Evidence is presented that is consistent with cucurbitacin B being biosynthesized via cycloartenol or parkeol but not through the intermediacy of lanosterol.THE chemistry and properties of the cucurbitacins have been well documented;l in contrast, the mechanisms by which these novel compounds are biosynthesized remain unknown. Of particular interest is the unusual methyl substitution pattern of ring B, and we have therefore investigated the mechanism of elaboration of this ring by means of labelled precursors.The biosynthesis of the cucurbitacins from [(4R)-4-,H1,2-14C]mevalonic acid (MYA) 1) can be expected to involve the initial formation of the cation (3) ; from this cation several routes are then possible (Scheme). Loss of the C-9p hydrogen, accompanied by the multiple migrations indicated will lead (path a) to lanosterol (4) , which may then rearrange to give the cucurbitacin skeleton (7a). Such a sequence would entail the loss of tritium from C-9 with the result that, together with the loss of the two tritium atoms from C-3 and C-20, cucurbitacin B (7) would have a 3H/14C ratio of 316 relative to that of the original MVA or squalene (2).
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