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Xanthones are the largest class of compounds in natural product chemistry. Many xanthones have been isolated from natural sources in higher plants, fungi, ferns and lichens. The isolation and structural resolution of xanthones can be performed by different physicochemical and instrumental methods such as liquid-solid and liquid-liquid extraction, TLC, fast chromatography, column chromatography, IR, 1H NMR and 13C NMR spectroscopy, GLC, HPLC, GC and LCMS. Xanthones are hepatoprotective, anticancer, antimalarial, antioxidant, anticholinergic, mutagenic, radioprotective, immunomodulatory, anti-bone resorption, antiparasitic, neuraminidase inhibitor, anti-complement, antibacterial, antifungal, algicidal, anti-HIV, cardioprotective, antitumor, antidiabetic, antihyperlipidemic, and antiatherosclerotic. Naturally occurring xanthones are also anti-inflammatory, anti-ulcer, anti-diabetic, hypolipidemic, analgesic, anti-asthmatic, antihistamine, diuretic and antidiarrheal.

Classification of Xanthones

Xanthones isolated from natural sources are classified into six major groups, namely, simple xanthones, xanthone glycosides, isoprenated xanthones, xanthones, dioxanthones, and mixed xanthones.

  • Simple xanthene-containing xanthene ketones

Simple xanthene-containing xanthene ketones are subdivided into non-, mono-, di-, tri-, tetra-, penta- and hexa-oxo substances according to the degree of oxygenation. In these xanthones, the substituents are simple hydroxyl, methoxy or methyl groups.

  • Xanthones glycosides

Sixty-one naturally occurring glycosylated xanthones, thirty-nine of which are new compounds, have been reported as C- or O- glycosides mainly in the families Gentianaceae and Teleostei. Detailed information on naturally occurring xanthones glycosides has been reviewed and a distinction has been made between C- and O-glycosides. In C-glycosides, the C-C bond connects the sugar portion to the xanthone nucleus and they are resistant to acidic and enzymatic hydrolysis, whereas O-glycosides have typical glycosidic bonds.

  • Isoprenated and related xanthones

Of the 285 isoprenylated xanthones, 173 are described as new compounds. The occurrence of isoprenated xanthones is restricted to plant species of the Guttiferae family. The major C 5 units of the substituents include the common 3-methylbut-2-enyl or isoprenediyl groups (e.g. isomorin) and the less common 3-hydroxy-3-methylbutyl (e.g. aniline flavonoid P and 1,1-dimethylprop-2-enyl).

  • Biflavones

To date, a total of 12 biflavones have been reported, five of which are from higher plants, one from lichens and six from fungi. These include jacarelhyperols A and B from Hypericum and dimeroxanthone, and globulixanthone E from the roots of Symphonia globulifera.

Biosynthesis of Xanthones

Biosynthetic xanthones are a mixed source of mangiferic acid and acetate. Thus, the phenylalanine formed from mangiferic acid loses two carbon atoms from the side chain and is oxidized to form m-hydroxybenzoic acid. This combines with three units of acetate (via the malonate) to give the intermediate. The manganate-acetate intermediate undergoes ring closure to give a substituted benzophenone, which is coupled by oxidation of phenol to produce the central ring of the xanthone portion. This oxidative coupling can occur in two ways depending on the folding position of the benzophenone in the adjacent or para position to the hydroxyl substituent in the potential B ring to give the 1,3,5-trihydroxyxanthenone or the 1,3,7-substituted analogue of gentioblastine, respectively. Thus, depending on the orientation of the intermediate, two different modes of hydroxylation can be found.

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