. used a dual column system consisting of a non-chiral pre-column (30 m X 0.25 mm X 0.38 p.m, PS-268) and a chiral (30 m X 0.32 mm X 0.64 p.m, heptakis(2,3-di-(9-methyl-6-(9-tert-butyldimethylsilyl)-(3-cyclodextrin) (TBDM-CD) analytical column to separate derivatized urinary organic acids that are indicative of metabolic diseases such as short bowel syndrome, phenylketonuria, tyrosinaemia, and others. They used a FID following the pre-column and an ion trap mass-selective detector following the... 

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... 

Possible differences are also well illustrated by 3-thio- and 3-methyl-thiohex-anols and their esters (Table 1). Among these compounds, there is a tendency for the (R) enantiomers to have a typical, fruity aroma. However, for 3-methylthiohexanol (an aroma component of yellow passion fruit) this situation is reversed the (S) enantiomer had the characteristic fruity aroma ( exotisch, fruchtig ).52 For the separation of enantiomers of odorous compounds, enan-tioselective GLC with chiral stationary phases, and MGDC techniques using a conventional capillary column and an enantioselective column are commonly used.53... 

Ekborg-Ott, K.H., Liu, Y, and Armstrong, D.W., Highly enantioselective HPLC separations using the covalently bonded macrocyclic antibiotic, ristocetin A, chiral stationary phase. Chirality, 10, 434, 1998. 

Berthod, A., Chang, S.-C., and Armstrong, D.W., Empirical procedure that uses molecular structure to predict enantioselectivity of chiral stationary phases. Anal. Chem., 64, 395,1992. 

Alcaro, S. et al., Enantioselective semi-preparative HPLC of two 2-arylpropionic acids on glycopeptides containing chiral stationary phases, Tetrahedron Asymmetry, 13, 69, 2002. 

Svensson, L.A. and Owens, P.K., Enantioselective supercritical fluid chromatography using ristocetin A chiral stationary phases. Analyst, 125, 1037, 2000. 

Aboul-Enein, H.Y. and Serignese, V., Optimized enantioselective separation of clenbuterol on macrocyclic antibiotic teicoplanin chiral stationary phase, J. Liq. Chrom. Rel. Technol, 22, 2177, 1999. 

Tesalova, E., Bosdkovi, Z., and Pacakova, V., Comparison of enantioselective separation of A -tert-butyloxycarbonyl amino acids and their non-blocked analogues on teicoplanin-based chiral stationary phase, J. Chromatogr. A, 838, 121, 1999. 

Fried, K.M., Koch, P, and Wainer, I.W., Determination of the enantiomers of albuterol in human and canine plasma by enantioselective high-performance liquid chromatography on a teicoplanin-based chiral stationary phase. Chirality, 10, 484, 1998. 

Aboul-Enein, H.Y. and Hefnawy, M.M., Enantioselective determination of arotino-lol in human plasma by HPLC using teicoplanin chiral stationary phase, Biomed. Chromatogr., 17, 453, 2003. 

Kleidernigg, O.P. and Kappe, C.O., Separation of enantiomers of 4-aryldihydropyrimidines by direct enantioselective HPLC. A critical comparison of chiral stationary phases. Tetrahedron Asymmetry, 8, 2057, 1997. 

Enantioselective separation by supercritical fluid chromatography (SFC) has been a field of great progress since the first demonstration of a chiral separation by SFC in the 1980s. The unique properties of supercritical fluids make packed column SFC the most favorable choice for fast enantiomeric separation among all of the separation techniques. In this chapter, the effect of chiral stationary phases, modifiers, and additives on enantioseparation are discussed in terms of speed and resolution in SFC. Fundamental considerations and thermodynamic aspects are also presented. 

Armstrong et al. ° first introduced chiral stationary phases based on macrocyclic antibiotics. Vancomycin, ristocetin A, teicoplanin, avoparcin, rifamycin B and thiostrepton are used as chiral selectors. They posses a broad enantiorecognition range, similar to protein based CSPs. However, CSPs based on macrocyclic antibiotics show higher stability and capacities.Underivatized amino acids, N-derivatized amino-acids, acidic compounds, neutrals, amides, esters and amines can be separated.The first four of the above-mentioned chiral selectors appear to have the largest enantiorecognition range.The selectors can also be derivatized to obtain different enantioselectivities. 

Levin, S., Abu-Lafi, S., The Role of Enantioselective Liquid Chromatography Separations Using Chiral Stationary Phases in Pharmaceutical Analysis, In Advances in Chromatography, Grushka, E. and Brown, P. R., Eds., Vol. 33, Marcel Dekker, New York, pp. 233-266, 1993. 

