Knipholone and related 4-phenylanthraquinones: structurally, pharmacologically, and biosynthetically remarkable natural products.

This review gives the first comprehensive overview of the rapidly emerging young class of 4-phenylanthraquinones isolated from the three genera Bulbine, Bulbinella, and Kniphofia, all from the plant family Aphodelaceae: their occurrence, structural elucidation (with particular emphasis on the phenomenon of axial chirality), partial and total synthesis, their pharmacological (i.a., antimalarial and antitumoral) activities, and their biosynthetic origin (from acetate units). Particular emphasis is given to the stereostructure of knipholone, the most abundant of these naturally occurring phenylanthraquinones. The most recent highlight is the discovery of the first dimeric representatives, named joziknipholones, involving interesting phenomena of--partially or fully restricted--rotation at the sp2-sp2 and sp2-sp3 axes.