Synthesis of Tribenzotropone by Ring Expansion of Phenanthrene-9,10-dione

Tribenzotropone was efficiently synthesized by a ring-expansion method from readily available phenanthrene-9,10-dione via a ring-opened diketone as a key intermediate; the diketone was prepared by nucleophilic addition of allyl and vinyl groups, followed by an oxidative ring-opening reaction with lead(IV) acetate. Ring closure by an intramolecular Diels–Alder reaction and subsequent dehydrogenation produced tribenzotropone in 38% overall yield. Ring closure by a Morita–Baylis–Hillman reaction, on the other hand, produced a dibenzo-fused nonanedione in 22% overall yield.