Polar Substituents in Benzobarrelene Photochemistry; The Photoreactivities of Dimethyl 10‐methoxy‐ and Dimethyl 10,11‐dimethoxy‐2,3‐benzobicyclo[2.2.2]octateraene‐5,6‐dicarboxylate

The direct irradiation of the monomethoxybarrelene 8 gives cyclooctatetraene product 18 (Φ = 0.075) via the anti ‐Epiotis aromatic‐vinyldiester [2π+2π] bridging route, and two semibullvalenes 16 (Φ = 0.075) and 17 (Φ = 0.075) from di‐π‐methane rearrangements initiating from vinyl‐vinyldiester bridging. Benzophenone sensitization yields 16 (Φ =0.11) and 17 (Φ = 0.11). The dimethoxybarrelene 9 gives only semibullvalene 10 , this resulting from di‐π‐methane rearrangement involving the vinyl and vinyldiester bridges, upon direct (Φ = 0.009) or benzophenone sensitized irradiations (Φ = 0.38). 10 undergoes a vinylcyclopropane rearrangement to give semibullvalene 11 upon direct or acetone sensitized irradiations (Φ = 0.008).