BIOSYNTHESIS OF SECOIRIDOID GLUCOSIDES IN OLEACEAE

The biosynthesis of the oleoside-type of glucosides (oleosides) has been investigated in two species of Oleaceac, namely Fraxinus excelsior and Syringa josikaea. Deuterated analogues of deoxyloganic acid, 8-epi-deoxyloganic acid, 7-epi-loganic acid and loganic acid were prepared. On administration of the above compounds high incorporations of deoxyloganic acid as well as of loganic acid and its 7-epimer were obtained, strongly implying that 7-ketologanic acid is an intermediate in the biosynthesis of oleosides. The incorporations were measured by H-2 NMR in several isolated secoiridoids all belonging to the oleosides. In addition, the content of acidic iridoids (both carbocyclic and seco-type) in two additional species of Oleaceae, namely Ligustrum ovalifolium and Phillyrea latifolia, were examined. Based on these results, a plausible biosynthetic route from deoxyloganic acid, 7-epi-loganic acid, 7-ketologanic acid, 8-epi-kingisidic acid, oleoside 11-methyl ester, 7-beta-1-D-glucopyranosyl 11-methyl oleoside and ligstroside to oleuropein can now be presented.