A xanthone and a phenylanthraquinone from the roots of Bulbine frutescens, and the revision of six seco-anthraquinones into xanthones

Two new compounds named 8-hydroxy-6-methylxanthone-1-carboxylic acid (1) and 6′,8-O-dimethylknipholone (2) were isolated from the root of Bulbine frutescens. The structures of six seco-anthraquinones were revised into xanthones. Knipholone and its derivatives showed cytotoxic activity against human cervix carcinoma KB-3-1 cells.