THE 1,7-CYCLOHEXENONORBORNADIENE SYSTEM

Article abstract of DOI:10.1016/S0040-4020(01)87596-7

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 with 4 equiv of an organolithium reagent, undergoes the Skatteboel 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 acetylenedicarboxylate and (E)-1,2-bis(phenylsulfonyl)ethylene.The question of face selectivity is raised.For the bissulfone adducts 21 and 22, reductive desulfonylation with 1-2percent sodium amalgam delivers hydrocarbon 23, the first known member of the 1,7-cycloalkenonorbornadiene class of molecules. (13)C-NMR measurements show that 23 is as polarized as its simple non-annulated prototype and is unstrained.