Polyketide-derived lichen metabolites

Orsellinic acid and its homologues form the most common mononuclear structural units of polyketide-derived lichen metabolites, but compounds derived from phloroacetophenone (107) (through an alternative cyclisation... 

Lichens had to evolve diverse biosynthetic pathways to produce such complex arrays of secondary metabolites polyketide, shikimic acid, and mevalonic acid pathways. Most of the lichen substances are phenolic compounds. Polyketide-derived aromatic compounds, depsides, depsidones, dibenzofurans, xanthones, and naphthoquinones, are of great interest. Compounds from other pathways are esters, terpenes, steroids, terphenylquinones, and pulvinic acid (Fahselt 1994 Cohen and Towers 1995 Muller 2001 Brunauer et al. 2006, 2007 Stocker-Worgotter and Elix 2002 Johnson et al. 2011 Manojlovic et al. 2012). So, many lichens and lichen products have proved to be a source of important secondary metabolites for food and pharmaceutical industries (Huneck 1999 Oksanen 2006)... 

Bruun and Motzfeld (35) isolated the novel peroxyergosteryl divari-catinate (525) from the lichen Haematomma ventosum. This product is the only known example of a steryl ester of an aromatic polyketide derived acid. The depside divaricatic acid (234) also present in this lichen contains the same A-ring component and it is possible that the ester formation in both of these metabolites is effected by the same enzyme. 

Of the more than 20,000 known species of lichens, only a few have been analyzed and identified as containing biologically active secondary compounds. Most of the unique secondary metabolites that are present in lichens are derived from the polyketide pathway, with a few originating from the shikimic acid and mevalonic acid pathways (Table 1.1). Previous studies have suggested that the para-depsides are precursors to mefa-depsides, depsones, diphenyl ethers, depsidones and dibenzofurans.9,12... 

By far the greatest number of lichen substances are derived by condensation of a starter unit, usually acetyl-SCoA( ),t and -malonyl-SCoAunits with concomitant decarboxylation. The putative poly-/S-ketothioesterthus formed undergoes modification to give the observed metabolites, also known as polyketides (Collie, 1907), a name best restricted to non-fatty-acid compounds. 

Several groups of characteristic polyketides occur in lichens. Biosyntheti-cally they are closely related, being derived mainly from 6-alkyl-or 6-( -keto) alkyl-2,4-dihydroxybenzoic acids which further vary in their details of extra ring substituents (Table I). /3-Orsellinic acid units are not so widespread as fungal metabolites. The only report of such a recognized depside component in a free-living fungus concerns the occurrence, biosynthesis, and metabolism of 3-methylorsellinic acid (R = CHg) in Penicillium stipi-tatum (Scott et aL, 1971). 

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