Read this article to learn about Xanthones. After reading this article you will learn about: 1. Significance of Xanthones 2. Biosynthesis of Xanthones 3. Isolation of Xanthone Derivatives.
Significance of Xanthones:
During last few decades there has been a tremendous growth in the study of various aspects of naturally occurring and synthetic xanthones.
This development in the field of chemistry and biology of xanthones was possible due to increase in number of naturally occurring xanthones, successful synthesis involving newer methods, availability of physical and spectral data, and an extensive research work on their biological and biochemical aspects.
Xanthones are widely distributed as poly-oxygenated, prenylated, methylated, furano, and chloro substituted derivatives in nature.
The number of families of higher plants notably, Guttiferae, Gentinaceae, Polygalaceae, Moraceae and Leguminosae has been reported to produce xanthones as secondary metabolites. Furano and chloro substituted xanthones have been found only in lower organisms, such as moulds and fungi as metabolites.
Although xanthone structure contains only four different locations for substituents, which are duplicated symmetrically. This unique backbone along with type and position of the attached chemical groups define specific xanthones. The oxygenation pattern of xanthones is important chemo taxonomically.
There is a correlation between the type and position of substituents in xanthone nucleus, plant family and species. The two families Centianaceae and Guttiferae are the important sources of substituted xanthones. Number of xanthone compounds has been identified in species of Centianaceae. Canscora, Gentiana, Polygdla, Swertia, Halenia, Hopcea, Enieostemna and Frasera are some important xanthone yielding genera.
Biosynthesis of Xanthones:
Xanthones are biosynthesized in higher plants by a mixed shikimate-acetate pathways. Studies indicated that m-hydroxy benzoic acid acts as an important precursor of xanthone biosynthesis, indicating that a C6C1 unit and three acetate unit forms the intermediate benzophenone.
This intermediate is further hydroxylated enzymatically to form 2, 3 4, 6-tetrahydroxy benzophenone which acts as a main intermediate in xanthone biosynthesis.
Tracer studies have suggested the fact that the benzophenone intermediates react intermolecular (oxidative coupling) to form xanthones.
The enzyme catalyzing this reaction is xanthone synthase. The c-glycosides are limited as compared to O-glycosides. There is a biogenetic relationship between flavonoid and xanthones. Mangiferin is derived from a p-coumarate and two acetate units.
Isolation of Xanthone Derivatives:
Bioactivity-guided fractionation of a dichloromethane soluble extract of Carcinia mangostana fruits has led to the isolation and identification of five compounds, including two xanthones, 1,2-dihy-dro-1,8,10-trihydroxy-2-(2-hydroxypropan-2-yl)- 9-(3-methylbut-2-enyl)furo[3,2-a]xanthen-11 -one and 6-deoxy-7-de-methylmangostanin, along with three known compounds, 1,3,7-trihydroxy-2,8-di- (3-methylbut-2-enyl)xanthone, mangostanin, and alpha-mangostin. (Chin et al. 2008)
Bioactivity guided fractionation of polygala alpestris L. (Rchb) extracts lead to the identification of two new xanthones, 1, 3, 7- trihydroxy 2,7- dimethoxy xanthones and 2,3-methylenedioxy-4,7-dihydroxyxanthones.
In addition five known compounds 3, 4- dimethoxy- 1,7-dihydroxyxanthone, 1,3-dihydroxy-7-methoxyxanthone, 1,7-dihydroxy-2,3- di-methoxyxanthone, 3-,6-0- disinapoyl sucrose, 3-5-dimethoxybiphenyl-4-ol were isolated (Dall’Acquaa etal., 2004).
Kielcorin and the Cadensins A and 8, isolated respectively from Kielmeyera coriacea and Caraipa densiflora (Guttiferae), were shown to be xanthonolignoids.
Three xanthones, polyanxanthone A, B and C have been isolated from the methanol extract of the wood trunk of Carcinia polyantha, along with five known xanthones 1,3,5- trihydroxyxanthone;1,5-dihydroxyxanthone;1,3,6,7-tetrahydroxyxanthone;1,6-dihydroxy- 5-dethoxyxanthoneand1,3,5,6-tetrahydroxyxanthone.
A new xanthone 1, 4, 8-trihydroxyxanthone (1, 4, 8-trihydroxy-9H-xanthen-9-one) was isolated from the roots of Vismia latifolia (Guttiferae). Four other known xanthones were isolated: 1, 5-dihydroxy-8-methoxyxanthone, 1, 7-dihydroxyxanthone, 1,6-dihydroxy-7-methoxyxanthone and 1,3,5,6-tetrahydroxyxanthone. The structures were established by UV, IR, MS, 1D and 2D NMR spectroscopic techniques.
A marine fungal isolate, identified as Wardomyces anomalus, on cultivation found to produce two new xanthone derivatives 2, 3, 6, 8-tetrahydroxy-1 –methylxanthone and 2, 3, 4, 6, 8-pentahydroxy-1 -methylxanthone in addition to the known xanthone derivative 3, 6, 8-tri- hydroxy-1-methylxanthone.