Sesquiterpenoids of the Basidiomycetes

Sesquiterpenoids are widespread metabolites of the Basidiomycetes. Lactarius species (milk or ink caps) are common Basidiomycetes that are found growing in woodlands. They are characterized by a latex which is exuded when the fruiting body is cut or broken. The colour and taste of the latex is of taxonomic significance. The latex of the edible species, Lactarius deliciosus, is at first carrot-coloured. On exposure to air it then darkens and turns a green-blue colour. The colour has been associated with the formation of an azulene, lac-taroviolin (5.55), which arises by the aerial oxidation of a dihydroazulene. 

A particularly interesting anti-tumour isoilludane, HMAF (5.78), has been obtained from the treatment of illudin S with formaldehyde and sulfuric acid. 

The biosynthesis of the illudins from C-acetates and variously stereo-specifically labelled mevalonates has been studied. The results are in accord with the cyclization of farnesyl diphosphate set out in 5.59 5.65. 

Another group of metabolites that have been isolated from C. illudens are the illudalanes. In these compounds the four-membered ring has been cleaved to give metabolites such as illudalic acid (5.79) and illudacetalic acid. Illudinine (5.80) is an alkaloid that can also be obtained from illudacetalic acid by treatment with ammonia. 

The fungus Fames annosus is the cause of another serious root decay of trees, particularly conifers. The phytotoxic metabolite, fomannosin (5.81) also arises by cleavage of a protoilludane, this time of the six-membered ring. The cleavage is followed by a lactonization. Labelling studies with [l,2- C2]acetate have confirmed this biosynthetic route. 

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