Mechanism of the Diels-Alder Reaction

The thermal interconversions of α- and β-1-hydroxydicyclopentadiene with syn- and anti-8-hydroxydicyclopentadiene, respectively, with retention of optical activity suggest that other Diels-Alder dimers might undergo analogous rearrangements. The conversions of S-ketodicyclopentadiene and a chlorinated derivative to the Species III might be the transition state or else an intermediate, but in either case its geometry is formulated as being highly restricted.