Stereochemical aspects of the bioreduction of the conjugated double bond of perillaldehyde

A study on the regioselective reduction of the conjugate double bond of perillaldehyde is described. The chemical reduction of this substrate was investigated in order to provide a straightforward access to the relevant natural flavour, dihydroperill-aldehyde. The biological reduction of both natural (S)-(-)-perillaldehyde and synthetic (R)-(+)-perillaldehyde was accomplished by means of fermenting baker's yeast. The latter microorganism converted, with different diastereoselectivity, the (S)- and (R)-enantiomers into the corresponding trans and cis saturated alcohols, respectively. The origin of the hydrogen atoms added to the double bond was studied by deuterium labelling experiments and H-2 NMR measurements that clearly demonstrate a different mechanism of the biohydrogenation of the two enantiomeric forms of perillaldehyde.