Cyclodextrins chromatographic separation

Cyclodextrins can solubilize hydrophobic molecules in aqueous media through complex formation (5-8). A nonpolar species prefers the protective environment of the CDx cavity to the hulk aqueous solvent. In addition, cyclodextrins create a degree of structural rigidity and molecular organization for the included species. As a result of these characteristics, these macrocycles are used in studies of fluorescence and phosphorescence enhancement (9-11), stereoselective catalysis (.12,13), and reverse-phase chromatographic separations of structurally similar molecules (14,15). These same complexing abilities make cyclodextrins useful in solvent extraction. 

For initial work on cyclodextrin-mediated chromatographic separation of enantiomers, see Hinze, W. L. Armstrong, D. W. ed. Ordered Media in Chemical Separations, ACS Symposium Series 342, 1986. 

Cyclodextrin-mediated chromatographic separation of enantiomers is also discussed in Braithwaite, A. Smith, F. J. Chromatographic Methods, 5th Edition, Blackie Academic Professional, London, New York, 1996. 

Investigations on the stereochemistry of chiral semiochemicals may be carried out by (gas) chromatographic separation of stereoisomers using chiral stationary phases, e.g. modified cyclodextrins [32]. Alter natively, formation of diastereomers (e.g. Mosher s ester or derivatives involving lactic acid etc.) may be followed by separation on conventional achiral stationary phases. Assignment of the absolute configuration of the natural product will again need comparison with an authentic (synthetic) reference sample. 

Armstrong, D.W. and DeMond, W., Cyclodextrin bonded phases for the liquid chromatographic separation of optical, geometrical, and structural isomers, J. Chromatogr. Sci., 22, 411, 1984. 

Elargitai, T., Kaida, Y., and Okamoto, Y., Preparation and chromatographic evaluation of 3,5-dimethylphenyl carbamoylated beta-cyclodextrin stationary phases for normal-phase high-performance liquid-chromatographic separation of enantiomers, J. Chromatogr., 628, 11, 1993. 

Schurig V, Nowotny HP, Gas chromatographic separation of enantiomers on cyclodextrin derivatives, Angew Chem 29, 939—957, 1990. 

Bicchi C, D Amato A, Rubiolo P, Cyclodextrin derivatives as chiral selectors for direct gas chromatographic separation of enantiomers in the essential oil, aroma and flavor J Chromatogr A 843 99-121, 1999. 

A chiral selector can also be dissolved in the IL solvent and be subsequently coated on the capillary wall [38]. In this approach, the achiral [C4CiIm]Cl was used to dissolve permethylated p-cyclodextrin (p-PM) and dimethylated P-cyclodextrin (p-DM). The chromatographic separations obtained from these two columns were compared to two commercially available CSPs based on p-PM and p-DM dissolved in polydimethylsiloxane. From a set of 64 chiral molecules separafed by fhe commercial p-PM column, only 21 of the molecules were enantioresolved by the IL-based p-PM column. Likewise, from a collecfion of 80 analytes separated by the p-DM column, only 16 analytes could be separated on the IL-based p-DM column. The authors also noted a considerable enhancement in the separation efficiency of fhe IL-based CSPs. This resulf, coupled to fhe loss of enantioselecfivify for mosf separations, suggests that the imidazolium cation may occupy the cavity of the cyclodextrin preventing the analyte-cyclodextrin inclusion complex-ation that is crucial for chiral recognition. The ability for ILs to form inclusion complexes wifh cyclodextrin molecules has been recently studied by Tran and coworkers using near-infrared spectromefry [39]. 

Hinze WL, Applications of cyclodextrins in chromatographic separations and purification methods, in Separations and Purification Methods (Van Oss C, Ed.), Vol. 10, p. 159, Marcel Dekker, New York (1981). 

For the application of label-free optical transduction principles like SPR or RIfS, a chiral receptor bound to a transparent polymer layer is required. As various types of these polymers have already been applied to chromatographic separation processes, a substantial wealth of knowledge was achieved during the last few decades. Stationary materials like bonded amide selectors or cyclodextrins were adopted as sensor coatings. Several different applications of these materials in various fields of interest have been reported in the literature [17]. 

