Extending the scope of enantiomer separation on diluted methylated β-cyclodextrin derivatives by high-resolution gas chromatography

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.

Investigation of a Quantitative Method for the Analysis of Chiral Monoterpenes in White Wine by HS-SPME-MDGC-MS of Different Wine Matrices

A valid quantitative method for the analysis of chiral monoterpenes in white wine using head-space solid phase micro-extraction-MDGC-MS (HS-SPME-MDGC-MS) with stable isotope dilution analysis was established. Fifteen compounds: (S)-(−)-limonene, (R)-(+)-limonene, (+)-(2R,4S)-cis-rose oxide, (−)-(2S,4R)-cis-rose oxide, (−)-(2R,4R)-trans-rose oxide, (+)-(2S,4S)-cis-rose oxide, furanoid (+)-trans-linalool oxide, furanoid (−)-cis-linalool oxide, furanoid (−)-trans-linalool oxide, furanoid (+)-cis-linalool oxide, (−)-linalool, (+)-linalool, (−)-α-terpineol, (+)-α-terpineol and (R)-(+)-β-citronellol were quantified. Two calibration curves were plotted for different wine bases, with varying residual sugar content, and three calibration curves for each wine base were investigated during a single fiber’s lifetime. This was needed as both sugar content and fiber life impacted the quantification of the chiral terpenes. The chiral monoterpene content of six Pinot Gris wines and six Riesling wines was then analyzed using the verified method. ANOVA with Tukey multiple comparisons showed significant differences for each of the detected chiral compounds in all 12 wines. PCA score plots showed a clear separation between the Riesling and Pinot Gris wines. Riesling wines had greater number of chiral terpenes in comparison to Pinot Gris wines. Beyond total terpene content it is possible that the differences in chiral terpene content may be driving the aromatic differences in white wines.

Sex pheromone of female african white rice stem borer,Maliarpha separatella (Lepidoptera: Pyralidae) from Sierra Leone: Identification and field testing

Analysis of ovipositor washings of femaleMaliarpha separatella from Sierra Leone by high-resolution gas chromatography (GC) linked to a male electroantennograph (EAG) indicated the presence of three electrophysiologically active compounds. The GC retention times of these compounds were consistent with those of (Z)-9-tetradecen-1-ol, (Z,E)-9,12-tetradecadien-1-ol, and (E,E)-10,12-tetradecadien-1-ol. Analysis by El and CI mass spectrometry of ovipositor washings confirmed these identifications and also indicated the presence of the saturated analog, tetradecan-1-ol. There was no evidence, from these analyses, of the corresponding aldehydes or acetates. The EAG-active compounds were present in the ovipositor washings in a ratio of approximately 2∶3.5∶1, with the major component constituting approximately 0.4 ng per ovipositor. GC-MS analysis of entrained female effluvia confirmed that the EAG-active compounds were released by virgin females. Field testing of the EAG-active compounds indicated that (Z,E)-9,12-tetradecadien-1-ol and (E,E)10,12-tetradecadien-1-ol were attractive to male moths in ratios of between 9:1 and 39:1, while (Z)-9-tetradecen-1-ol was found to reduce trap catch when added to blends of the other two compounds.

Determination of interconversion barriers by dynamic gas chromatography: epimerization of chalcogran

