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.