Modified cyclodextrin phases

Capillary gas chromatography on optically active modified cyclodextrin phases is a simple, fast, accurate and highly sensitive method for the enantiomeric analysis of chiral volatile compounds. 

From our own experience, it should be emphasised that the enantioselectivity of modified cyclodextrin phases is considerably influenced by the polarity of the (non-chiral) polysiloxane solvents used. 

Kreis P, Dietrich A, Mosandl A (1996) Elution order of the furanoid linalool oxides on common gas chromatographic phases and modified cyclodextrin phases. J Essent Oil Res 8 339 Weinert B, Wiist M, Mosandl A Hanssum H (1998) Stereoisomeric flavour compounds LXX-Vlff. Separation and structure elucidation of the pyranoid linalool oxide stereoisomers using common gas chromatographic phases, modified cyclodextrin phases and nuclear magnetic resonance spectroscopy. Phytochem Anal 9.T0... 

It is in the study of this phenomenon where two-dimensional GC offers by far the most superior method of analysis. The use of chiral selector stationary phases, in particular modified cyclodextrin types, allows apolar primary and atropisomer selective secondary separation. Reported two-dimensional methods have been successful... 

These interesting results are attributed to the formation of an alkene/ cyclodextrin inclusion complex as well as the solubility of the chemically modified cyclodextrin in both phases. Prior to this study, hydroformylation in the presence of unmodified cyclodextrins had been studied by Jackson, but the results were rather disappointing.174... 

Another chiral stationary phase is modified cyclodextrin. Cyclodextrins are cyclic chiral carbohydrates composed of six, seven, or eight glucopyranose... 

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. 

The advantage of biphasic systems over the more common CHJCN/H2O mixtures (see 6.5) is in the easier and cleaner product isolation. However, practically useful rates can be achieved only in the presence of such reverse phase transfer agents like the various chemically modified cyclodextrins, of which 2,6-di-OMe-P-CD proved the best. 

Enantiomer separation on modified macrocyclic cyclodextrin phases via inclusion into chiral cavities. ... 

V Schurig, D Wistuba. Recent innovations in enantiomer separation by electrochromatography utilizing modified cyclodextrins as stationary phases. Electrophoresis 20 2313-2328, 1999. 

Capillary gas chromatography (GC) using modified cyclodextrins as chiral stationary phases is the preferred method for the separation of volatile enantiomers. Fused-silica capillary columns coated with several alkyl or aryl a-cyclo-dextrin, -cyclodextrin and y-cyclodextrin derivatives are suitable to separate most of the volatile chiral compounds. Multidimensional GC (MDGC)-mass spectrometry (MS) allows the separation of essential oil components on an achiral normal phase column and through heart-cutting techniques, the separated components are led to a chiral column for enantiomeric separation. The mass detector ensures the correct identification of the separated components [73]. Preparative chiral GC is suitable for the isolation of enantiomers [5, 73]. 

A real breakthrough in this field occurred when enantio-cGC became more and more available. In particular, since 1988 selectively modified cyclodextrins have been synthesised, serving as chiral stationary phases in enantio-cGC, reported by Schurig and Novotny [I], Konig et al. [2, 3], Armstrong et al. [4], Dietrich et al. [5,6 ], Saturin et al. [7], and Bicchi et al. [8]. 6-O-silylated modified /I-cyclo dextrin and y-cyclodextrin derivatives of well-defined structure and purity were synthesised and have proved to be chiral stationary phases of unique selectivity and versatility and, therefore, are successfully used in simultaneous enantio-cGC analysis [5,6]. Further derivatives were recently reported by Taka-hisa and Engel [9, 10], dealing with 2,3-di-0-methoxymethyl-6-0-tert-butyldi-methylsilyl modified cyclodextrins as chiral stationary phases in enantio-cGG. 

Using a chiral column, coated with a definite modified cyclodextrin as the chiral stationary phase, the elution orders of furanoid and pyranoid linalool oxides are not comparable [11, 12]. Consistently, the chromatographic behaviour of diastereomers and/or enantiomers on modified cyclodextrins is not predictable (Fig. 17.1, Table 17.1). Even by changing the non-chiral polysiloxane part of the chiral stationary phase used, the order of elution may significantly be changed [13]. The reliable assignment of the elution order in enantio-cGC implies the coinjection of structurally well defined references [11-13]. 

