SYNTHESIS AND ENANTIODIFFERENTIATION OF RIESLING ACETALS

The cyclic acetals 1b/c were prepared in a biomimetic-type reaction vi a the Mosher esters 5a/b of their natural precursor 0,10-tetramethyl-1 -oxaspiro[4.5]dec-6-ene-2,8-diol (6) which were available from 3,4-did ehydro-beta-ionone (2) by photosensitized oxidation and subsequent red uction with thiourea, The racemic cis-diols 3 so-obtained were esterif ied with ).alpha-methoxy-alpha-trifluoromethyl-phenylacetyl chloride a nd the resulting diastereomeric esters (4a/b) isolated in pure form (d iastereomeric excess >95%) by preparative high performance liquid chro matography. Configuration at C-6 was determined after bio-oxidative cl eavage of 4a/b to dehydrovomifoliols 8a/b, The hemiacetals 5a/b obtain ed from esters 4a/b by hydrogenation (Pd/BaSO4) were subjected to ther mal treatment at pH 3 yielding two optically enriched isomers of Riesl ing acetal which were subsequently obtained in optically pure form by preparative gas chromatography using a modified cyclodextrin column. T heir relative configuration was established by NOE-experiments. In add ition, the structures and the heats of formation of the different dias tereomers were calculated using a force field program and the semi-emp irical MNDO, AM1 and PM3 methods. The absolute configurations were fou nd from the results of the NOESY experiment, the calculations and the stereoselective synthesis. Riesling acetal enantiomers differed distin ctly in their odour properties, (+)-Riesling acetal 1b showed a weak ' 'woody, fruity, ionone, flowery'' note, whereas the (-)-isomer 1c exhi bited a slight ''camphoraceous, flowery'' note. Enantiodifferentiation of 1b/c in a number of natural sources with the help of multidimensio nal gas chromatography (DB-Wax/C-Dex B) revealed a nearly racemic comp osition of 1b/c.