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.

This review covers the phytochemical, biological properties, and synthesis of naturally occurring homoisoflavonoids. Homoisoflavonoids are a very important class of secondary metabolites whose numbers have grown from 20 in 1981 to 157 at the present time. They are found to occur in seven plant families. For the purpose of this review they are classified into five groups: 3-benzylchroman-4-ones, 3-benzylflavans, Δ3,9 and Δ2,3 3-benzylchroman-4-ones, benzocyclobutenes (scillascillins) and rearranged homoisoflavonoids (brazilin and related compounds). Biosynthetically, the 3-benzylchroman-4-ones and the 3-hydroxy-derivatives have been shown to arise from a chalcone precursor (sappanchalcone) and there is strong evidence that this isolable intermediate can be converted into the diverse structures such as the benzocyclobutenes (scillascillins) and the rearranged, brazilin-type compounds. Homoisoflavonoids possess a wide range of biological activities, including, antimicrobial, antimutagenic, anti-inflammatory, antidiabetic, etc, properties. The review also surveys the chemical synthesis of natural homoisoflavonoids. Analytical methods for the determination of these important metabolites are also reviewed. The last section is devoted to a brief review of the diagnostic NMR spectroscopic features of homoisoflavonoids. A comprehensive Table has also been compiled listing all known metabolites, their sources, melting points and optical rotation values (where available) and references.

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From the roots of the African plant Bulbine frutescens (Asphodelaceae), two unprecedented novel dimeric phenylanthraquinones, named joziknipholones A and B, possessing axial and centrochirality, were isolated, together with six known compounds. Structural elucidation of the new metabolites was achieved by spectroscopic and chiroptical methods, by reductive cleavage of the central bond between the monomeric phenylanthraquinone and -anthrone portions with sodium dithionite, and by quantum chemical CD calculations. Based on the recently revised absolute axial configuration of the parent phenylanthraquinones, knipholone and knipholone anthrone, the new dimers were attributed to possess the P-configuration (i.e., with the acetyl portions below the anthraquinone plane) at both axes in the case of joziknipholone A, whereas in joziknipholone B, the knipholone part was found to be M-configured. Joziknipholones A and B are active against the chloroquine resistant strain K1 of the malaria pathogen, Plasmodium falciparum, and show moderate activity against murine leukemic lymphoma L5178y cells.