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 bicyclo[4.4.1]undecane, bicyclo[4.3.1]decane, bicyclo[5.4.1]dodecane, and bicyclo[6.4.1]]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.