The Interconversions of Pyrylium Salts, Pyrans, Pyrones, and Their Open-chain Forms

A general scheme is proposed for the interconversions of six-membered oxygen-containing heterocylces (pyrylium salts, pyrans, and pyrones) and their open-chain forms, having selected pyrylium salts as compounds of the key type. The latest data concerning the mechanism of a number of the above transformations are examined. It is shown that ring opening is one of the most characteristic manifestations of the reactivity of six-membered oxygen-containing heterocylces.
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