In the first pathway, ( + )-costunolide undergoes enzymatic epoxidation forming parthenolide.
2.
It was proposed that the methylene of ( + )-Costunolide is reduced before the second ring closure.
3.
This species contains the tridepside compound tenuiron, and the sesquiterpenes (-)-frullanolide and costunolide, the latter of which has been shown to be damaging to DNA.
4.
The second pathway includes the enzymatic hydroxylation of ( + )-costunolide followed by dehydration and cyclization to give the guaianolide skeleton . ( "'Fig . 3 "') Further epoxiation of the guaianolide skeleton would yield the desired sesquiterpene lactone, arglabin.
5.
Germacrene acid could then be hydoxylated and undergo lactonization to form ( + )-costunolide, a branching point for the biosynthesis of sesquiterpene lactones . ( "'Fig . 2 "') From here, the biosynthesis of guaianolides can follow two proposed pathways.
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