Isolation and Structure Elucidation of Highly Antioxidative 3,8-Linked Biflavanones and Flavanone-C-glycosides from Garcinia buchananii Bark

The aim of this study was to identify antioxidants from Garcinia buchananii bark extract using hydrogen peroxide scavenging and oxygen radical absorbance capacity (ORAC) assays. LC-MS/MS analysis, 1D- and 2D-NMR, and circular dichroism (CD) spectroscopy led to the unequivocal identification of the major antioxidative molecules as a series of three 3,8 ''-linked biflavanones and two flavanone-C-glycosides. Besides the previously reported (2R,3R,2 '' R,3 '' R)-naringenin-C-3/C-8 '' dihydroquercetin linked biflavanone (GB-2; 4) and (2R,3S,2 '' R,3 '' R)-manniflavanone (3), whose stereochemistry has been revised, the antioxidants identified for the first time in Garcinia buchananii were (2R,3R)-taxifolin-6-C-beta-D-glucopyranoside (1), (2R,3R)-aromadendrin-6-C-beta-D-glucopyranoside (2), and the new compound (2R,3S,2 '' S)-buchananiflavanone (5). The H2O2 scavenging and the ORAC assays demonstrated that these natural products have an extraordinarily high antioxidative power, especially (2R,3S,2 '' R,3 '' R)-manniflavanone (3) and GB-2 (4), with Egg) values of 2.8 and 2.2 mu M, respectively, and 13.73 and 12.10 mu mol TE/mu mol. These findings demonstrate that G. buchananii bark extract is a rich natural source of antioxidants.