This review describes current analytical technology for the analysis of chiral constituents in essential oils and other natural volatiles, flavor and fragrance compounds, and covers some important results achieved by natural compound chemists, food chemists, perfumers, and molecular biologists. The technique of enantioselective gas chromatography (GC) is described and applied for assigning absolute configuration of chiral natural compounds, which is strongly connected to differences in odor properties of their enantiomers. In addition, some recent results to facilitate the handling of GC-mass spectrometry data of known and unknown plant volatiles are discussed.

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The influence of substituents on the rotational energy barrier of a series of 2,2'-bis(trifluoromethy1)-and 2,2'-diisopropylbiphenyl derivatives with various substitution patterns was investigated by dynamic gas chromatography (DGC) with selectively modified cyclodextrins and by high-performance liquid chromatography (HPLC) on microcrystalline cellulose triacetate combined with polarimetry. Investigation of the steric effect of substituents was possible by gradually increasing the bulkiness of substituents in the ortho positions of 2,2'-disubstituted biphenyls and determination of the corresponding energy barriers. The buttressing effect of alkyl groups adjacent to the ortho substituents is discussed. Comparison of the energy barriers of 2,2'-bis(trifluoromethy1)biphenyl (1) with its derivatives possessing alkyl groups in positions 3 and 5, respectively, allows a discrimination between the electronic and steric contributions of the meta substituents and thus the determination of the influence on the energy barrier. The electronic effect on the rate of rotation, i.e. the rate of enantiomerization, of axially chiral biphenyls bearing substituents in the para positions depends on the aromatic system to which they are attached. Electron-donating groups decrease the rotational energy barrier of 2,2'-bis(trifluoromethy1)biphenyl derivatives but increase the barrier of 2,2'-diisopropylbiphenyl derivatives. Electron-accepting groups exhibit the opposite behavior. The unexpected effects in the case of para-substituted 2,2'-diisopropylbiphenyl derivatives are possibly related to C-H/n interactions.Atropisomeric biphenyls were successfully separated by high-performance liquid chromatography (HPLC) on microcrystalline cellulose triacetate on a preparative scale ['] and by capillary gas chromatography using modified cyclodextrins as chiral stationary phases[2,3]l, respectively. Rotation about the central bond of an axially chiral biphenyl causes enantiomerization. Accordingly, enantiomer separation requires configurational stability of at least 90 kJ mol-I. Rotamers possessing suitable configurational stability and volatility for dynamic gas chromatographic (DGC) investigations show temperature-dependent interconversion profiles and coalescence phenomena as a result of competition between resolution and enantiomerization in the gas chromatographic column. Computer simulation of experimentally obtained elution profiles provides rate constants and energy barriers. In this way, dynamic The configurational stability of atropisomeric biphenyls depends on nature, position, and number of the substituents. The bulkiness of the substituents in the ortho positions plays the most important role with respect to conformational stability of biphenyls. Similarly, steric interactions are decisive for the buttressing effect of meta substituents. They reduce the flexiblity of the adjacent ortho substituents and therefore support their steric interactions in the almost coplanar transition state. Nevertheless, electronic contributions to the i...