Mechanism of the Thermal Isomerization of 5-Norbornene-2,3-endo-Dicarboxylic Anhydride

The thermal interconversions of α and β-1-hydroxydicyclopentadiene with syn- and anti-8-hydroxydicyclopentadiene, respectively, with retention of optical activity and analogous rearrangements have been considered in mechanistic descriptions of the Diels-Alder reaction. In the case of deuterium-labeled methacrolein dimer, an internal stereospecific pathway for rearrangement and complete dissociation of the dimer are very much in competition at 180°, and both reactions occur simultaneously. As was noted, the possibility that different energy paths are involved in the two processes remained unsettled.