Syntheses and Thermal Isomerizations of Bicyclo[3.2.2]nona-2,6,8-triene and Its Benzo Analog

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Abstract
A convenient synthesis of bicyclo[3.2.2]nona-2,6,8-triene (1) was devised starting from the cycloheptatrienemaleic anhydride adduct. Benzo[f]bicyclo[3.2.2]nona-2,6,8-triene (2) was prepared starting from benzo[f] bicyclo[3.2.2]nona-3,6,8-trien-2-one. Heating 1 in n-hexane at 160–180°C gave almost quantitatively 3-vinyl-cycloheptatriene (9) besides a trace of 7-vinylcycloheptatriene (10). It was found however that 10 undergoes thermal isomerization into 9 under milder conditions. The result from 1 is thus rationalized by the existence of an equilibrium between 1 and 10 preceding the conversion of 10 into 9. On the other hand, benzo analog 2 does not isomerize in a similar manner and instead, is transformed by heating at 270–290°C into benzobarbaralane (14) and hydrobenzindene (15). The contrast in behavior is discussed.

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