Investigations of cascade cyclizations of terpenoid polyalkenes via radical cations. A biomimetic-type synthesis of (+/-)-3-hydroxy-spongian-16-one

A short and efficient synthesis of the tetracyclic oxygenated diterpene ana log of 8, i. e., of 3-hydroxy-spongian-16-one (7), has been achieved in onl y five steps via a cascade cyclization of the functionalized terpenoid poly alkene 4. Photoinduced electron trans fer serves as the pivotal step for se lectively creating a radical cation intermediate. Such polycyclizations are radical-driven upon regio-and stereoselective trapping of the radical cati on by a nucleophile, such as water in the present case. The overall reactio n sequence mimics the non-oxidative biosynthesis of terpenes.