Stationary phases chiral separations

Most chiral chromatographic separations are accompHshed using chromatographic stationary phases that incorporate a chiral selector. The chiral separation mechanisms are generally thought to involve the formation of transient diastereomeric complexes between the enantiomers and the stationary phase chiral ligand. Differences in the stabiHties of these complexes account for the differences in the retention observed for the two enantiomers. Often, the use of a... 

There is a wide variety of commercially available chiral stationary phases and mobile phase additives.32 34 Preparative scale separations have been performed on the gram scale.32 Many stationary phases are based on chiral polymers such as cellulose or methacrylate, proteins such as human serum albumin or acid glycoprotein, Pirkle-type phases (often based on amino acids), or cyclodextrins. A typical application of a Pirkle phase column was the use of a N-(3,5-dinitrobenzyl)-a-amino phosphonate to synthesize several functionalized chiral stationary phases to separate enantiomers of... 

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. 

Sun, Q. and Olesik, S.V., Chiral separation by simultaneous use of vancomycin as stationary phase chiral selector and chiral mobile phase additive, J. Chromatogr. B, 745, 159, 2000. 

Torok, G. et al.. Direct chiral separation of unnatural amino acids by high performance liquid chromatography on a ristocetin A-bonded stationary phase. Chirality, 13, 648, 2001. 

CE has been touted as a replacement for HPLC in the pharmaceutical industry. This was a shame, since the techniques are so different. For many measurements, it is an orthogonal technique to HPLC. Whereas HPLC separates based on interaction with the stationary phase, CE separates based on the ratio of charge to mass. There are numerous examples of where CE exceeds the resolving power of HPLC (e.g., ion analysis, chiral analysis, DNA quantification, separation, large molecule analysis, etc.). 

Another method for creating a chiral environment is lo add an optically pure chiral selector to a bulk liquid phase. Chiral additives have several advanlages over chiral stationary phases and continue lo be the predominant mode for chiral separations by tic and capillary electrophoresis (cc). First of all, the chiral selector added to a bulk liquid phase can be readily changed. The use of chiral additives allows chiral separations lo be done using less expensive, conventional stationary phases. A wider variety of chiral selectors are available [ be used as chiral additives than are available as chiral stationary phases, thus, providing the analyst with considerable flexibility. Finally, the use of chiral additives may provide valuable insight into (he chromatographic conditions and/or likelihood ol success with a potential chiral stationary-phase chiral selector. This is particularly important for the development of new chiral stationary phases because of the difficulty and cosl involved. 

Okamoto, Y., Aburatani, R., Kaida, Y., Hatada, K., Inotsume, N., and Nakano, M. (1989) Direct chromatographic separation of 2-arylpropionic acid enantiomers using tris(3,5-dimethylphenylcarbamate)s of cellulose and amylose as chiral stationary phases, Chirality 1, 239-242. 

Analytical Properties CSP (chiral stationary phase) 1 — separates some chiral binaphthyl derivatives when mixtures of hexane diethyl ether, dichloromethane, or dioxane are used as the mobile phase CSP 2 — separates compounds with carbamate or amide functions (mixtures of n-hexane and 2-propanol can be used as mobile phase) CSP 3 — separation of compounds separated by CSP 2, as well as separation of compounds with carbonyl or amide functions and some amino alcohols that have pharmaceutical relevance ((3-blockers)... 

The first electrically driven enantioseparations involved the addition of a chiral selector to the mobile phase in CE. This selector is usually a complexing agent and acts as a pseudo-stationary phase. The separation is accomplished by the difference in the distibution equilibria between the pseudo-stationary phase and the enantiomers [134], The most common additives incorporated into these CE experiments were cyclodextrins and cyclodextrin derivatives [135-138], However, these experiments required the replacement of the chiral selector after each electrophoretic run. 

T. Loughlin, R. Thompson, G. Bicker, R Tway, and N. Grinberg, Use of subcritical fluid chromatography for the separation of enantiomers using packed column cellulose based stationary phase. Chirality 8 (1996), 157. 

Enantiomeric Stationary Phases. Chiral nonracemic chromatographic stationary phases prepared from p-cyclodextrin, derivatized with (R)- and (S)-NEI, and covalently bonded to a silica support are useful for the direct separation of enantiomers of a wide variety of compounds in both normal-phase and reversed-phase HPLC. ... 

With chiral stationary phases, chromatographic separation of enantiomeric ferrocene derivatives is possible. An apparatus for the resolution of ferrocenyl alcohols and other compounds on triacetylcellulose has been described [60]. Analytical enantiomer separation of ferrocenyl alcohols, ethers, sulfides, and amines for the determination of enantiomeric excesses is best achieved on cyclodextrin bonded phases [61]. 

The large number and variety of applications of the chiral derivatization approach attest to the success, viability, and importance of the technique. It is expected that despite the predictable advances to be realized in the near future in the development of direct chromatographic— mainly chiral-stationary-phase-based—separations of enantiomers, the indirect approach will continue to be widely used to solve stereochemical problems in the pharmaceutical, pharmacological, and toxicological arenas. 

Stereoisomers can be classified into two types enantiomers and dia-stereomers. Enantiomers (mirror images) have identical physical and chemical properties and therefore are not separated on the conventional reversed-phase stationary phases. Their separation will not be discussed. Diastereomers are isomers which are not mirror images of the parent. They have slightly different physical and chemical properties and can often be separated on conventional stationary phases. There are two classes of diastereomers optically active isomers when the API has two or more stereocenters and non-optically active geometric isomers, such as cis-trans, syn-anti, etc. Stereoisomers of chiral molecules must be included in the peak set. 

Lord, B. S., Memory effect of diethyl-amine mobile phase additive on chiral separations on polysaccharide stationary phases, Chirality, 2004,16, 493-498. 

Using a chiral recognition rationale, Plrkle, et fd (10) designed a chiral fluoroalcohollc bonded stationary phase which separates the enantiomers of sulfoxides, lactones, and derivatives of alcohols, amines, amino acids, hydroxy acids, and mercaptans. 

Caccamese, S. Direct high-performance liquid chromatography (HPLC) separation of etodolac enantiomers using chiral stationary phases. Chirality, 1993, 5, 164-167... 

Summarizing, the separation of enantiomeric pairs is achieved by the close selective interaction of one enantiomer with a stationary phase chiral center, resulting in stronger molecular interactions between the enantiomer and the neighboring groups or atoms round the stationary phase chiral center. 

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