New Highly in Vitro Antioxidative 3,8-Linked Biflav(an)ones and Flavanone-C-glycosides from Garcinia buchananii Stem Bark

Very recently, we described highly antioxidative polyphenols isolated from the stem bark extract of the Garcinia buchananii tree. In this study, we describe additional antioxidants from Garcinia buchananii bark extract using hydrogen peroxide scavenging, oxygen radical absorbance capacity (ORAC), and trolox equivalent antioxidant capacity (TEAC) assays. UPLC-HR-ESI-TOF-MSe analysis, 1- and 2D-NMR, and circular dichroism (CD) spectroscopy led to the unequivocal identification of the antioxidative molecules as a series of five 3,8 ''-linked biflav(an)ones and two flavanone-C-glycosides. (2S,3R)-Taxifolin-6-C-beta-D-glucopyranoside (2), (2R,3S,2 '' S,3 '' S)-manniflavanone (3), (2R,3S)-buchananiflavonol (4), and (2S,3R,2 '' R,3 '' R)-GB-1 (6) are new compounds, and (2S,3S)-taxifolin-6-C-beta-D-glucopyranoside (1) was described so far only in one other plant. The structure of (2R,3S,2 '' R,3 '' R)-GB-1 (5) and (2R,3S,2 '' S)-GB2a (7) were confirmed. The H2O2 scavenging, TEAC, and the ORAC assays demonstrated that these natural products have an extraordinarily high antioxidative power, especially (2R,3S,2 '' S,3 '' S)-manniflavanone (3) with an EC50 value of 3.0 mu M, 4.00 mmol TE/mmol, and 10.30 mu mol TE/mu mol.