The four stereoisomers of chalcogran 1 ((2RS,SRS)-2-ethyl-1,6-di-oxaspiro[4.4]nonane), the principal component of the aggregation pheromone of the bark beetle pityogenes chalcographus, are prone to interconversion at the spiro center (C5). During diastereo- and enantioselective dynamic gas chromatography (DGC), epimerization of 1 gives rise to two independent interconversion peak profiles, each featuring a plateau between the peaks of the interconverting epimers. To determine the rate constants of epimerization by dynamic gas chromatography (DGC), equations to simulate the complex elution profiles were derived, using the theoretical plate model and the stochastic model of the chromatographic process. The Eyring activation parameters of the experimental interconversion profiles, between 70 and 120 C in the presence of the chiral stationary phase (CSP) Chirasil-beta-Dex, were then determined by computer-aided simulation with the aid of the new program Chrom-Win: (2R,5R)-1: deltaG(++) (298.15 K) = 108.0 +/-0.5 kJ mol(-1), deltaH(++) = 47.1+/-0.2 kJ mol(-1), deltaS(++) = -204+/-6 JK(-1) mol(-1): (2R,5S)-1: deltaG(++) (298.15 K) = 108.5+/-0.5 kJ mol(-1), deltaH(++) = 45.8+/-0.2 kJ mol(-1), deltaS(++) = -210 +/-6 J K mol(-1); (2S,5S)-1: deltaG(++) (298.15 K)= 108.1+/-0.5 kJ mol(-1), deltaH(++) = 49.3+/-0.3 kJ mol(-1), deltaS(++) = -197+/-8 J K(-1) mol(-1); (2S,5R)-1: deltaG(++) (298.15 K)=108.6+/-0.5 kJ mol(-1), deltaH(++) = 48.0+/-0.3 kJ mol(-1), deltaS(++) = -203+/-8 J K(-1) mol(-1). The thermodynamic Gibbs free energy of the E/Z equilibrium of the epimers was determined by the stopped-flow multidimensional gas chromatographic technique: deltaG(E/Z) (298.15 K)= -0.5 kJ mol(-1), deltaH(E/Z) = 1.4 kJ mol(-1) and deltaS(E/Z) = 6.3 J K(-1) mol(-1). An interconversion pathway proceeding through ring-opening and formation of a zwitterion and an enol ether/alcohol intermediate of 1 is proposed.

Consequences of variable purity of heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin determined by liquid chromatography-mass spectrometry on the enantioselective separation of polychlorinated compounds

The composition of 10 batches of heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (PM-CD) from different suppliers was determined by high-performance liquid chromatography combined with atmospheric pressure chemical ionisation mass spectrometry (MS). Considerable differences were found. Some batches consisted of more than 95% pure PM-CD, whereas others were not completely derivatised or contained a significant amount of by-products. Some suggestions about the structures of these impurities are given though neither nuclear magnetic resonance spectroscopy nor MS-MS investigations could completely reveal their nature. Capillaries for high-resolution gas chromatography were coated with the batches of most differing composition. They demonstrated widely varying column performance and separation properties for selected chiral polychlorinated substances such as chlordane compounds, o,p'-DDT, o,p'-DDD, alpha-hexachlorocyclohexane and atropisomeric polychlorinated biphenyls. The best enantioselectivity was obtained with the purest PM-CD. Compared to separations reported in the literature, a broader enantioselectivity was observed and also trans-heptachlor epoxide and oxychlordane could be resolved into enantiomers.

Enantioselective determination of chiral organochlorine compounds in biota by gas chromatography on modified cyclodextrins

Approaches to the gas chromatographic enantiomer separation of chiral organchlorines (alpha-hexachlorocyclohexane, cis- and trans-chlordane, heptachlor, heptachlorepoxide, oxychlordane, o,p'-DDT, compounds of technical toxaphene and stable atropisomeric polychlorinated biphenyls) are reviewed. Chiral stationary phases based on cyclodextrin derivatives and used for the gas chromatographic enantiomer separation of the chiral organochlorines are described. Enantiomeric ratios of chiral organochlorines in technical mixtures and biological samples are reported and discussed.

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.

CHIRBASE, a molecular database for the separation of enantiomers by chromatography

A selected review on the separation of the stereoisomers of menthol by gas chromatography serves to demonstrate a new approach to tackle the increasing number of publications on enantiomer separation. The graphical molecular database CHIRBASE covers information (structural, bibliographic and chromatographic data) on liquid chromatography, supercritical fluid chromatography and gas chromatography. CHIRBASE is based on standard database software, thus meeting the requirements of "state of the art" information management in the chemical and pharmaceutical industries and academia. The scrutiny required to create and maintain a factual database revealed that the information content of the contemporary literature with respect to chromatographic data requires improvement.