The selectivity between [2 + 2] and [2 + 4] cycloadditions of cyclopentyne with spiro[2.4]nona-2,4-diene depends on the mode of generating the cycloalkyne. Using diazomethylenecyclobutane and (bromocyclobutylidenemethyl)trimethylsilane as precursors affords cyclopentyne in an unencumbered form, whereas using dibromomethylenecyclobutane and 1,2-dibromocyclopentene yields a cyclopentyne-lithium bromide complex.
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.
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