Biosynthesis of Sordarin Revealing a Diels–Alderase for the Formation of the Norbornene Skeleton

Abstract Sordarin ( 1 ) is a fungal diterpene glycoside that displays potent antifungal bioactivity through inhibition of elongation factor 2. The structures of sordarin and related compounds feature a highly rearranged tetracyclic diterpene core. In this study, we identified a concise pathway in the biosynthesis of sordarin. A diterpene cyclase (SdnA) generates the 5/8/5 cycloaraneosene framework, which is decorated by a set of P450s that catalyze a series of oxidation reactions, including hydroxylation, desaturation, and C−C bond oxidative cleavage, to give a carboxylate intermediate with a terminal alkene and a cyclopentadiene moiety. A novel Diels–Alderase SdnG catalyzes an intramolecular Diels–Alder (IMDA) reaction on this intermediate to forge the sordarin core structure. Subsequent methyl hydroxylation and glycosylation complete the biosynthesis of sordarin. Our work discloses a new strategy used by nature for the formation of the rearranged diterpene skeleton.