APPLICATION OF 3-OXABICYCLO[3.2.0]HEPT-6-ENE-2,4-DIONE (CYCLOBUT-3-ENE-1,2-DICARBOXYLIC ANHYDRIDE) AS AN ACETYLENE EQUIVALENT IN CYCLOADDITIONS

The effectiveness and limitations of dioxo-3-oxabicyclo[3.2.0]hept-6-ene-2,4-dione (cyclobut-3-ene-1,2-dicarboxylic anhydride) as an acetylene equivalent in both 1,3-dipolar and Diels-Alder cycloadditions is reported; it reacted readily with a variety of reagents, including N-benzylideneaniline N-oxide, nitrile oxides, diazomethane, cyclopentadiene, tetracyclone, anthracene, 1,2,5-triphenylphosphole 1-oxide and 1,3-diphenylisobenzofuran. The structures and stereochemistry of the adducts were deduced from their NMR data; in all cases, the sterically favoured anti-isomers are formed exclusively. The configuration of the Diels-Alder adducts are assigned as endo with the exception of that from tetracyclone (and possible 1,3-diphenylisobenzofuran) for which an exo-structure is assumed on the basis of steric arguments. Adducts were not obtained with several other reagents; possible reasons for this lack of reactivity are discussed. When subjected to flash vacuum pyrolysis, the adducts underwent thermal fragmentation, either by a retro-cleavage, or by loss of maleic anhydride to form products that are derived formally from reaction of acetylene in the cycloaddition step. A concerted pathway is proposed for the pyrolytic conversion into the 'formal acetylene cycloadduct' rather than a stepwise radical mechanism.