2′-Hydroxyflavylium: introducing flavanones into the flavylium network of chemical reactions

Chalcones possessing a hydroxyl group in position 2 cyclize to form flavylium salts in acidic media, this reaction being reversible under neutral–basic conditions. On the other hand, chalcones possessing a hydroxyl group in position 2′ cyclize to form flavanones in basic media. By synthesizing 2′-hydroxyflavylium tetrafluoroborate, it was possible to obtain trans-2,2′-dihydroxychalcone that in solution can evolve to 2′-hydroxyflavanone or back to 2′-hydroxyflavylium depending on the pH. The several equilibria established in aqueous solution were fully characterized. The importance of including flavanones into the flavylium network of chemical reactions is briefly exploited.