A xanthone and a phenylanthraquinone from the roots of Bulbine frutescens, and the revision of six seco-anthraquinones into xanthones - Abdissa, N; Heydenreich, M; Midiwo, J O; Ndakala, A; Majer, Z; Neumann, B; Stammler, H G; Sewald, N; Yenesew, A (Phytochem Lett, 2014)