Reaction of bicyclic dienes with 4-substituted-1,2,4-triazoline-3,5-diones

Reaction of 1,2-dihydrophthalic anhydride and several of its imide derivatives, 1,2-dihydrophthalide and 1,2-dihydrophthalan with N-methyl- and N-phenyl-1,2,4-triazoline-3,5-dione affords two configurational families of adducts through attack from both possible directions. Major attack in the first three cases occurs mainly syn- to the hetero-ring but in the two latter cases anti- to it. These results may be explained by invoking stabilization of the transition state in the former cases through secondary orbital overlap and by simple steric effects in the latter.