Photoreaction of Spirofurans and Furan-Fused Benzocycloalkanes. Synthesis of Dihydrophenalene Derivatives and Helicene-Type Compounds

The photo-induced benzannulation of spiro[furan-2(3H),1\u27-benzocycloalkane]s afforded dihydrophenalene derivatives. Benzo[kl]xanthene was also formed by a similar photoreaction of spiro[furan-2(3H),9\u27-xanthene]. The photoreaction of the benzocycloalka[1,2-b]furans gave helicene-type compounds in good yields. The reaction pathway was elucidated by the 1H NMR spectrum of the reaction mixture and the alternative synthesis of the intermediate