Covalent anthocyanin-flavone dimer from leaves of Oxalis triangularis.

The anthocyanin-flavone C-glycoside, (malvidin 3-O-(6(II)-O-alpha-rhamnopyranosyl(AIV)-beta-glucopyranoside(AII))-5-O-beta-glucopyranoside(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 (1)H NMR spectrum of 1 dissolved in CD(3)OD-CF(3)CO(2)D (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 '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 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.

Acylated anthocyanins from leaves of Oxalis triangularis.

The novel anthocyanins, malvidin 3-O-(6-O-(4-O-malonyl-alpha-rhamnopyranosyl)-beta-glucopyranoside)-5-O-beta-glucopyranoside (2), malvidin 3-O-(6-O-alpha-rhamnopyranosyl-beta-glucopyranoside)-5-O-(6-O-malonyl-beta-glucopyranoside) (3), malvidin 3-O-(6-O-(4-O-malonyl-alpha-rhamnopyranosyl)-beta-glucopyranoside)-5-O-(6-O-malonyl-beta-glucopyranoside) (4), malvidin 3-O-(6-O-(4-O-malonyl-alpha-rhamnopyranosyl)-beta-glucopyranoside) (5) and malvidin 3-O-(6-O-(Z)-p-coumaroyl-beta-glucopyranoside)-5-O-beta-glucopyranoside (6), in addition to the 3-O-(6-O-alpha-rhamnopyranosyl-beta-glucopyranoside)-5-O-beta-glucopyranoside (1) and the 3-O-(6-O-(E)-p-coumaroyl-beta-glucopyranoside)-5-O-beta-glucopyranoside (7) of malvidin have been isolated from purple leaves of Oxalis triangularis A. St.-Hil. In pigments 2, 4 and 5 a malonyl unit is linked to the rhamnose 4-position, which has not been reported previously for any anthocyanin before. The identifications were mainly based on 2D NMR spectroscopy and electrospray MS.