Cucurbitacins biosynthesis

Alfhough bearing a sterane nucleus, fhe biosyntheses of cucurbitacins and dammaran-type saponins will not be reviewed here. Both groups belong to the triterpenoids, the chemistry, biosynthesis and biological activities of which have been described elsewhere (Charlwood and Banthorpe, 1991 ... 

Cucurbitacin B (99) biosynthesis has been studied in Cucurbita pepo. When [2- " C,3/ ,4/ - H]mevalonate was used, four tritium atoms were incorporated, of which one was at C-10. Using [2- C,2- H2]mevalonate, ten tritium atoms were incorporated. Thus only the expected losses from C-3, C-20, and C-22 occurred. The presence of tritium at C-8 excludes lanosterol as an intermediate. Either cycloartenol (which is present in this plant) or parkeol may be intermediates. 

Widespread reports indicate that, in addition to their anti-inflammatory role, triterpenes have other activities. Triterpenoids have been found to possess cytotoxic, antimicrobial and interesting effects on metabolism. Triterpenoids with antitumour activity include oleanane, lanostane, lupane, friedelane, hopane and quassionoid types. Glycyrrhetinic acid has been described as an antiviral, hypolipidemic and anti-atherosclerotic agent. Cucurbitacins B and E, and oleanolic acid possess a potent protective action on the liver, and ganoderic acid and its derivatives have been shown to be inhibitors of cholesterol biosynthesis. Lanostane derivatives, like suberosol, have also been found to inhibit HIV replication in H9 lymphocytes [1,3]. 

Cattel, L., Balliano, G., Caputo, O., Viola, F. Biosynthesis of cucurbitacins in Bryonia dioica seedlings. Planta Med. 41, 328-336 (1981)... 

The common precursor in the biosynthesis of limonoids and cucurbitacins is squalene-2,3-oxide (IV). Based on some identified intermediary compounds, the biosynthetic pathway is probably as postulated in Reaction 18.7. 

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