Structural changes of flavylium salts. III. Polarographic and spectrometric examination of 3,7,4'-trihydroxyflavylium perchlorate

Abstract

The molecular changes undergone by 3,7,4?-trihydroxyflavylium perchlorate in aqueous methanolic solutions with increasing pH were followed by polarography and spectrometry, and the effect of the hydroxyl at C3 observed by comparison with the behaviour of 7,4?- dihydroxyflavylium perchlorate. The trihydroxyflavylium ion is less stable above pH 2 than the dihydroxy compound and forms the open-chain α-diketone, probably via the 3-keto pseudobase. The stability of the enolic form of the pseudobase increases at higher pH and is favoured from pH 5 to pH 9. At pH 6.5 the anhydrobase appears in the equilibrium mixture and at pH 9 becomes stabilized by ionization. Although the α- hydroxychalcone was not detectable at lower pH, at pH 10 ring opening gives the ionized a-hydroxychalcone, rather than the ionized diketone, and equilibrium shifts in this direction as the pH is raised further.