Biosynthesis of cucurbitacins in Bryonia dioica seedlings.

The biosynthesis of cucurbitacins during the seed germination of Bryonia dioica was studied by analysis of the cucurbitacin-triterpenoid fraction and by tracer experiments with acetate-[2- (14)C]. Isolation of 10alpha-cucurbita-5,24-dien-3beta-ol (9a), the simplest tetracyclic triterpene with a cucurbitane skeleton, supports the view that (9a) is the general precursor of cucurbitacins. Moreover, following the tracer experiments, cucurbitacin E (1a) was the first cucurbitacin formed, whereas the less oxygenated bryodulcosigenin (4a) was not detectable during germination of the plant. In the course of the present investigation, a new pentacyclic triterpene, isomultiflorenol (11a) (possible precursor of bryonolic acid (5a)), was also isolated.