13-Step Total Synthesis of Atropurpuran

Herein, we report a concise total synthesis of atropurpuran, a unique and synthetically challenging pentacyclic diterpene that bears a tetracyclo­[5.3.3.04,9.04,12]-tridecane skeleton that is unprecedented among natural terpenes. This 13-step approach features a strategy that include early stage rapid skeleton formation and the late-stage introduction of reactive functional groups, thus allowed a high overall synthetic efficiency with minimal use of PGs. The key transformations of our work include a facile construction of the spiro[5.5]­undecane moiety through an ring-closing enyne metathesis reaction and an efficient formation of the tetracyclo­[5.3.3.04,9.04,12]-tridecane scaffold via an regioselective double oxidative dearomatization/intramolecular Diels–Alder reaction cascade. This efficient approach should also inspire further advances in the synthesis of related complex diterpenes and diterpenoid alkaloids.