Synthetic study of ellagitannins based on coupling reactions between two aromatic rings

Although ellagitannins, which are natural polyphenol compounds, composed of only limited structures, glucose and gallic acid, many kinds of compounds have been known. Since ellagitannin compounds often exhibit interesting biological activities, they have been expected to be medicinal seeds. However, studies on organic synthesis of ellagitannins have insufficiently progressed, and a few examples of total synthesis of simple ellagitannins have been reported. In this study, we challenged syntheses of complex-structured ellagitannins based on coupling reactions between two gallic acids, involving the C-C or C-O oxidative coupling.