Type II Intramolecular [5+2] Cycloaddition: Facile Synthesis of Highly Functionalized Bridged Ring Systems

A type II intramolecular oxidopyrylium‐mediated 5+2] cycloaddition reaction allows the efficient and diastereoselective formation of various highly functionalized and synthetically challenging bridged seven‐membered ring systems (such as bicyclo4.4.1]undecane, bicyclo4.3.1]decane, bicyclo5.4.1]dodecane, and bicyclo6.4.1]]tridecane). This simple, thermal, direct transformation has a broad substrate scope and is high yielding, with high functional‐group tolerance and unique endo selectivity. The highly strained tricyclic cores of ingenol mebutate (picato) and cyclocitrinol are synthesized efficiently and diastereoselectively using this methodology.