H. Ihmels et al., Regioselectivity of the di-pi-methane rearrangements of 1,4-dialkoxy-9,10-bis(methoxycarbonyl)dibenzobarrelenes in solution and in the solid state, SYNTHESIS-S, (8), 2001, pp. 1175-1180
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.