Triterpenoids of the Basidiomycetes

anthracophilus found on a Eucalyptus species were established at the same time. The work on eburicoic acid was carried out by Robertson in Liverpool. The elucidation of the structure of polyporenic acid A (5.161) benefited from experience gained in studies on the structure of lanosterol and on the penta-cyclic triterpenes. The identification of some structural features also showed the developing role of UV and IR spectroscopic methods in the 1950s. However, the purification of these compounds was impeded by the difficulty of separating the 8(9)-enes from the 7(8),9(ll)-dienes that often accompanied them. Initial studies on polyporenic acid A revealed the presence of a carboxylic acid, two hydroxyl groups and two double bonds, each of differing reactivity. One double bond was a terminal methylene since formaldehyde was obtained on ozonolysis whilst the other was more hindered. Typical of the tetracyclic triterpenes. 

The position of the carboxyl group in the side-chain followed from its easy decarboxylation as a p,y-unsaturated acid and from the isolation of acetaldehyde on cleavage of the alkene of the decarboxylation product. The hydroxyl group on ring A was shown to possess the less-common axial C-3a configuration. 

Whilst this work was in progress comparable studies were carried out on eburicoic acid. In this case the relationship of the acid at C-21 to the alkene at C-24 was established by oxidation with Se02 to form a y-lactone (5.163). The size of the lactone ring was established by its IR absorption, correlations that at that time had only recently been established. The degradation of eburicoic acid linked it to lanosterol via a common hydrocarbon, a standard strategy in 

Polyporenic acids A and C have anti-inflammatory activity and, in a bioassay for potential anti-tumour activity, suppressed the oedema induced by 12-O-tetradecanoylphorbol-13-acetate. 

An interesting feature of these triterpenes is the presence of the extra methylene at C-24 that is typical of ergosterol, indicating that this group may be introduced relatively early in the biosynthetic modification of lanosterol. 

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