Biosynthesis of Daphnetin in Daphne mezereum L.

Shoots of Daphne mezereum synthesized daphnetin (7,8-dihydroxycoumarin) more efficiently from [2- 14 C]umbelliferone (7-hydroxycoumarin) than from [2- 14 C]p-coumaric acid, and [2- 14 C]caffeic acid was more poorly utilized still. These findings do not support the idea of derivation of daphnetin via hydroxylation of the caffeic acid ring at the 2 position, followed by lactone ring formation; instead they are consistent with the concept of daphnetin formation by an additional hydroxylation of umbelliferone at C-8. Umbelliferone was recovered with little l4C dilution from emulsin-hydrolysed extracts of shoots fed labelled umbelliferone, and TLC of extracts from untreated shoots revealed two substances yielding umbelliferone on hydrolysis. Evidence is presented from TLC and HP LC analysis that one of these is skimmin (7-O-β-ᴅ -glucosylumbelliferone), not previously reported from Daphne. The tracer experiments further support the theory that umbelliferone is the general precursor of coumarins bearing two or more hydroxyl functions on the aromatic ring.