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The reaction of the hydrofluoroborate salt of an open‐chain analogue of a Reissert compound with some α,β‐ethylenic esters does not give a [4 + 2] cycloadduct, as previously described in the case of ethyl acrylate. The reaction starts with a 1,3‐dipolar cycloaddition of a münchnone imine 5c, d. The [3 + 2] cycloadducts 13 evolve via a rearrangement–condensation sequence to give a substituted 2‐pyridone derivative 18 or 19. The proposed mechanism has been verified by the isolation and structural X‐ray analysis of some compounds of the reaction sequence.