Covalent anthocyanin–flavone dimer from leaves of Oxalis triangularis

Abstract

The anthocyanin–flavone C-glycoside, (malvidin 3-O-(6IIOa-rhamnopyranosylAIV-b-glucopyranosideAII)-5-O-b-glucopyranoside AIII) (apigenin 6-C-(2II-O-b-glucopyranosylFIII-b-glucopyranosideFII)) malonateAV (AIV-4! AV-1, FIII-6! AV-3) (1), has been isolated from leaves of Oxalis triangularis A. St.-Hil. In the 1D 1H NMR spectrum of 1 dissolved in CD3OD–CF3CO2D (95:5), MTFA, recorded 45 min after sample preparation, this covalently linked dimer occurred mainly as flavylium cation (38%) and two equilibrium forms assigned to be quinonoidal bases (54%), whereas only minor amounts of the hemiacetal forms were present. After five days storage at 300 K, the hemiacetals (39%) and flavylium cation (38%) constituted the main forms of 1. More simple anthocyanins are normally considered to be on the flavylium cation form in acidified deuterated methanol. The cross-peaks observed in NOESY NMR spectra of 1 indicated the presence of vertical ’p–p’ stacking between the B-ring of the flavone unit and the A-ring of each of the two forms assigned to be quinonoidal bases. It was not possible to discriminate between inter- or intramolecular association mechanisms. The equilibria between the various forms of 1 were studied by two-dimensional NOESY and ROESY NMR spectroscopy. 2D HSQC-TOCSY NMR spectroscopy was among the methods used for characterization of the various forms.