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-cyclooctadiene-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.