Irradiation of solutions of the title compounds led to the regioselective formation
of one of the two possible dibenzosemibullvalenes, as a consequence of the stabilization
of intermediate biradicals by the alkoxy groups present. Continued irradiation of
the reaction mixture converted the main product into a dibenzocyclooctatetraene as
secondary photoproduct. Irradiation of the solid neopentoxy-substituted dibenzobarrelene
afforded, almost exclusively, the dibenzosemibullvalene that is the minor photoproduct
on irradiation in solution. Presumably, due to steric constraints in the crystal,
the formation of the other isomer is prevented.
photochemistry - rearrangements - isomerizations - polycycles - biradicals
