Oxidative formation and structural characterisation of new α-pyranone (lactone) compounds of non-oxonium nature originated from fruit anthocyanins

A new group of pyranoanthocyanin-derived polyphenolic compounds was recently found to occur in aged wines and named as oxovitisins, the spectrum of which displayed only a pronounced broad absorption peak around 370 nm in the UV region, being quite different from that of anthocyanin-like chromenylium pigments which present maximum absorption in the visible wavelength ranges. The possibilities and reaction conditions for the oxidative transformation of anthocyanin type flavylium cations into new type of stable pyranone structures of non-oxonium nature was studied through two-step reactions using anthocyanins obtained from different fruit extracts. The irreversible change of pyranoflavyliums to the neutral pyranone compounds by hydration and further oxidation reactions in the second step were strongly related to pH. The reaction took place only in mildly acidic solutions with the most favourable pH range 4.0–5.5, as the hemiacetal formation by the nucleophilic attack of water could be hindered at higher or lower pH. The reaction rate increased significantly with increasing temperature. The new non-oxonium compounds resulting from malvidin-3-coumaroylglucoside and cyanidin-3-rutinoside were structurally characterised by MS and NMR spectroscopy and shown similarly to possess the common α-pyranone (lactone) ring between C-4 and the hydroxy group at C-5 of the anthocyanin core, which confers them with unique spectrum characteristics. The interest in new anthocyanin derivatives of non-oxonium nature with additional pyranone (lactone) ring will go beyond wine chemistry and bring expectations concerning their use in the food and pharmaceutical industry due to their spectral and structural similarity to flavones as well as their naturally occurring nature.