Pigments of Fungi. XXI. Synthesis of (±)-6-Demethoxyaustrocortirubin

Abstract

6-Demethoxyaustrocortirubin (6) is synthesized via the epoxide (21) which is available in 73% yield over four steps from naphthazarin (10). Hydrolysis of the epoxide (21) yields the diol (25) which on hydrogenolysis affords 6-demethoxy-1-deoxyaustrocortirubin (7). Stereoselective benzylic hydroxylation of (7) gives (6). Cleavage of the epoxide (21) with sodium acetate in acetic acid affords a mixture of the esters (22)-(24), while methanolysis yields the isomeric ethers (29) and (30). Hydrogenolysis of (22), (23) and (29) gives high yields of (7). Potentially more direct routes to the alcohol (7) involving oxymercuration, epoxidation and bromohydrin formation from the alkene (8) are not viable.