Synthetic studies on breviones: construction of the CDE ring system

Breviones A–E (1–5), allelopathic agents isolated from Penicillium brevicompactum Dierckx, are structurally unique pentacyclic or hexacyclic diterpenoid derivatives. The synthesis of a structurally simplified model compound (8), corresponding to the characteristic spiro-fused CDE ring portion of 1–4, was accomplished by employing sequential, double nucleophilic substitution reactions as the key steps.