No abstract

The depsidones divaronic acid (24) (8-hydroxy-3-methoxy-1,6-dipropyl-11- oxo -11H- dibenzo [ b,e ][1,4]dioxepin-7-carboxylic acid) and stenosporonic acid (25) (8-hydroxy-3-methoxy-6-pentyl-l-propyl-ll-oxo-l1H-dibenzo[ b,e ][1,4]dioxepin-7-carboxylic acid) have been prepared by unambiguous synthesis, and have been shown to cooccur with colensoic acid, atranorin and chloroatranorin in the lichen Paraparmelia mongaensis. The syntheses employed a novel biomimetic-type approach which involved a Smiles rearrangement of a precursor meta-depside in the key step. This rearrangement has important biosynthetic implications and may account for the natural occurrence of isostructural depside-depsidone pairs.

The dibenzofurans schizopeltic acid (1) and pannaric acid (2) have been synthesized by utilizing a biomimetic approach. The depside methyl 4-O-benzyllecanorate (6) has been converted into the diphenyl ether (7) in high yield by treatment with base. Appropriate modification of this diphenyl ether followed by cyclization with palladium acetate afforded the polysubstituted dibenzofurans (11) and (12) which were converted into the natural acids.

The structurally unique depsidone furfuric acid (1) has been prepared in one step by the acid-catalysed alkylation of methyl p- orsellinate (6) or of atranorin (4) with physodalic acid (7). This mode of synthesis gives credence to the proposal that (1) is an artefact of the isolation procedure rather than a true metabolite of the lichen Pseudevernia furfuracea .

The new lichen depsides, methyl 5-chloronorobtusatate (4) and 4-O-demethyldiffractaic acid (5), have been detected in an Erioderma sp. And Xanthoparmelia duplicata Hale respectively, and the structures confirmed by total synthesis. In addition, the lichen depsides, 2'-O- methylatranorin (1), 2-O-methylsquamatic acid (2) and elatinic acid (3), have been prepared by unambiguous synthesis for the first time.