Hexacyclo[8.7.0.03,8.05,15.06,13.012,16]heptadeca-1(10),5-diene-3,8-dicarboxylic anhydride, a tetrahydro[4]beltene with two σ-conjugated double bonds

Abstract

The tetrahydro[4]beltene 8 has been synthesized via a sequence of three Diels–Alder reactions. As shown in the crystal structure of 8, the two double bonds in the cage diene are positioned face-to-face at a distance of 2.92 Å, which is smaller than the sum of their van der Waals radii. The resulting σ-conjugation in 8 is evident from the long wavelength UV adsorption and from the formation of a stable radical cation at low potential. Hydrogenation increases the strain of 8 such that only one double bond can be saturated under forcing conditions.