Synthesis and enantiodifferentiation of riesling acetals

Abstract

AbstractThe cyclic acetals 1b/c were prepared in a biomimetic‐type reaction via the Mosher‐esters 5a/b of their natural precursor 2,6,10,10‐tetramethyl‐1‐oxaspiro[4.5]dec‐6‐ene‐2,8‐diol (6) which were available from 3,4‐didehydro‐β‐ionone (2) by photosensitized oxidation and subsequent reduction with thiourea. The racemic cis‐diols 3 so‐obtained were esterified with (–)‐α‐methoxy‐α‐trifluoromethyl‐phenylacetyl chloride and the resulting diastereo‐meric esters (4a/b) isolated in pure form (diastereomeric excess >95%) by preparative high performance liquid chromatography. Configuration at C‐6 was determined after bio‐oxidative cleavage of 4a/b to dehydrovomifo‐liols 8a/b. The hemiacetals 5a/b obtained from esters 4a/b by hydrogenation (Pd/BaSO4) were subjected to thermal treatment at pH 3 yielding two optically enriched isomers of Riesling acetal which were subsequently obtained in optically pure form by preparative gas chromatography using a modified cyclodextrin column. Their relative configuration was established by NOE‐experiments. In addition, the structures and the heats of formation of the different diastereomers were calculated using a force field program and the semi‐empirical MNDO, AMI and PM3 methods. The absolute configurations were found from the results of the NOESY experiment, the calculations and the stereoselective synthesis. Riesling acetal enantiomers differed distinctly in their odour properties. (+)‐Riesling acetal 1b showed a weak “woody, fruity, ionone, flowery” note, whereas the (‐)‐isomer 1c exhibited a slight “camphoraceous, flowery” note. Enantiodifferentiation of 1b/c in a number of natural sources with the help of multidimensional gas chromatography (DB‐Wax/C‐Dex B) revealed a nearly racemic composition of 1b/c.