Polyporic acid

The dimethoxy derivative of polyporic acid, betulinan A (4), is another of the few known />-terphenyls with unsubstituted side rings. It has recently been isolated from fruiting bodies of Lenzites betulina, together with the related betulinan B (113, see Section 4.1) [27], Betulinan A and B are potent inhibitors of lipid peroxidation in rat liver microsomes (thiobarbituric acid method), and 4 showed the highest activity (IC50 = 0.46 pg/mL). 

The structure of the 4,3 ,4 -trihydroxy derivative of polyporic acid was assigned in 1942 by Akagi to the brown pigment leucomelone (47) isolated from the black edible mushroom Polyporus leucomelas (.Boletopsis leucomelaena) collected in Japan [68], From the same fungus, Akagi obtained also a colourless compound named protoleucomelone (48), established as the leuco-peracetate of 47. 

Polyporic acid (3), the above cited terphenylquinone occurring in several species of basidiomycetes, has also been found in lichens of the Sticta genus [19]. Also thelephoric acid (82, see Section 3.2), widely distributed within Thelephoraceae, has been found in lichens of Lobaria genus [3], From the lichen Relicina connivens (Parmelia butleri) the butlerins A-F (51 - 56) were isolated [72, 73], These polyporic acid-related metabolites have not yet been found in fungi. 

Among the early synthetic works, we wish to mention here the synthesis of polyporic acid (3) [115] and thelephoric acid (82) [78]. As summarised in Scheme 3a, several terphenylquinones have been synthesized with moderate yields starting from 2,5-dichlorobenzoquinone by arylation with V-nitrosoacetanilides or diazonium salts and subsequent alkaline hydrolysis. This method allowed preparation of symmetrical and unsymmetrical terphenylquinones, this latters with low yields. 

Mushrooms and various fungi and lichens are rich in enol metabolites and many exhibit significant bioactivities , such as usnic acid (14) which serves as a regulator for plant growth and shows antitumor and antibiotic activities " . The widely distributed quinone polyporic acid (15a, PPA) from the Purple-Dye Polypore mushroom (Hapalopilus nidu-lans) and other sources is a weak inhibitor (IC50 = 0.1 to >1.5 mM " ) of dihydroorotate... 

Polyporic acid (1) is converted in 95 % yield into pulvinic acid dilactone (2) on... 

TTie rearrangements of hydroxy-1,4-benzoquinones in alkali are more complex. Under conditions in which polyporic acid (34) is converted, by way of retro-aldol ring cleavage and benzilic rearrangement, into a-benzyl-P-phenylsuccinic acid, cis- and tranr-a-benzylcinnamic acid and oxalic acid, atromentin... 

The authors investigated the possible relationship between poisoning due to ingestion of H. rutilans and DHO-DH inhibiton by 3, and concluded that the intoxication symptoms are due to the high content of polyporic acid. 

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