Cycloadditions, 27 . The Thermal Rearrangement of the Bicyclo[2.2.2]‐ to Bicyclo[3.2.1]octadiene Moiety in Tricyclic Lactams

The tricyclic lactams 4a–h , possessing a bicyclo[2.2.2]octadiene moiety, obtained by intramolecular Diels‐Alder reaction of the vinylidene malondiamides 1a–h , generally isomerize in boiling xylene to give the new lactams 7a–h , now bearing a bicyclo[3.2.1]octadiene part. The difference in the rates of the isomerizations 4 → 7 and the results with the nitro ‐ and methoxy‐substituted derivatives 4i–l are in accordance with the proposed intermediate zwitterion 6 , which is formed by the migration of one vinyl group from position 1 to position 5 of the lactam moiety.