A series of novel sesterterpenes (2–6) have been isolated from the roots of Aletris farinosa and structurally characterized by MS, NMR, and X-ray crystallography in conjunction with computational modeling. Their structures provide new insights into the mechanisms of sester-terpene biosynthesis. Specifically, we propose with support from density functional theory compu-tations that the configuration at a single stereocenter determines the fate of a key tetracyclic carbo-cationic intermediate, derived from an oxidogeranylfarnesol precursor. Whereas one epimer of the carbocation undergoes H+ elimination to give 6, the other undergoes a spectacular cascade of seven 1,2 methyl and hydride migrations leading to the previously unreported carbon skeleton of 5. Theo-retical calculations suggest that the cascade is triggered by substrate preorganization in the enzyme active site.