BIOSYNTHESIS OF IRIDOIDS IN SYRINGA AND FRAXINUS - CARBOCYCLIC IRIDOID PRECURSORS

Feeding experiments with deuterium-labelled precursors to Fraxinus exc elsior, Syringa josikaea and S. vulgaris have shown that the biosynthe sis of the oleoside-type secoiridoids (e.g. oleuropein) proceeds via i ridodial, iridotrial, deoxy-loganic acid aglucon and deoxy-loganic aci d. Hydroxylation of the 7 alpha-position is followed by oxidation and methylation of C-11 to give 7-ketologanin, the last carbocyclic iridoi d precursor of the oleosides. The sequences of the steps between deoxy -loganic acid and 7-ketologanin may differ with plant species and time of year. 8-Epi-kingisidic acid and 8-epi-kingiside can be formed from 8-ketologanic acid (and its methyl ester). The secoiridoids, fliedero side and lilacoside, from S. vulgaris have been characterized.