Sequential annulation approach to sterpuric acid and sterpurene-3,13,14-triol, metabolites of the silver leaf fungus Stereum purpureum

Article abstract of DOI:10.1021/ja00225a037

Sterpuric acid (1) and sterpurene-3,12,14-triol (2), unusual oxygenated sesquiterpenes produced by the fungus Stereum purpureum, have been stereoselectively synthesized from the common precursor 5b. Our approach begins with a Diels-Alder reaction involving diene 14. Direct Jones oxidation of the product mixture delivers chiefly 13, thereby setting the stage for [2 + 2] photocycloaddition to ethylene and the acquisition of 18, an intermediate already possessing the complete carbocyclic skeleton of the target molecules. The tert-butyldimethylsilyl enol ether of 18 underwent peracid oxidation to afford principally 29, reductive desulfonylation of which delivered 32. Condensation of this ketone with methylenetriphenylphosphorane accomplished introduction of the final carbon atom. The resulting exo methylene derivative 33 proved to be the branching point in the two syntheses. To arrive at sterpuric acid, 33 was subjected to sequential ene reaction with N-sulfinylbenzesulfonamide and Raney nickel desulfurization. Final deblocking of the silyl ether and ester saponification delivered 1 in 11 steps with 11% efficiency. To acquire 2, it was necessary to avoid a strong proclivity of the system for framework isomerization to linear triquinane derivatives. The key step involved oxidation of 33 with selenium dioxide in the presence of tert-butyl hydroperoxide. The tertiary allylic alcohol so formed (42) could then be oxidatively rearranged in the presence of Cr(VI). Dibal reduction of aldehyde 43 and unmasking of the tertiary hydroxyl afforded 2. The overall efficiency in this instance was 5.5%.