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

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 LiAlH4 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 CD3OD/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.