Anthocyanidins and related compounds—VIII Condensation reactions of flavylium salts with 5,5-dimethyl-1,3-cyclohexanedione in acid solutions

4′-Hydroxyflavylium salts, unsubstituted at position 3, react with 5,5-dimethyl-1,3-cyclohexanedione in acid solutions to yield para-hydroxyphenacyl-5-keto-tetrahydroxanthene derivatives, e.g., Va, isomeric with the flavan derivatives, e.g., IIa, formed at pH 5·8. These flavans isomerize to the tetrahydroxanthenes in both acid and alkaline media.

7-Hydroxy- and 3-methyl-4′-hydroxyflavylium salts yield flavan derivatives, eq. IIIa, IId, when condensed either in acid solutions or at pH 5·8. Isomerization of these flavans to corresponding tetrahydroxyanthenes has not been detected.