Separation of Isomers and Enantiomers

Separation of Isomers and Enantiomers by Bile Acid Derivatives... 

CD derivatives have wide applications in analogue enzyme and analysis chemistry. CD derivative with special functional group can be used as a simple enzyme model which is convenient for studying the action mode and functional groups of the enzyme. The fluorescent and chromophore groups linked to the CDs can be seen as an indicator or a probe which are beneficial for separating the isomers and enantiomers. 

Mueller-Mulot (1976) described a one-dimensional system that separates all tocopherols and tocotrienols but cannot resolve a-tocopherol from 3-tocotrie-nol the system uses three successive developments of a silica gel 60 plate with the mobile phase n-hexane-ethane acetate (92.5 7.5). Sliwick et al. (1993) separated tocopherol isomers and enantiomers by TLC and studied the relationship between Rf values and topological indexes. 

Chiral Chromatography. Chiral chromatography is used for the analysis of enantiomers, most useful for separations of pharmaceuticals and biochemical compounds (see Biopolymers, analytical techniques). There are several types of chiral stationary phases those that use attractive interactions, metal ligands, inclusion complexes, and protein complexes. The separation of optical isomers has important ramifications, especially in biochemistry and pharmaceutical chemistry, where one form of a compound may be bioactive and the other inactive, inhibitory, or toxic. 

A simple and rapid method of separating optical isomers of amino acids on a reversed-phase plate, without using impregnated plates or a chiral mobile phase, was described by Nagata et al. [27]. Amino acids were derivatized with /-fluoro-2,4-dinitrophenyl-5-L-alanine amide (FDAA or Marfey s reagent). Each FDAA amino acid can be separated from the others by two-dimensional elution. Separation of L- and D-serine was achieved with 30% of acetonitrile solvent. The enantiomers of threonine, proline, and alanine were separated with 35% of acetonitrile solvent and those of methionine, valine, phenylalanine, and leucine with 40% of acetonitrile solvent. The spots were scraped off the plate after the... 

An equimolar mixture of two enantiomers is called a racemate. The separation of two enantiomers that constitute a racemate is called optical resolution or resolution. Their crystalline forms best characterize types of racemates. A racemic mixture is a crystal where two enantiomers are present in equal amounts. A conglomerate is a case where each enantiomer has its own crystalline form. Sometimes their crystals have so-called hemihedral faces, which differentiate left and right crystals. For over a hundred years, crystallization processes have been used for the separation and purification of isomers and optical resolution, both in the laboratory and on an industrial scale. 

Kassai, Cs., Balint, J., Juvancz, Z., Fogassy, E., and Kozma, D. Isomer and enantiomer separation of 2-and 4-alkylcyclohexonols by stereoselective complex formation with 0,0 -dibenzoyl-(2i ,3i )-tartaric acid, Synth. Commun. 2001, 31, 1715-1719. 

The geometrical isomers and enantiomers of the overcrowded alkenes 15-18 can readily be separated using chiral HPLC. Recently, an asymmetric synthesis of overcrowded alkenes has been developed, involving chirality transfer from an axial single bond to an axial double bond (Scheme 8).32 This methodology is particularly attractive for preparation of larger quantities of enantiomerically pure chiral switches based on overcrowded alkenes. The orientation of the two xanthylidene moieties is dictated by a binaphthol template. After a coupling step and separation of the diastereomers, the bi-xanthylidene is obtained with 96 % e.e. after removal of the template. 

It seems tc, also be worth mentioning that the described procedure has been used for micro-preparative separations of mephenytoin and hexobarbital enantiomers (26) p -CD solutions were also successfully used for resolution of 1-[2-(3-hydroxyphenyl)-l-phenylethylJ-4-(3-me-thyl-2-buteny1) piperazine enantiomers in RP systems (.20). An especially interesting example of the application of -CD is the separation of optical isomers of D,L - norgestrel (27). 

It is known that 1,4-dioxocines 4 are paratropic and exist in the equilibrium with syn-hcn/.cnc dioxides 5 (see Equation (1), Section 14.06.2). NMR analysis showed that the equilibrium mixture contained both the residual syn-benzene dioxide 161 (R = I) as a minor component (12%), and the corresponding 1,4-dioxocine isomer 162 (R = I) as a major component (88%) (Scheme 17) <2000CC2151>. Chromatographic separation of 161 and 162 followed by heating either component yielded the same equilibrium mixture as before separation. This unusual example of a concerted racemization of four chiral centers in one enantiomer was not observed for the 7 //-benzene dioxide 163. 

While distillative separation of enantiomers in contact with optical selectors is the most exciting issue, the separation of isomers or mixtures with very close boiling points by distillation in the presence of structural selectors (that do not need to be chiral for that purpose) is also of high interest, because it again minimizes waste and abuse of energy. Very difficult separations, some of industrial importance, have been performed correspondingly. [21]... 

In 1960, a patent was granted for the separation of D,L-prollne on a lactose column (56). Davankov s laboratory was the first to report separation of amino acid Isomers on polymeric resins derlvatlzed with optically active amino acids (57). However, separation of amino acid enantiomers by these techniques has been hampered by long separation times (ca. 10 hr) and the difficulty In synthesizing supports of sufficient quality for modern HPLC (spherical particles, small size, uniform chemical modification). Separation of amino acid Isomers on a column consisting of silica bonded with L-amlno acids and complexed with copper (II) has been reported by Gubltz and Jellenz (42). Short analysis times for separation of mixtures of single D,L-amlno acids were reported (ca. 30 min), but complex mixtures have not been separated. 

Aqueous and methanolic solutions of cyclodextrins have been employed as mobile phases in high performance liquid chromatography (HPLC), and as stationary phases by bonding the cyclodextrin to silica packing by several workers (1-8). They have been shown to be especially suitable for separation of structural isomers, cis-trans geometric isomers, and enantiomers. Cyclodextrins (CD) are toroidal-shaped, cyclic, oligosaccharides made up of o-l,4 linked,... 

A mixture of the (+) and (-) enantiomers in equal proportions is called a racemic modification (racemate) and is optically inactive. The optical inactivity results from the rotation caused by one enantiomer canceling out that produced by its complementary enantiomer. The racemic modification is designated as ( ) e.g. ( ) lactic acid). As the enantiomers of a substance have identical physical properties, they cannot be easily resolved employing the usual separation techniques such as fractional distillation. As a result, the isolation of optical isomers often pose difficult separation problems and it is usually necessary to resort to some very special techniques to achieve a satisfactory resolution hence the raison d etre for this book. 

Enantiomers of a single substance are essentially chemically identical and only differ in the spatial arrangement of chemical groups or atoms around a single atom, referred to as the stereogenic center or the chiral center(s). Such differences impart mostly very subtle physical chemical variations between the individual isomers and thus must be carefully exploited if a chromatographic separation of the individual enantiomers is to be achieved. 

An example of the recycling process is demonstrated in the separation of the Warfarin enantiomers as shown in figure 12.7. The chromatogram shows the effect of three cycles. In the first cycle there is little or no visible separation. On the second cycle, the two enantiomers are beginning to separate. In the third cycle, the separation is improved further, and is sufficient to allow the collection of significant quantities of the individual isomers at a high purity. It is also seen that the process is fairly rapid as three cycles are completed in less than 12 minutes. 

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