Synthesis and enantiodifferentiation of isomeric theaspiranes.
1992
Schmidt G. | Full G. | Winterhalter P. | Schreier P.
The spiro ethers 6a-d were prepared in a biomimetic-type reaction from their natural precursor 4-hydroxy-7,8-dihydro-beta-ionol (5) that was available from 4-oxo-beta-ionone (1) by NaBH4 and subsequent H2/Pd reduction. The so-obtained racemic keto alcohol 3 was esterified with (R)-(-)-2-phenylpropionic acid, and the resulting diastereomeric esters (4a/b) were isolated in pure form (de > 90%) by preparative HPLC. Configuration at C-9 was determined by 1H NMR spectroscopy. The isomeric diols 5a/b obtained from esters 4a/b by LiAlH4 reductive cleavage were subjected to thermal treatment at pH 3.4, yielding two pairs of diastereomeric theaspiranes (6a/b and 6c/d) which were subsequently obtained in optically pure form by preparative HPLC. The absolute configuration at C-5 was established by NOE experiments. The enantiomers distinctly differed in their odor properties. Whereas spiro ether 6a showed a weak camphoraceous note, isomer 6b exhibited an intense fresh-fruity (black currant or cassis) odor. The odor of isomer 6c was dominated by a fresh camphoraceous note; this note almost became naphthalene-like in isomer 6d. Using on-line coupled multidimensional gas chromatography-mass spectrometry (DB-5/C-Dex B) with SIM mode, enantiodifferentiation of 6a-d in a number of natural sources revealed a high variation in the distribution of the enantiomers.
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