An unusual ring-expansion occurs upon reacting 1-methyloxindole with a Vilsmeier reagent. The Vilsmeier-Haack reaction on 4-alkylideneisoxazolin-5-ones 7 gives 1,3-oxazin-6-ones 8 which are useful precursors of a-pyrones, 2-pyridones and pyridines. Ring-opening, subsequent ring-closure, and lastly hydrolysis affords the 1,3-oxazin-6-one. Formation of the 5-chloro-4-formylisoxazole 10 evidently proceeds via phosphorylation of the exocyclic oxygen atom of 10. 3-Arylisoxazole-5–4H-ones 13 have also been reported to rearrange on reaction with DMF-POCl3 to afford a series of 2,4-dichloroisoquinolines. The precise reaction conditions are crucial to the outcome of the reaction; contrast the reaction of 9 in scheme 5.6 under similar conditions. An a-methyl group of pyrylium species can be iminoalkylated; reaction of 14 with DMF-POCl3 gave the enaminic salt 15. Reaction of 14 with the NH4Cl or p-methylaniline gave pyrimidinones 17. This elegant application of the Vilsmeier chemistry could find use in preparing other heterocyclic rings.