Two Galloylated Flavonoids as Antioxidants in Rosa gallica Petals

Abstract

MCl gel eluted with the solvent system H 2 O–MeOH–acetone, and five fractions were obtained. The fractions obtained by stepwise gradient with H 2 O–MeOH contained flavones with different degrees of galloylation. Fractions 1 and 2 eluted with 90 and 80% H 2 O, respectively, contain two galloylated flavonoids (compounds 1 and 2, respectively) and were isolated by repeated chromatography on Sephadex LH-20 and on a reversed phase column using Wakosil 40C 18 . In the 1 H NMR spectrum of compound 1, two meta-coupled signals were observed at 6.15 and 6.32, and three signals due to 1,3,4-trisubstituted benzene ring protons at 7.56, 7.51, and 6.82. Moreover, an anomeric proton of a sugar moiety was observed at 5.73 (d, J = 7.9) together with six signals of sugar protons at 5.11–3.33, and the coupling constants of these signals indicated the presence of glucose. The specific optical rotation of 1 also indicated the presence of the -D-glucosyl moiety. In the 1 H– 1 H COSY spectrum, the cross peaks of sugar protons were confirmed, but the signal of the H-2 proton of the glucosyl moiety was observed down field at 5.11. This indicated that the hydroxyl group at the C-2 position of glucose was acylated. The molecular weight of compound 1 and the signal of two proton singlets at 7.12 implied the presence of the galloyl group at C-2 of glucose. The signals at 167.8, 146.3, 139.2, 121.0, and 110.6 in the 13 C NMR spectrum also supported the presence of a galloyl group. These data indicated the presence of quercetin [3], the glycosyl unit [4], and the galloyl group. In the HMBC spectrum, H-1 ( 5.73) of the glycosyl moiety showed a correlation with the signal of C-3 ( 134.9) of quercetin. The carbonyl carbon ( 167.8) of the galloyl moiety also showed correlations with H-2 ( 5.11) and H-2 (7.12). Comparing these data with the literature [5, 6], we characterize compound 1 as quercetin 3-O-(2-O-galloyl)-D-glucoside. Compound 2 showed 1 H MNR spectral data similar to those of compound 1, except for the ortho-coupled two proton signals at 7.95 and 6.89 instead of the signals of 1,3,4-trisubstituted benzene. These data indicated the presence of kaempferol as aglycone. Comparing the 1 H and 13 C NMR data and the specific optical rotation with those in the literature [6], we characterize compound 2 as kaempferol 3-O-(2-O-galloyl)--D-glucoside. The two galloylated flavonoids have been isolated from Diospyros kaki Thunb., Geranium tuberosum subsp. tuberosum and Polygonum lapathifolium L. [7–9], but this is the first report on the isolation of these compounds from the plant belonging to the genus Rosa.