Baeckein E, a new bioactive C-methylated biflavonoid from the roots of Baeckea frutescens

A phytochemical study of Baeckea frutescens afforded a new biflavonoid named as baeckein E (1), along with four known C-methylated flavonols, baeckeins A (2) and B (3), 6-methylquercetin (6) and 6-methylquercetin-4'-O-β-D-glucopyranoside (7), and two known biflavonoids baeckeins C (4) and D (5). The structure of compound 1 was elucidated on the basis of spectroscopic analysis, including HR-ESI-MS, 1D and 2D NMR spectra, and the absolute configurations of C-2 and C-3 for compound 1 were assigned by circular dichroism spectrometry combined with quantum chemical calculations. The antioxidant and cytoprotective activities of compounds 1-7 were also investigated.