Enantioselective Formal Synthesis of the Cytotoxic Topoisomerase II Inhibitor Deoxythysanone, Catalyzed by Chiral Spiroborate Ester

Bioactive pyranonaphthoquinone analogs, (1R,3S)-deoxythysanone, (1R,3S)-thysanone, and (1R,3S)-demethoxythysanone can be efficiently synthesized from a common intermediate product, (S)-3-methyl-3,4-dihydro-1H-isochromene-5,8-dione. We have developed a short synthetic route to pyranonaphthoquinone antibiotics, which involves enantioselective reduction of homobenzylic ketone in the presence of a chiral spiroborate catalyst with 87% enantiomeric excess as the key step. The subsequent oxa-Pectet-Spengler reaction, followed by oxidative demethylation, afforded deoxythysanone.