Xanthones are secondary metabolites found in a few higher plants, fungi, and lichens. The xanthone skeleton (the word “xanthon” is derived from the Greek word xanthos, meaning yellow) is a planar, conjugated ring system composed of carbons 1-4 (aromatic ring A) and carbons 5-8 (aromatic ring B), fused through a carbonyl group and an oxygen atom. The simplest member of the class, 9H-xanthen-9-one, is a symmetrical compound with a dibenzo-γ-pyrone skeleton. The numbering starts from ring A, while ring B is given prime locants or consecutively numbered from ring A.
Xanthones have diverse pharmacological properties, mainly due to their oxygenation nature and diversity of functional groups. The biological activities discussed in some review articles on xanthones during 2000-2012 include antibacterial, antiviral, antioxidative, antiinflammatory, antiproliferative, antihypertensive, antithrombotic, in vitro and in vivo antitumor, cytotoxic, coagulant, monoamine oxidase (MAO) inhibition, gastro-protective effects, antiatherosclerotic activity, inhibition of hypotension, cardioprotection, inhibition of cholinesterase, cyclooxygenase activity, immunosuppression, and binding to transthyretin (TTR). The α-glucosidase inhibitory activity may lead to xanthones and their sources being used against diabetes and HIV/AIDS. Xanthones have shown the potential to prevent disease development by their concerted action of protecting cells from oxidative stress damage and acting as phytoalexin to impair pathogen growth. A biological activity that is of significance for tropical Africa is the antimalarial property.
Xanthones isolated from natural sources are classified into six main groups, namely, simple xanthones, xanthone glycosides, prenylated xanthones, xanthonolignoids, bisxanthones, and miscellaneous xanthones.
Simple oxygenated xanthones are subdivided according to the degree of oxygenation into non-, mono-, di-, tri-, tetra-, penta-, and hexaoxygenated substances. In these xanthones the substituents are simple hydroxy, methoxy, or methyl groups.
Sixty-one naturally occurring glycosylated xanthones, thirty-nine of which are new compounds, have been reported predominantly in the families Gentianaceae and Polygalaceae as C- or O-glycosides. The details of naturally occurring xanthone glycosides have been reviewed and distinction between C-glycosides and O-glycosides has also been made. In C-glycosides, C-C bond links the sugar moiety to the xanthone nucleus and they are resistant to acidic and enzymatic hydrolysis whereas the O-glycosides have typical glycosidic linkage.
Among 285 prenylated xanthones, 173 were described as new compounds. The occurrence of prenylated xanthones is restricted to the plant species of the family Guttiferae.
Naturally occurring xanthonolignoids are rare, so only five compounds are known. The first xanthonolignoid was isolated from Kielmeyera species by Castelão Jr. et al.. They also isolated two other xanthonolignoids named Cadensins A and B from Caraipa densiflora. A xanthonolignoid Kielcorin was obtained from Hypericum species. Recently, kielcorin was also isolated from Vismia guaramirangae, Kielmeyera variabilis, and Hypericum canariensis, whereas cadensin C and cadensin D from Vismia guaramirangae and Hypericum canariensis have been reported.
Mazimba, O., Nana, F., Kuete, V., & Singh, G. S. (2013). Xanthones and anthranoids from the medicinal plants of Africa. In Medicinal Plant Research in Africa (pp. 393-434). Elsevier.
Negi, J. S., Bisht, V. K., Singh, P., Rawat, M. S. M., & Joshi, G. P. (2013). Naturally occurring xanthones: chemistry and biology. Journal of Applied Chemistry, 2013.