Mechanism of the Diels-Alder Reaction. The Question of the “Internal” Pathway for the Thermal endo-exo Isomerizations of the 5-Norbornene-2,3-dicarboxylic Anhydrides

Much information bearing on the mechanism of the Diels-Alder reaction has been obtained from studies of isomerizations of various adducts of cyclopentadiene. One of the most interesting and important of these isomerizations is the conversion of the endocyclopentadiene-maleic anhydride adduct [5-norbornene-2,3-endo-dicarboxylic anhydride (N)12] to the corresponding exo isomer (X) at 190º