Whereas N-phenylmaleimide and 6-morpholino-5-phenyl-1,3-oxazin-2-one (1) yield the pyridine derivative (3) by a Diels–Alder reaction, the action of 1-diethylaminopropyne on the oxazinone results in the isomeric adducts (5) and (9), which are thought to be formed by initial 1,4- and 1,2-cycloaddition, respectively, of the ynamine to the acyclic valence isomer (4) of the oxazinone; the 1,2-cycloadduct rearranges to the final product by a sequence of pericyclic reactions.