Antioxidative compounds from Garcinia buchananii stem bark.

An aqueous ethanolic extract of the stem bark of Garcinia buchananii showed strong antioxidative activity using H2O2 scavenging, oxygen radical absorbance capacity (ORAC), and Trolox equivalent antioxidant capacity (TEAC) assays. Activity-guided fractionation afforded three new compounds, isomanniflavanone (1), an ent-eriodictyol-(3α→6)-dihydroquercetin-linked biflavanone, 1,5-dimethoxyajacareubin (2), and the depsidone garcinisidone-G (3), and six known compounds, (2″R,3″R)-preussianon, euxanthone, 2-isoprenyl-1,3,5,6-tetrahydroxyxanthone, jacareubin, isogarcinol, and garcinol. All compounds were described for the first time in Garcinia buchananii. The absolute configurations were determined by a combination of NMR, ECD spectroscopy, and polarimetry. These natural products showed high in vitro antioxidative power, especially isomanniflavanone, with an EC50 value of 8.5 μM (H2O2 scavenging), 3.50/4.95 mmol TE/mmol (H/L-TEAC), and 7.54/14.56 mmol TE/mmol (H/L-ORAC).