SummaryThis paper reports the differentiation between the enantiomers of theaspiranes and theaspirones, potent flavor compounds widely used in the flavor industry. Optically pure reference compounds of isomeric theaspiranes were obtained by enantioselective synthesis. Enantiomerically pure reference theaspirones were isolated from quince fruit; their absolute stereochemistry was assigned by CD spectroscopy. For both types of compounds the order of elution was elucidated by using authentic reference compounds. These data enabled the determination of the enantiomeric distribution of both types of compounds in a variety of plant tissues. Because of the complexity of the natural flavor isolates, compounds were identified by mass spectrometry after multidimensional gas chromatography employing a Sichromat 2 double oven chromatograph. After separation of the target compounds on the first, achiral, column (DB-5), they were transferred to a chiral column (C-Dex B) for determination of the enantiomeric distribution. Multiple ion detection (MID) enabled the determination of the enantiomeric distribution even for complex mixtures containing the target compounds at extremely low levels.
Water-mediated (pH 3) model experiments were performed with citral (nerallgeranial la/lb) under oxygen atmosphere. After complete decomposition of l d l b and solvent extraction of the products formed, separation into neutral and acid fraction was carried out by bicarbonate treatment. Subsequent HRGC, HRGC-MS, and HRGC-FTIR analyses revealed the Occurrence of more than twenty compounds, among them geranic acid in the acid fraction and the 2-formylmethyl-2-methyl-5-( I-hydroxy-1-methylethyl)-tetrahydrofuran diastereomers 3d3b in the neutral fraction were the major products comprising approximately 15% and 80%, respectively, of the total volatiles formed. The determination of the absolute configuration of 3d3b and their analytical stereodifferentiation was achieved by synthesis and chromatography of the (2R) configured diastereomers, starting from ( R ) (-)-linalol.KEY WORDS Citral Citral decomposition 2-Formylmethyl-2-methyl-5-( 1-hydroxy-1-methylethyl)-tetrahydrofuran 2,8DMP-P-cyclodextrin
Enantioselective gas chromatography (GC) with 5 modified cyclodextrins was applied to chiral organochlorines. A prerequisite for determining GC elution orders of enantiomers is the availability of enantioenriched standard solutions. In addition to compounds reported before (e.g., α-HCH, PCB 174, oxychlordane), we determined the sign of optical rotation of enantioenriched solutions of e-aeee-pentachlorocyclohexene-1 (β-PCCH), perdeuterated α-HCH (α-PDHCH), perdeuterated β-PCCH, and the persistent compound of technical toxaphene—2-exo,3-endo,5-exo,9,9,10,10-heptachlorobornane (B7-1453)—by liquid chromatography (LC) with a chiral detector. An enantioenriched solution of (β-PCCH was obtained by enantioselective degradation of α-HCH with (-)-brucine. In addition to forming an enantiomeric excess of (-)-α- HCH, we formed enantioenriched (+)-(α-PCCH. In a similar study, α-PDHCH showed the same behavior with respect to enantioselectivity. Dextrorotation of an enantioenriched solution of B7-1453 was also confirmed by LC with a chiral detector. Enantioseparation of chiral organochlorines on 5 chiral stationary phases resulted in several reversed elution orders. These results indicate that a careful check of elution orders of organochlorine enantiomers is necessary prior to comparison of literature data for the study of enantioselective processes in the environment.
The benzopyran derivatives la-d were prepared in a biomimetic-type reaction from their natural precursor 3-hydroxy-retro-a-ion01 (6) which was available from a-ionol by tert-butyl chromate oxidation, reduction with NaBH4, and subsequent rearrangement of the 7,8 double bond. The soobtained geometrical isomers 3-oxo-retro-a-ionol4a/b were separated by preparative and analytical multilayer coil countercurrent chromatography. The racemic 3-oxo-retro-a-ionol4a was esterified with (R)-(-)-2-phenylpropionic acid, and the resulting diastereomeric esters (5a/b) were isolated in pure form (de 90%) by preparative HPLC. Configuration at C-9 was determined by 'H NMR spectroscopy. The isomeric diols 6a/b obtained from esters 5a/b by LiAlH4 reductive cleavage were subjected to thermal treatment (simultaneous distillation extraction), yielding two pairs of diastereomeric edulans (la/b and ldd) which were subsequently obtained in optically pure form by analytical HPLC. The absolute configuration at C-8a was established by NOE experiments. Using on-line coupled multidimensional gas chromatography-mass spectrometry [DB-Waxheptakis-(2,6-di-O-methyl-3-O-pentyl)-~-cyclodextrinl with selected ion monitoring mode, enantiodifferentiation of la-d in a number of natural sources revealed predominance of the 2 s enantiomers.
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