First identification of the odour-active unsaturated aliphatic acid (E)-4-methyl-3-hexenoic acid in yuzu (Citrus junos Sieb. ex Tanaka)

We have investigated the aroma of yuzu (Citrus junos Sieb. ex Tanaka). Application of the aroma extract dilution analysis technique allowed us to determine a sequence of the odour-active compounds in yuzu volatile oil. In this study, we confirmed the presence of an unknown odorant that possesses a sweaty odour and a relatively high flavour dilution factor. In order to identify the unknown compound, the concentrated volatile acid fraction of yuzu volatile oil was obtained, using an enrichment procedure consisting of the combination of the sub-extraction of an acidic component with a basic aqueous solution and subsequent affinity chromatography with an Oasis MAX cartridge. Gas chromatography–olfactometry and multi-dimensional gas chromatography–mass spectrometry analyses revealed that the unknown compound was (E)-4-methyl-3-hexenoic acid [(E)-4M3H]. (E)-4M3H was identified for the first time as an odour-active unsaturated aliphatic acid in yuzu. Additionally, both (Z)- and (E)-4M3H were stereoselectively synthesised over two steps comprised of the iron-catalysed carbometalations of homopropargylic alcohols with Grignard reagents, followed by oxidation. On the basis of sensory evaluation, the orthonasal odour detection threshold of the (E)-isomer was 10-fold to 100-fold lower than that of the (Z)-isomer. Copyright © 2012 John Wiley & Sons, Ltd.