Pana Charumilind

The 1,7-cyclohexenonorbornadiene system

Abstract 2,3-Dimethylene-7-oxabicyclo[2.2.1]heptane, available by reduction and dehydration of the furan-fumaroyl chloride Diels-Alder adduct, undergoes ready dibromocyclopropanation at one of its exocyclic double bonds. The corresponding carbenoid, generated by reaction of the dibromide wilh 4 equiv of an organolithium reagent, undergoes the Skattebol rearrangement to provide a fused cyclopentadiene ring which is immediately deprotonated. This anion fragments to a fulvene alkoxide to which the organolithium reagent subsequently adds. The resulting anion, namely 9, provides fulvenes of type 3 on workup. The use of 3 as a diene in Diels-Alder cycloadditions is demonstrated by the condensation of methyl derivative 3a with dimethyl acctylenedicarboxylate and (E)-1,2-bis(phenylsulfonyl)ethylene. The question of face selectivity is raised. For the bissulfone adducts 21 and 22, reductive desulfonylation with 1–2% sodium amalgam delivers hydrocarbon 23, the first known member of the 1,7-cycloalkenonorbornadiene class of molecules. 13C-NMR measurements show that 23 is as polarized as its simple non-annulated prototype and is unstrained.