Figure 18 Gas-chromatographic separation of the enantiomer of both substrate (30) (as carbamate) and product (31) on heptakis-(2,3-di-6>-methyl-6-6>-tert-butyldimethylsilyl)-/J-cyclodextrin of the Pseudomonas fluorescens lipase (PFL) catalyzed transesterification of (30) in toluene att=9 hrs, ees =99.9%, eep=92.2%, conv. =52%, E...

Lindstrom, M., Norin, T., and Roeraade, J. (1990) Gas chromatographic separation of mo-noterpene hydrocarbon enantiomers on a-cyclodextrin. J. Chromatogr. 513, 315-320. 

Schiirch, S., Saxer, A., Claude, S., Tabacchi, R., Trusch, B., and Hulliger, J. (2001) Semi-preparative gas chromatographic separation of ah-trans-perhydrotriphenylene enantiomers on a chiral cyclodextrin stationary phase, J. Chromatogr. A 905, 175-182. 

Staerk, D. U., Shitangkoon, A., and Vigh, G. (1994a) Gas chromatographic separation of the enantiomers of volatile fluoroether anesthetics by derivatized cyclodextrins II. Preparative-scale separations for isoflurane. J. Chromatogr. A 663, 79-85. 

Armstrong, D.W., DeMond, W., Alak, A., Hinze, W.L., Riehl, T.E., and Bui, K.H., Liquid chromatographic separation of diastereomers and structural isomers on cyclodextrin-bonded phases, Anal. Chem., 57, 234, 1985. 

Chang, C.A., Wu, Q., and Tan, L., Normal-phase high-performance liquid chromatographic separations of positional isomers of substituted benzoic acids with amine and P-cyclodextrin bonded-phase columns, J. Chromatogr., 361, 199, 1986. 

Displacement Chromatographic Separations on / -Cyclodextrin—Silica Columns... 

The chromatographic separation of positional isomers (26-31), geometrical isomers (27,32-36) and enantiomers (37-49) has been achieved by utilizing the concerted action of inclusion complex formation, additional primary and secondary hydrogen-bond formation and steric hindrance effects between the solutes and the cyclodextrins (11,12,14-23,50). There is an abundant literature on the analytical applications of cyclodextrin-silicas (13-50), but not on their preparative chromatographic use. 

Cyclodextrin-silicas have three major liabilities in preparative chromatographic separations in the elution mode (i) the chiral selectivity factors are generally lew, (ii) the loading... 

In the initial experiments reported here we did not attempt to optimize the separation in terms of yield and production rate. Rather, cur intent was to demonstrate that displacement chromatographic separations are feasible on a chiral stationary phase, cyclodextrin-silica, and gather preliminary information regarding the structure of displacers which cam De used with cyclodextrin-sil icas. The method development sequence described in the previous paragraph will be followed in the discussion of the results. 

Since the chloroanilines are sufficiently retained (k >5) in a 10 % v/v methanol water eluent, and the Ibuprofen enantiomers are sufficiently retained in a 30 % v/v acetonitrile buffer eluent, these solvents were selected as carrier solvents for the displacement chromatographic separations. Also, these solvents were used to determine the adsorption isotherms of p-nitrophenol and 4-t-butylcyclohexanol on beta-cyclodextrin silica. The isotherms were determined from frontal chromatographic measurements as described in (56). The isotherms are shown in Figs. 7 and 8. Since both isotherms are downwardly convex, p-nitrophenol and 4-t-butylcyclohexanol might prove useful displacers for our test solutes, provided that they are more strongly adsorbed that the solutes. 

Fig. 24. Asymmetric oligomerization of propylene at various temperatures gas-chromatographic separation of 2,4-dimethyl-2-heptene (trimer) in a capillary column coated with octakis(6-0-methyl-2,3-di-0-pentyl)cyclodextrin) (204).

A characteristic feature of non-chromatographic separations utilizing cyclodextrins is that they are aimed at preparative separations. Unfortunately only incomplete separations or enrichments can be attained. By repeating the separations in multistage processes, the required component can be enriched on preparative, and even industrial, scale. Many examples have been published both for partial separation of compounds, isomers, or enantiomers through selective crystallization of their complexes (3). 

Figure 6. Gel inclusion chromatographic separation of amino acids on a-, 6-, y-cyclodextrin bead polymer columns, and on Sephadex G-25 column (1.6x88 cm, pH 5.0 phosphate buffer, flow rate 10 ml/h, 20 °C).

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