TfOH-promoted Classical Nazarov-type Cyclization of Benzofulvenes: Synthesis of Polycyclic 5H,10′H‑spiro[benzo[k]phenanthridine-5,6′-dibenzopentalenes]

The reaction of o-benzofulvene with TfOH leads to intramolecular cyclization through novel C–C and C–N bond formation, resulting in the formation of 5H,10′H-spiro­[benzo­[k]­phenanthridine-5,6′-dibenzopentalene]. This protocol provides a new molecular framework with reasonable to excellent yields and tolerates various electron-withdrawing/donating substituents. This method yields diastereoselectivity of up to >20:1. Furthermore, it is free of bases, oxidants, and metals and proceeds under mild reaction conditions, which are favorable for synthetic organic chemistry.