I n benzene at 60, 1 and cyclopentadiene gave adduct 5 exclusively. The isomeric adduct 7 was synthesized by an alternative procedure, and was shown to isomerize quantitatively to 5 at this temperature. Repetition of this Diels-Alder reaction at 23" gave 5 and 7 in an approximate ratio of 4: 1, together with some endo-dicyclopentadiene (8)) as shown by nmr analysis. The adduct 7 was stable under these conditions. In addition, a sample of 1 in ...