Vinylindenes and Some Heteroanalogs in the Diels-Alder Reaction. X. Dibenzofurans From 3-Vinylbenzofurans

3-Vinylbenzofuran, 3-isopropenylbenzofuran and 2-methyl-3-vinylbenzofuran give normal [4p+2p] adducts with ethenetetracarbonitrile, accompanied, in the case of the last named diene, by the isomeric cyclobutane. Normal adducts also result from these dienes and 4-phenyl-1,2,4-triazoline-3,5-dione, and from 3-vinylbenzofuran and both maleic anhydride and N- phenylmaleimide. 3-Vinylbenzofuran gives the rearranged adduct dimethyl 1,2-dihydrodibenzofuran-3,4-dicarboxylate with dimethyl acetylenedicarboxylate.