Furan derivatives. Part 10. Synthesis of cyclohepta[cd]benzofuran

Cyclohepta[cd]benzofuran 2 was synthesized by heating (5‐oxo‐5H‐benzocyclohepten‐4‐yloxy)acetic acid 16 with sodium acetate in acetic anhydride or by photocyclization of 16 in acetonitrile. Several reactions of cyclohepta[cd]benzofuran 2 were examined. Protonation of 2 with trifluoroacetic acid occurred at the 2‐position to give a tropylium ion 17. Catalytic hydrogenation of 2 with palladium on charcoal proceeded smoothly to give tetrahydrocyclohepta[cd]benzofuran 18. The Diels‐Alder reaction of 2 with tetracyano‐ethylene produced an adduct 19. Formylation of 2 with phosphorus oxychloride and dimethylformamide occurred easily at the 2‐position to afford compound 20. Cyclohepta[cd]benzofuran 2 has both properties of heptafulvene and benzofuran.