Unveiling the 6,8‐Rearrangement in 8‐Phenyl‐5,7‐dihydroxyflavylium and 8‐Methyl‐5,7‐dihydroxyflavylium through Host–Guest Complexation

Abstract

8‐Phenyl‐5,7‐dihydroxyflavylium and 8‐methyl‐5,7‐dihydroxyflavylium were synthesized to observe the 6,8‐rearrangement. 8‐Phenyl and 8‐methyl residues were introduced by Suzuki–Miyaura reaction of 8‐iodochrysin, then reduced by LiAlH 4 to give the corresponding 3‐deoxyanthocyanidins. At pH 1.0 the stable form is the 8‐substituted flavylium cation in both compounds. At higher pH values the quinoidal bases are the stable species and some evidence for the 6,8‐rearrangement was obtained, but the respective spectral variations are very small. This was overcome by using a modified cyclodextrin (captisol), which favors the formation of the trans ‐chalcone at the expense of the quinoidal bases. The trans ‐chalcone was isolated and dissolved in CD 3 OD/DCl (pD < 1) to give a mixture of the two flavylium isomers. This was confirmed by ESI‐MS analysis (recorded in positive ion mode) of the two isomers after separation by HPLC, which gave the same peak ([M] + m/z 253). The 6‐isomer slowly reverts to the most stable 8‐isomer. The 6,8‐rearrangement was also observed after irradiation of the trans ‐chalcone (in the presence of captisol) at pH 5. Acidification of this photostationary state gave a mixture of both flavylium cations. The UV/Vis absorption of the flavylium cation (6‐isomer) was blueshifted in comparison to the 8‐isomer.