Benzodioxans by oxidative phenol coupling. Synthesis of silybin

Abstract

Oxidative coupling of substituted catechols with isoeugenol or coniferyl alcohol in the presence of silver oxide affords 2,3-trans-1,4-benzodioxans in good yield. The reaction is highly regioselective when the catechol bears an alkyl substituent, much less so in the case of an electrophilic one. A free-radical coupling mechanism is proposed. A one-step biomimetic synthesis in high yield of the natural flavanolignans silybin and isosilybin from 2R,3R-dihydroquercetin and coniferyl alcohol is reported.