Cytotoxic flavonoids from the flowers of Achillea millefolium

A new natural product, 8,8'-bi-3-O-methylquercetin (9), and eight known flavonoids were isolated from the flowers of Achillea millefolium. Their structures were established on the basis of spectral evidence. All the spectral data of these compounds were reassigned after interpretation of their 2D NMR spectroscopic data. Compounds 2, 5, and 7 exhibited antiproliferative activity against human breast cancer cell line MCF7WT. In addition, 2 and 5 showed antiproliferative activity against human prostatic cancer cell line PC-3, while the other compounds did not show inhibitory effects in two human tumor cell lines tested.