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.

具有顺,syn,cis相对立体构型的高度官能化线性聚环丁烷的热碎裂反应，可快速构建二环戊[a,d]并环辛烯基（dicyclopenta[a,d]cyclooctenyl）5-8-5环系。带有顺,syn,cis或顺,anti,cis构型的稠环环丁烷的热解反应，可通过顺,trans-环辛二烯中间体，以形式上的"对称性允许"路径进行。然而，若在热解步骤中存在用于在分子内[2+2]光环合反应中构建顺,syn,cis立体构型的桥连连接臂（tether），则会观察到形式上的"对称性禁阻"碎裂反应，生成含有顺,cis-环辛二烯的5-8-5环系产物。总体而言，此类碎裂反应中观察到的立体化学现象，可为获取这类5-8-5环系中的多种立体构型关系提供新的机遇。