Biogenetic studies in Mentha x piperita. 2. Stereoselectivity in the bioconversion of pulegone into menthone and isomenthone

Mentha x piperita shoot tips and first leaf pairs were fed with aqueous sol utions of different deuterium-labeled pulegone and various enantiomeric dis tributions. The essential oil was extracted by solid-phase microextraction and analyzed using enantioselective multidimensional gas chromatography/mas s spectrometry. The genuine p-menthan-3-ones (-)-menthone and (+)-isomentho ne as well as their labeled analogues were analyzed simultaneously. Both en antiomers of labeled pulegone were converted into the corresponding labeled p-menthan-3-ones by Mentha x piperita, indicating an unspecific reduction process. The generation of 4S- and 4R-configured p-menthan-3-ones differed in their stereoselectivities. Labeled (S)-pulegone was reduced by Mentha x piperita more rapidly rather than (R)-pulegone. From a comparison of labele d pulegone enantiomers the bioconversion preferrably led to 4S-configured d iastereomers.