Synthesis, Structure, and Properties of Twofold Bridged Sesquinorbornenes

The twofold bridged sesquinorbornenes 2 and 6 were prepared using sequential [4 + 2] cycloadditions of benzoquinone with 1,5-dihydropentalenes 1 and 5. These syntheses were improved using dilution conditions or a more reactive substituted benzoquinone. Results from semiempirical and ab initio DFT calculations indicated remarkably high pyramidalization angles (ϕ = 46-47°) for the central C–C double-bond atoms. The chemical reactivity with triplet and singlet oxygen, dimethyldioxirane and N-methyl-1,2,4-triazoline-3,5-dione supports these structural assignments.