Synthesis of dl-shikonin by vanadium(II)-assisted cross-coupling and electrooxidation of aromatic nuclei

Article abstract of DOI:10.1246/bcsj.68.2917

Vanadium(II)-assisted cross-coupling of 1,4,5,8-tetramethoxynaphthalene-2-carbaldehyde and 3-methyl-2-butenal was employed for introduction of the side chain of dl-shikonin. 2-(1-Hydroxy-4-methyl-3-pentenyl)-1,4,5,8-tetramethoxynaphthalene was prepared by the pinacol coupling and the subsequent palladium-catalyzed hydrogenolysis of the carbon-oxygen bond at the allylic position of the diol carbonate. Electrochemical oxidation of the 2-substituted 1,4,5,8-tetramethoxynaphthalene, followed by reductive acetylation with zinc and the subsequent electrooxidation of the resulting 5,8-diacetoxy-1,4-dimethoxynaphthalene, afforded the corresponding 5,8-diacetoxy-1,4-naphthoquinone, whose alkaline hydrolysis furnished dl-shikonin.