Maleimide-Fused Cyclobutadienes and Dewar Furans: Trapping With Dienes as a Route to Propellane Photosubstrates With Potential for Oxiren Production

Abstract

N-Methylmaleimide-fused cyclobutadiene (15) and its 1,2-dimethyl derivative (41) have been generated by zinc/titanium tetrachloride treatment of the vicinal dibromide (39) and (24) respectively. Prior epoxidations of (24) and (39) produce mixtures of dibromo epoxides which yield the related N-methylmaleimide-fused Dewar furans (16) and (17) upon similar debromination (Zn/TiCl₄). None of these reactive intermediates is sufficiently stable for direct observation, but each was trapped in adduct form by Diels-Alder reaction with furan. Attempts to form fused Dewar furan (17) by flash vacuum pyrolysis of the furan adduct (34) led to the production of the isomeric furo [3,4-c] pyrrole (43), the structure of which was confirmed by separate synthesis. Stereochemical assignments have been made to most adducts, and AM1 calculations have been used to predict the stereoselectivities and facial selectivities in these cycloaddition reactions. Reaction of these Dewar furans with cyclopentadienones produced propellanes which were decarbonylated to their respective cyclohexadienes and used as photosubstrates for the generation of oxiren or dimethyloxiren . Attempts to trap oxiren, generated by photolysis of propellane (12), with isobenzofuran were unsuccessful.