Total Synthesis of Sculponin†U through a Photoinduced Radical Cascade Cyclization.

We have accomplished the total synthesis of sculponin U, a polycyclic C-20-oxygenated kaurane diterpenoid featuring a 7,20-lactone-hemiketal bridge, through a radical cascade cyclization triggered by photoinduced electron transfer (PET) of a silyl enolate to form the cyclohexanone-fused bicyclo[3.2.1]octane skeleton. Other key points in our synthetic strategy encompass a Diels-Alder reaction to construct the middle six-membered ring of sculponin U, and an intramolecular radical cyclization induced by iron-catalyzed hydrogen atom transfer to close the western cyclohexane ring. Successful preparation of the enantiopure silyl enolate as the PET precursor enables the asymmetric total synthesis of sculponin U, opening a new avenue for divergent syntheses of structurally related C-20-oxygenated kaurane congeners and pharmaceutical derivatives thereof.