... Soc., Chem. Commun., 1984, 702-703. On the preparation of 2-benzopyrylium-4-oxide and its cycloaddition properties. (Note: The full text of this document is currently only available in the PDF Version ). Peter G. Sammes and Richard J. Whitby. Treatment of 1-acetoxy-1H-2-benzopyran- 4-one with either base or heat generates the parent 2-benzopyrylium-4-oxide system which gives intermolecular cycloadducts with a variety of dipolarophiles, including olefins.
