Orsellinic acid, structural type

Cyclisation to the aromatic ring of the methylphloroacetophenone precursor of the usnic acids is basically different from the orsellinic acid cyclisations leading to precursors of dibenzofuran derivates, but the oxidative coupling and cyclodehydration reactions of pairs of units are similar in the two cases. With the usnic acids, a C-methyl substituent prevents rearomatisation of one ring. The structurally isomeric usnic acids are related by the orientation of the A ring about the carbon-carbon bond. In the two types of dibenzofuran derivates, the second unit also remains fixed but the A ring is reoriented with respect to the ether linkage. 

Xanthones are known in free-living fungi, and recent studies indicate that they are rather common in lichens too. Unlike the fungal xanthones, many lichen xanthones have one or more nuclear chlorine substituents. The fundamental structure of the known lichen xanthones could be derived directly by linear condensation of seven acetate and malonate units with one orsellinic acid-type cyclisation. The two rings are joined by a ketonic carbon and by an ether-oxygen arising from cyclodehydration. Xanthones include arthothelin, concretin,... 

Fig. 4. Structural relationships and possible formation of the five types of condensation products of orsellinic acid (6) or its homologues taking place in lichens. (9a) = olivetoric acid, (9b) = dihydropicrolichenic acid, (10) = physodic acid, (11) = picrolichenic acid, (12) = pannaric acid, (13) = pyxiferin.

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