Table 17.1 Elution order of the furanoid linalool oxides using different modified cyclodextrins (CD) as chiral stationary phases [11, 13]...

The aqueous-organic two-phase system was successfully applied to perform hydrocarboxylation.300 Palladium complexes of trisulfonated triphenylphosphine ligands were shown to exhibit high activity.301-303 The application of cosolvents and modified cyclodextrins allow to eliminate solubility problems associated with the transformation of higher alkenes.304... 

In new studies heteropoly acids as cocatalysts were found to be very effective in combination with oxygen in the oxidation of ethylene.1311 Addition of phosphomo-lybdic acid to a chloride ion-free Pd(II)-Cu(II) catalyst system results in a great increase in catalytic activity and selectivity.1312 Aerobic oxidation of terminal alkenes to methy ketones can be performed with Pd(OAc)21313 or soluble palladium complexes. Modified cyclodextrins accelerates reaction rates and enhance selectivities in two-phase systems under mild conditions.1315 1316... 

Modified cyclodextrins are the most versatile and widely used chiral stationary phases. The most widely used columns contain 10% to 50% cyclodextrin dissolved in either OV-101 or SE-52 polysiloxane. They are thermally stable up to 230°C but require some care in use because cyclodextrins are soluble in many solvents and can be washed off the column if they are exposed to too much solvent. Considerations in selecting a column... 

SPR spectroscopy and RIfS were used as optical transduction principles for the detection of halogenated diethers as chiral guests with modified cyclodextrin as the stationary phase [7]. 

Yoshida, K. Nakagama, T. Uchiyama, K. and Hobo, T. (1998) Photo responsive chromatographic behavior of dansyl amino acid enantiomers using azobenzene-modified cyclodextrin stationary phases in... 

Name p-Cyclodextrin, hydroxypropyl ether modified bonded phase Structure ... 

Enantiomer separation of various compounds such as barbituric acids, benzoin, MTH-proline, glutethimide, a-methyl-oc-phenyl-succinimide, y-phenyl-y-butyrolac-tone, methyl-mandelate, l-(2-naphthyl)ethanol, mecoprop methyl, diclofop methyl and fenoxaprop methyl by pressure supported CEC on a permethyl-P-cyclodextrin modified stationary phase was described by Wistuba and Schurig [42-44]. Three different separation beds were used (i) permethyl-P-cyclodextrin was covalently attached via a thioether to silica (Chira-Dex-silica) [42], permethyl-P-cyclodextrin was linked to a dimethylpolysiloxane and thermally immobilized (ii) on silica (Chirasil-Dex-silica) [43] or (iii) on a silica monolith (Chirasil-Dex-monolith) [44], respectively. 

Investigation on the enantiomeric composition of chiral secondary alcohols will, however, require either derivatization with an optically active reagent and separation on a conventional column or enantioselective GC using an optically active stationary phase. Today, the latter approach most frequently involves modified cyclodextrins.135 Enantioselective HPLC has also been successfully applied to separate enantiomers.136,137 Several reagents have been used in the transformation of chiral alcohols into diastereomers. Among these, acetyllactic acid138 or chlorofluoroacetic acid139 furnish volatile derivatives of pheromone... 

The topic of molecular recognition should gain increased attention in catalyst design. For example, specific structural interactions of higher olefins (e. g., 1-de-cene) with chemically modified -cyclodextrins allow efficient hydroformylation in a two-phase aqueous system even though the olefin is completely insoluble in water at the same time, olefin isomerization at the rhodium catalyst is hampered [81]. 

H.-G. Schmarr, A. Mosandl, H.-P. Neukom and K. Grob. Modified Cyclodextrins as Stationary Phases for Capillary GC Consequences of Dilution in Polysiloxanes. J. High Resolut. Chromatogr., 14, 207-210(1991). 

A. Dietrich, B. Maas, G. Brand, V. Karl, A. Kaunzinger and A. Mosandl. Stereoisomeric Flavor Compounds, Part LX. Diluted modified cyclodextrins as chiral stationary phases - the influence of the polysiloxane solvent. J. High Resolut. Chromatogr. IS, 769-772 (1992). 

Konig, W. A., Lutz, S., Wenz, G. Modified cyclodextrins-novel, highly enantioselective stationary phases for gas chromatography. Angew. Chem. Int. Ed. Engl. 1988, 27, 979-980. 

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