Proposed Biosynthesis
The full biosynthesis of α-santonin has not been elucidated but α-santonin bears much similarity to parthenolide. The proposed biosynthesis begins with the cyclization of farnesyl diphosphate (FPP) to (+)-germacrene A by a sesquiterpene synthase. (+)-germacrene A hydroxylase then hydroxylates the isopropenyl side chain. The oxidation of germacratrien-12-ol to germacratrien-12-oic acid via the intermediate germacratrien-12-al is done by NADP+-dependent dehydrogenase(s). Germacratrien-12-oic acid is then hydroxylated at C6 subsequently followed by lactonization forming (+)-costunolide.
It was proposed that the methylene of (+)-Costunolide is reduced before the second ring closure. The bicyclic decalin ring system is formed via the eudesmyl cation followed by hydroxylation at C1. Further oxidation at C3 forms the β-ketohydroxyl which upon elimination of H2O completes the proposed biosynthetic pathway of α-santonin.
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