Phenolics and Other Aromatic Compounds

Several mushrooms produce chlorinated phenolic compounds which often exhibit antimicrobial properties (Figure 1). For example, 2-chloro-4-nitrophenol (4) from the carrot truffle (Stephanospora caroticolor, Figure 2) is released from the storage precursor stephanosporin (1) after tissue wounding and serves the gasteromycete as an effective fungicide. Interestingly, 2-chloro-4-nitrophenol (4) has been used previously for leather conservation and seed protection. Another aromatic chloro derivative pterulinic acid (6) isolated from a mycelial culture of a Pterula species exhibits antifungal properties by acting as inhibitor of the NADH:ubiquinone oxidoreductase. 4-Chloro-3-methoxybenzaldehyde (5) isolated from cultures of Lepista diemii has antibacterial potential that may help the mushroom deter unwanted microorganisms from its surrounding.

Several higher fungi of the family Tricholomataceae such as the pinecone cap (Strobilurus tenacellus) produce compounds that very efficiently prevent the growth of other fungi in their surrounding. This observation led to the discovery of strobilurins, for example, strobilurin A (7), that turned out to be highly active fungicides which served as lead structures for a new class of commercially widely used fungicides. It has also been shown that stro-bilurins are produced by the fungi in vivo in amounts high enough to prevent growth of competitors proving their ecological relevance. Strobilurins interfere with the respiratory chain of fungi by inhibition of the cytochrome bci complex. The E-S-methoxyacrylate structure element of strobilurins has been established as their crucial active principle. For their own resistance against strobilurins, strobilurin-producing fungi adapted their cytochrome bc1 complexes by point mutations of only a few amino acids.

The fruiting bodies of the velvet pax Paxillus atroto-mentosus contain esterified leucomentins, such as 8, that are readily oxidized to flavomentins, such as 9; the esters easily undergo hydrolysis followed by epoxide opening finally yielding osmunda lactone (10). The lactone 10 is a potent insect antifeedant which explains why the fungus is not attacked by insects. Accordingly the phenolic fla-vomentin 9 may also exhibit antifeedant properties similar to phenolic compounds known from plants (see Plant Defense).

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