BIOSYNTHESIS OF IRIDOIDS IN SYRINGA AND FRAXINUS - SECOIRIDOID PRECURSORS

Several deuterium-labelled secoiridoids have been prepared and tested as possible precursors for the iridoids in Fraxinus excelsior, Syringa josikaea and S. vulgaris. Oleoside Ii-methyl ester was an efficient p recursor for the oleosides, whereas secologanin-type iridoids gave onl y significant incorporation in 5. josikaea. In this plant low incorpor ations into the oleosides were also seen for kingiside and 8-epi-kingi side. The major pathway to the oleosides therefore seems to proceed vi a a direct ring fission of ketologanin to oleoside 11-methyl ester. A Baeyer-Villiger-like mechanism which explains the different compounds found in the plants is proposed, and the taxonomy of the Oleaceae is d iscussed. Due to the unique presence of the usual pathway leading to s ecologanin and its congeners in Fontanesia, the Oleaceae is considered to be a member of the Gentiananae rather than Scrophulariales/Lamiana e.