Regioselectivity of the Di-π-Methane Rearrangements of 1,4-Dialkoxy-9,10-bis(methoxycarbonyl)dibenzobarrelenes in Solution and in the Solid State

Abstract

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.

Key words

photochemistry - rearrangements - isomerizations - polycycles - biradicals

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