Cyclodextrins as chiral stationary phases in capillary gas chromatography. Part IV: Heptakis(2,3,6-tri-O-pentyl)-β-cyclodextrin

Chiral stationary phases and applications in gas chromatography

Chiral compounds are ubiquitous in nature and play a pivotal role in biochemical processes, in chiroptical materials and applications, and as chiral drugs. The analysis and determination of the enantiomeric ratio (er) of chiral compounds is of enormous scientific, industrial, and economic importance. Chiral separation techniques and methods have become indispensable tools to separate chiral compounds into their enantiomers on an analytical as well on a preparative level to obtain enantiopure compounds. Chiral gas chromatography and high-performance liquid chromatography have paved the way and fostered several research areas, that is, asymmetric synthesis and catalysis in organic, medicinal, pharmaceutical, and supramolecular chemistry. The development of highly enantioselective chiral stationary phases was essential. In particular, the elucidation and understanding of the underlying enantioselective supramolecular separation mechanisms led to the design of new chiral stationary phases. This review article focuses on the development of chiral stationary phases for gas chromatography. The fundamental mechanisms of the recognition and separation of enantiomers and the selectors and chiral stationary phases used in chiral gas chromatography are presented. An overview over syntheses and applications of these chiral stationary phases is presented as a practical guidance for enantioselective separation of chiral compound classes and substances by gas chromatography.

Separation of racemic sulfoxides and sulfinate esters on four derivatized cyclodextrin chiral stationary phases using capillary gas chromatography

The separation of 17 chiral sulfoxides and eight chiral sulfinate esters by gas chromatography (GC) on four derivatized cyclodextrin chiral stationary phases (CSPs) (Chiraldex G-TA, G-BP, G-PN, B-DM) is presented. Many of these compounds are structural isomers or part of a homologous series. Differences in enantioselectivity of the methyl phenyl sulfoxide isomers on the derivatized gamma cyclodextrin and the heptakis 2,6-di-O-methyl-beta-cyclodextrin (i.e. B-DM) CSPs are discussed. Under the conditions of this study, the molecular mass cut-off for the GC separation of these compounds was approximately 230. Compounds of higher molecular mass were not eluted from the CSPs at reasonable times and temperatures, but these higher molecular mass enantiomers can be separated by liquid chromatography and capillary electrophoresis. The enantiomeric separation and elution order of a sulfinate ester containing two stereogenic centers as well as 15 chiral sulfoxides is presented. The G-TA and B-DM CSPs generally gave opposite elution orders for most of the compounds studied.

Enantiomer separation of mandelates and their analogs on cyclodextrin derivative chiral stationary phases by capillary GC

Enantiomer separation of mandelates and their analogs, which are important intermediates in asymmetric synthetic and pharmaceutical chemistry, was investigated by capillary gas chromatography using different cyclodextrin derivative chiral stationary phases (CD CSPs). The used cyclodextrin derivatives included permethylated beta-CD (PMBCD), permethylated gamma-CD, heptakis(2,6-di-O-butyl-3-O-butyryl)-beta-CD, heptakis(2,6-di-O-pentyl-3-O-acetyl)-beta-CD and heptakis(2,6-di-O-nonyl-3-O-trifluoroacetyl)-beta-CD (DNTBCD), respectively. Among all the CSPs used, PMBCD and DNTBCD exhibited the broadest and best enantioselectivity for all the racemates investigated. Some thermodynamic parameters were evaluated and an enthalpy-entropy compensation effect was observed in enantiomer separation processes of mandelates and their analogs. Based on thermodynamic data and molecular mechanics calculations, the chiral recognition mechanism of mandelate derivatives on CD CSPs is discussed.

Direct chiral resolution of lactic acid in food products by capillary electrophoresis

Chiral resolution of native DL-lactic acid was performed by capillary electrophoresis using 2-hydroxypropyl-beta-cyclodextrin as a chiral selector. Various factors affecting chiral resolution, migration time, and peak area of lactic acid were studied. The running conditions for optimum separation of lactic acid were found to be 90 mM phosphate buffer (pH 6.0) containing 240 mM 2-hydroxypropyl-beta-cyclodextrin with an effective voltage of -30 kV at 16 degrees C, using direct detection at 200 nm. In order to enhance the sensitivity, sample injection was done under a pressure of 50 mbar for 200 s. On-line sample concentration was accomplished by sample stacking. With this system, D- and L-lactic acids in food products were analyzed successfully.

Effect of column temperature and nitrogen flow rate on the separation of cis-permethrinic acid isomers on a (2,3,6-tri-O-methyl)-beta-cyclodextrin coated capillary column

The separation efficiency of a capillary column coated with (2,3,6-tri-O-methyl)-beta-cyclodextrin was determined at the temperature and flow rate ranges of 80-120 degrees C and 0.3-1.5 mL/min respectively using (+) and (-) cis-permethrinic acid methylesters as test compounds. The dependence of the retention differences, sum of peak widths and separation factor on the temperature and flow rate was calculated with step regression analysis. The column exhibited acceptable isomer separation capacity at each temperature and flow rate proving the excellent separation power of the cyclodextrin coating. The differences between the retention times of isomers depended on the logarithm and on the reciprocal value of temperature, the effect of flow rate was negligible. The sum of their peak widths depended reciprocally on the temperature and logarithmically on the flow rate, the impact of temperature being higher than that of flow rate. The significant impact of the reciprocal value of temperature can be explained by the inclusion complex formation between cyclodextrin and cis-permethrinic acid methylester isomers. The resolution depended on the square of column temperature and flow rate, indicating local optimums at each temperature and flow rate.