Unexpected Participation of an Unconjugated Olefin during Nazarov Cyclization of Bridged Bicyclic Dienones

Surprising rearrangements in a rigid system: A cyclopropyl ring is formed unexpectedly when a trienone substrate is subjected to the Nazarov cyclization (see scheme). The participation of the unconjugated alkene unit present in the substrate might be a consequence of the complete torquoselectivity of the initial electrocyclic closure. A subsequent rearrangement by a homo-1,5-hydrogen shift occurs cleanly to give the final product.