Optical isomers, adsorption

The concept of using continuous chromatography for the separation of stereoisomers or optical isomers is very old and was probably proposed for the first time by Martin and Kuhn in 1941 [28]. The suggested implementation was different from today s SMB technology, though the basic concept is the same. The chromatographic media is moved continuously in a conveyor belt, the feed is injected continuously at a fixed point, and the pure enantiomers are recovered at fixed points. In the idea of Martin and Kuhn, benefits were taken from the possibility of modulating the adsorption of the products at different temperatures. 

The OTCEC capillaries described in this chapter have been fabricated in a manner so that the major problems associated with packed capillaries are not present. The open tubular approach greatly reduces the likelihood of bubble formation so that pressurization of the system is not necessary. The other major problem, strong adsorption of basic compounds on the typical support material, is eliminated through the modification scheme, silanization/ hydrosilation, that removes silanols and replaces them with hydride groups. This type of separation medium also eliminates the need for any additives in the mobile phase to suppress adsorption of basic compounds, a technique that is often used in packed capillaries as the only means to elute such analytes. Therefore, the bulk of the applications developed to date have centered on the elution characteristics of compounds and separation of mixtures that are difficult to obtain in the packed capillary format. The major exception is the resolution of optical isomers that often can be done equally as well or often better with packed capillaries. The main objective of the chiral separations is to illustrate the presence of... 

Several barbiturates (e.g. 60-62) having two chiral centers exist in both enantiomeric and diastereomeric forms. The stereoisomers of 60 were obtained by Doran211 and exhibited different pharmacological activity.212 Four possible optical isomers of 61 were obtained by Miyano and Toki by enzymatic and chemical methods.193 Separations of diastereomeric mixtures were achieved by TLC and column chromatography on silica gel. The use of an adsorption of glucuronides of 61 on XAD-2 resin was advantageous for preliminary isolation of these compounds from urine.194... 

That the adsorption of the cyclopentane ring seems to proceed mainly flatly in deuterium exchange on films has been stated above (see Section I,D). Of considerable interest are the investigations on asymmetric catalysis initiated by Schwab et al. (273). In their work, one of the optical isomers reacted a little faster than the other in a racemic mixture. Terent yev and Klabunovskii (274, 273) carried out the catalytic asymmetric synthesis from optically inactive substances. The reactions were accomplished on metals deposited on dextro- and levorotatory quartz. Organic optically active carriers and admixtures give a still greater effect. On this problem see Klabunovskii (276). At the present time still more active catalysts for the reaction of asymmetric hydrogenation and polymerization have been revealed (277-279). 

Specific adsorbents have been prepared for a large number of compound types at the present time. In some cases these adsorbents seem capable of effecting quite difficult separations [e.g., optical isomers (123,131,132) and certain stereoisomers (110,111)]. Despite their unique promise, however, specific adsorbents have not yet been used to any significant extent in solving practical problems. Their preparation is invariably tedious and time consuming, their capacity for selective adsorption is unusually low, nonlinear isotherms are typical, and the specific selectivity is frequently slight. Furthermore, the technique can be applied only to those compounds which are appreciably adsorbed under the conditions of gel preparation. In almost all cases it is more profitable to attempt the chromatographic... 

Jackson T.A. (1971) Evidence of selective adsorption and polymerization of the L-optical isomers of amino acids relative to the D-optical isomers on the edge of facets of kaolinite, Experientia, 27,242-243. 

Kamai G.Kh., Klabunovskii E.I., Gatilov Yu.F. and Khodakov G.S. (1961) Resolution of tertiary arsenic compounds into optical isomers using asymmetric adsorption on natural dissymmetric adsorbents, Dokl. Acad. Sci. USSR, 139, 1112-1113, Chem. Abstr. 1962, 56,41f Chernyaev I.I., Korablina L.S. and Muraveiskaya G.S. (1965) On optical resolution and photochemical isomerization of as5mimetric c/s -diammines, Zh. neorg. khim. (J. Inorg. Chem. (russ.), 10, 1045-1052. 

However, adsorption of the protein on the membrane allows to its separation from water and can be used for other useful goals. In [117] adsorption of BSA on hollow fibers was used to separation of optical isomers of phenylalanine. BSA links the L-isomer with the binding constant of 1.810 M, that is one order higher than for D-isomer. Separation factor two-three times is higher for immobilized BSA, demonstrating channel proteins for optical resolution. Many other examples of useful applications of systems consisting of proteins adsorbed on ultrafiltration membrane can be expected. 

The direct separation method of a racemate into its enantiomers is based upon the complex formation between the optical isomers of the solute and a chiral selector, resulting in the formation of labile diastereoisomers [50,53]. These differ in their thermodynamic stability, provided that at least three active points of the selector participate in the interaction with corresponding sites of the solute molecule. The rule of the three-point interaction model is generally valid for enan-tioselective chromatography, with the extension to the rule, starting that one of the required interactions may be mediated by the adsorption of the two components of the interacting pair onto the sorbent surface [50,55], The separation of labile diastereoisomers can be accomplished if the complexes possess different stability constants. The major approaches in the formation of diastereomeric complexes are transition metal ion complexes, ion pairs, and inclusion complexes (diastereomeric complex/salt) (Figure 8.11). In this case, only the chiral purity of the selector influences the resolution [53]. 

By subjecting the same solution to the chromatographic adsorption process, it is found that the levorotary ergotamin is retained within the adsorption column, whereas the difficultly soluble dextrorotary ergotaminin runs first through it. As the optical activity of these isomers in chloroform is very different—... 

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