Base-catalyzed oxidation of silybin and isosilybin into 2,3-dehydro derivatives

The base-catalyzed oxidation of the flavonolignans, silybin, and isosilybin into the corresponding 2,3-dehydro derivatives has been studied and optimized. Various bases, solvents, and reaction conditions were tested to achieve the best yields. The influence of water on the course of the reaction was also observed. It was found that this oxidation reaction was probably based on the disproportionation of the (iso)silybin (or some intermediate) molecule, as the reaction also proceeds in the absence of oxygen. We report here for the first time the preparation of optically pure 2,3-dehydroisosilybins A and B.