Enantiomeric distribution of odour-active epoxyaldehydes in yuzu (Citrus junos Sieb. ex Tanaka)

The odour-active epoxyaldehydes cis-4,5-epoxy-(E)-2-decenal (1), trans-4,5-epoxy-(E)-2-decenal (2), and trans-4,5-epoxy-(E,Z)-2,7-decadienal (3) are present in yuzu (Citrus junos Sieb. ex Tanaka) volatile oil. These epoxyaldehydes have two chirality centres at the C4 and C5 positions in the epoxide moiety. To date, little is known about the enantiomeric distributions of the expoxyaldehydes in natural products. In this study, we examined the enantiomeric distributions of the epoxyaldehydes in yuzu volatile oil using multidimensional gas chromatography. The standard substrates used for 1 and 3 were synthesised enantioselectively using the Katsuki-Sharpless epoxidation. Standard substrates used for 2 were made via the asymmetric organocatalytic epoxidation with a proline-derivative catalyst. The 4S,5R- and 4R,5S-enantiomers of 1 were obtained in moderate enantiomeric excess (44–66 % ee), while 4S,5S- and 4R,5R-enantiomers of 2 and 3 were obtained in high enantiomeric excess (2, 90–92 % ee; 3, 96 % ee). Enantioselective multidimensional gas chromatography-mass spectrometry (enantio-MDGC-MS) revealed that the chiral centres at C4 and C5 in the cis-form (1) have the (4R,5S) absolute configuration, while trans-forms (2 and 3) have the (4S,5S) configuration. The finding of the absolute configuration at C5 (S) suggests that the optically active epoxyaldehydes may be formed along with retention of the S configuration at C13 of the lipid hydroperoxides 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoic acid (13S-HPOD) or 13(S)-hydroperoxy-9(Z),11(E),15(Z)-octadecatrienoic acid (13S-HPOT) during enzymatic lipid oxidation. Finally, we propose a pathway for the formation of the optically active epoxyaldehydes in yuzu.