ANTHOCYANIDINS AND RELATED COMPOUNDS .15. EFFECTS OF SUNLIGHT ON FLAVYLIUM SALT-CHALCONE EQUILIBRIUM IN ACID SOLUTIONS

The position of the equilibrium between flavylium salts and 2-hydroxychalcones in aqueous solutions is markedly effected by light. NMR and UV spectral data indicate that the chalcones formed at equilibrium have the trans-configuration. In sunlight these photoisomerize rapidly to cis 2-hydroxychalcones, which then cyclize in acid solutions to flavylium salts. In the absence of light the necessary, initial trans to cis isomerization is acid catalyzed and occurs readily only with-those chalcones with stabilizing, electron-donating groups, e.g. methoxyl, at the 4 and/or 6 positions. Prolonged exposure of cis 2-methoxychalcones to light results in a photochemical demethylation with formation of the cis 2-hydroxychalcone or cyclic pseudo base. © 1969.