Karlsson, C., Wikstrom, H., Armstrong, D. W, and Owens, P. K. (2000). Enantioselective reversed-phase and non-aqueous capillary electrochromatography using a teicoplanin chiral stationary phase.. Chromatogr. A 897(1-2, 3), 349-363. 

Lammerhofer, M., Tobler, E., Zarbl, E., Lindner, W., Svec, E, and Frechet, J. M. J. (2003). Macroporous monolithic chiral stationary phases for capillary electrochromatography new chiral monomer derived from cinchona alkaloid with enhanced enantioselectivity. Electrophoresis 24, 2986-2999. 

Konig WA, Icheln D, RungeT, Pforr I, Krebs A, Cyclodextrins as chiral stationary phases in capillary gas chromatography, VII, Cyclodextrins with an inverse substitution pattern — Synthesis and enantioselectivity, /Resolut Chromatogr 13 702-707, 1990. 

The enantioselective separation of racemic synthetic dihydrofurocoumarins in which the stereogenic center is located in the furan ring was accomplished by cyclodextrin-based LC <2003JCH(1011)37>. Hydroxypropyl-/3-cyclodextrin was the most effective chiral stationary phase, showing some enantioselectivity for 22 of the 28 dihydrofurocoumarins, and baseline resolving 16 of them in reversed-phase mode. 

Serious errors can result when there is preferential decomposition of one enantiomer during its residence time in the column. The enantiomer which spends a longer time in the column will be lost preferentially and, as a consequence, the ee will be biased in favor of the first eluted enantiomer. This effect can be recognized by a deviation from the expected 1 1 ratio for a racemic mixture, i.e., the peak area of the second peak is diminished. Decomposition of the sample on the chiral stationary phase also has the same effect in that the enantiomer which spends a longer time in the column will be lost preferentially. Once again the ee will be biased in favor of the first eluted enantiomer. If the decomposition is catalyzed by the chiral stationary phase in an enantioselective manner, either the second or the first peak can be diminished. 

The ability to design chiral ILs in which the cation and anion is of fixed chirality represents additional tuning features of ILs. Two approaches have incorporated ILs as new stationary phases for chiral GC. One method involves the use of chiral ILs as stationary phases in WCOT GC [37]. In the second approach, chiral selectors (e.g., cyclodextrins) were dissolved in an achiral IL and the mixture coated onto the wall of the capillary colunm [38]. Both approaches can separate a variety of different analytes, but the observed enantioselectivities and efficiencies do not rival those observed with commercially available chiral stationary phases (CSPs). 

A. Berthod, W. Li, and D. W. Armstrong, Multiple Enantioselective Retention Mechanisms on Derivatized Cyclodextrin Gas Chromatographic Chiral Stationary Phases, Anal. Chem. 1992,64, 873 K. Bester, Chiral Analysis for Environmental Applications, Anal. Bioanal. Chem. 2003,... 

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]. 

Generally, CD-based chiral stationary phases have been used in the reversed-phase mode. Earlier, it was assumed that in the normal phase mode, the more nonpolar component of the mobile phase would occupy the CD cavity, thereby blocking inclusion complexation between the chiral analyte and CD [4,11], But with the development of CD derivatives, it has become possible to use the normal phase mode too [45,74], Among the various CSPs based on CD derivatives, one based on a naphthylethyl carbamoylated derivative has shown excellent enantioselectivity in the normal phase mode [46,59]. Armstrong et al. [45] synthesized several /CCD derivatives and had them tested in the normal phase mode to resolve the enantiomers of a variety of drugs hexane-2-propanol (90 10, v/v) served as the mobile phase. The authors discussed the similarities and differences of the enantioselectivities on the native and derivatized CD phases. 

Ruderisch, A., Pfeiffer, J., and Schurig, V. (2003) Mixed Chiral Stationary Phase Containing Modified Resorcinarene and Beta-Cyclodextrin Selectors Bonded to a Polysiloxane for Enantioselective Gas Chromatography,/. Chromatogr. 994, 127-135. 

The preparative-scale separation of enantiomers on chiral stationary phases (CSPs) by GC cannot match the overwhelming success achieved in the realm of liquid chromatography (LC) (Francotte, 1994, 1996 and 2001). Modern commercial instrumentation for preparative-scale GC is not readily available. In contrast to LC, separation factors a in enantioselective GC are usually small (a = 1.01 - 1.20). This is beneficial for fast analytical separations but detrimental to preparative-scale separations. Only in rare instances are large chiral separation factors (a > 1.5) observed in enantioselective GC. Only in one instance, a separation factor as high as a = 10 was detected in enantioselective GC for a chiral fluorinated diether and a modified 7-cyclodextrin (Schurig and Schmidt, 2003) (vide supra). 

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