Bicyclo[2.2.2]octene-based molecular spacers. Construction of U-shaped syn-facial etheno-bridged polyhydrononacenyl frameworks

Based on the methodology of repetitive Diels–Alder reactions, the rack- and U-shaped polycyclic compounds , , , , and were constructed utilizing 1,2,3,4-tetrachloro-5,5-dimethoxycyclopentadiene (TDCp), p-benzoquinone, and norbornadiene as starting materials. These spacer molecules are composed of nine linearly syn-fused polyhydrobenzene rings (i.e., polyhydrononacene), in which all the etheno-bridge double bonds are arranged on the same side of the carbon skeletons. Upon irradiation with an ordinary tungsten lamp, and underwent [2+2]photocyclization to form quadruple- and double-caged compounds and , respectively. The structure of was analyzed by X-ray crystallography and found to have U-shaped framework.