Regioselectivity of the di-pi-methane rearrangements of 1,4-dialkoxy-9,10-bis(methoxycarbonyl)dibenzobarrelenes in solution and in the solid state

Irradiation of solutions of the title compounds led to the regioselective f ormation of one of the two possible dibenzosemibullvalenes, as a consequenc e of the stabilization of intermediate biradicals by the alkoxy groups pres ent. Continued irradiation of the reaction mixture converted the main produ ct into a dibenzocyclooctatetraene as secondary photoproduct. Irradiation o f the solid neopentoxy-substituted dibenzobarrelene afforded, almost exclus ively, the dibenzosemibullvalene that is the minor photoproduct on irradiat ion in solution. Presumably, due to steric constraints in the crystal, the formation of the other isomer is prevented.