Synthesis of 4-desmethyl-rippertenol and 7-epi-rippertenol via photoinduced cyclization of dienones

Abstract

The synthesis of cycloheptanoid-based fused polycyclic frameworks is a challenge for organic chemists due to unfavorable entropic factors and ring strains. Herein, a concise synthesis of 4-desmethyl -rippertenol and 7-epi-rippertenol bearing a unique, [6,6,5,7]-fused tetracyclic framework is reported. The route features a novel photoinduced intramolecular cyclization of α-cyclopropyl dienone followed by an unexpected thermal 1,5-hydrogen migration, which provides efficient access to the fused seven-membered ring system in a stereochemically well-defined manner. Further density functional theory (DFT) calculations disclose that the stereoselectivity of this photoinduced process is mainly attributed to transition state conformation and steric effects.