Covalent anthocyanin-flavone dimer from leaves of Oxalis triangularis

The anthocyanin-favone C-glycoside, (malvidin 3-O-(6(II)-O-alpha-rhamnopyranosyl(A1V)-beta-glucopyranoside(AII))-5-O-beta-glueopyranoside(AIII)) (apigenin 6-C-(2(II)-O-beta-glucopyranoSyl(FIII)-beta-glucopyranoside(FII))) malonate(AV) (A(IV)-4 -> A(V)-1, F-III-6 -> A(V)-3) (1), has been isolated from leaves of Oxalis triangularis A. St.-Hil. In the 1D H-1 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 I indicated the presence of vertical 'pi-pi 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 I 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. (c) 2006 Elsevier Ltd. All rights reserved.