Intramolecular [2+2] Photocycloaddition/thermal fragmentation: Formally "allowed" and "forbidden" pathways toward 5-8-5 ring systems

The thermal fragmentation of highly functionalized, linear polycyclobutanes with a cis,syn,cis-relative stereochemistry is shown to offer a rapid entry into the dicyclopenta[a,d]cyclooctenyl (5-8-5) ring system. The thermolysis of polyfused cyclobutanes with a cis,syn,cis- or a cis, anti, cis-relationship proceeds in a formally "symmetry-allowed" manner through the intermediacy of a cis, trans-cyclooctadiene. When a bridging tether used to establish the cis, syn, cis-stereochemistry in the intramolecular [2 + 2] photocyclization is present in the thermolysis step, however, the result of a formally "symmetry-forbidden" fragmentation is observed yielding cis, cis-cyclooctacliene-containing 5-8-5 products. In general, the stereochemical observations noted in these fragmentations offer new opportunities for accessing a variety of stereochemical relationships in these 5-8-5 ring systems.