6,8-Di-C-methyl-flavonoids with neuroprotective activities from Rhododendron fortunei.

Six 6,8-di-C-methyl-flavonoids, (2R,3R)-6,8-di-C-methyl-5,7,4'-trihydroxyflavanonol 7-O-β-d-gluco-pyranoside (1), (2R,3R)-6,8-di-C-methyl-5,7,4'-trihydroxyflavanonol 7-O-β-d-xylopyranosyl(1→6)-β-d-glucopyranoside (2), 6,8-di-C-methylkaempferol 7-O-β-d-glucopyranoside (3), (2R)-farrerol (4a), (2R/2S)-farrerol 7-O-β-d-glucopyranoside (5), and (2R/2S)-farrerol 7-O-β-d-xylopyranosyl(1→6)-β-d-glucopyranoside (6), and four known analogues, farrerol (4b), (2R,3R)-6,8-di-C-methyldihydrokae-mpferol (7), 6,8-di-C-methylkaempferol 7-O-β-d-glucopyranoside (8), and 6,8-di-C-methylkaempferol (9), were isolated from the twigs and leaves of Rhododendron fortunei. The structures of compounds 1-9 were determined by spectroscopic analyses (HRESIMS, 1D and 2D NMR, and CD) and chemical methods. Compounds 1-9 were evaluated for their neuroprotective effects on the human neuroblastoma SH-SY5Y cells apoptosis induced by hydrogen peroxide (H2O2) and amyloid β peptide (Aβ), respectively. Compounds 1-3 and 5-9 exhibited significant neuroprotective effects against H2O2-induced SH-SY5Y cell apoptosis, and compound 8 exhibited the strongest activity with a improvement of cell viability by about 30% at the concentration of 10μM. Compounds 1-9 showed significant neuroprotective effects against Aβ-induced SH-SY5Y cell apoptosis.