3, 4-Didehydropyridine (2) generated from two precursors (6 and 7) of the diazonium carboxylate 5 reacts with cyclopentadiene to give the [4+ 2]-cyclo-addition product 3 rather than the [2+ 2]-adduct 4 proposed in the literature. The structure of 3 was supported by complete proton and carbon NMR assignments made with the aid of decoupling and 2D experiments.