Bioinspired Total Syntheses of Grayanotoxins II/III, Kalmanol, and Rhodomollacetals A-C with Three Distinct Skeletons

Grayanane diterpenoids, which have intricate molecular architecture and interesting pharmacological properties, have attracted considerable interest of organic chemists since 1970s. However, the total syntheses of grayanane diterpenoids have been accomplished by only seven groups, which have mainly focused on the same carbon skeleton. Herein, we disclose the bioinspired total syntheses of six grayanane diterpenoids with three distinct carbon skeletons: grayanotoxins II/III, kalmanol, and rhodomollacetals A-C. Among them, the total syntheses of rhodomollacetals A-C were achieved for the first time. The total syntheses feature a Ni-catalyzed alpha-vinylation of beta-ketoester to construct a bicyclo [3.2.1]octane ring system, an intramolecular Pauson- Khand reaction of enyne to construct a 7/5 -bicyclic ring system, a biomimetic 1,2 -rearrangement of grayanane-type 5/7/6/5 skeleton to generate kalmane-type 5/8/5/5 skeleton, and a biomimetic oxidative cleavage of C2-C3 and C5-C6 bonds/skeletal reconstruction to construct a rhodomollacetal A skeleton.