Reaction of triacetonamine with ethyl cyanoacetate and ammonium acetate gives, in addition to the normal Knoevenagel reaction product (2,2,6,6-tetramethyl-4-ethoxycarbonylcyanomethylenepiperidine), its conversion product 3-cyano-4-(β,β-dimethylvinyl)-6,6-dimethyl-Δ3-dehydro-2-piperidone. Acid hydrolysis of the latter gives a dihydro-α-pyrone derivative. The mechanism of the formation of the new compounds is discussed, and their structures are proved by their NMR and mass spectra.