抄録
Ellagitannins are polyphenolic natural products with a wide range of biological activities and remarkable structural diversity. In this class of polyphenolics, rugosin B and isorugosin B possess a unique part structure called valoneoyl group, which is generated by a C-C/C-O oxidative coupling of galloyl groups in plants. Although, during the last two decades, a number of chemists have challenged to synthesize ellagitannins, there is no example of the synthesis of valoneoyl-containing ellagitannin. In this context, we attempted to synthesize rugosin B and isorugosin B. Generally, there are two strategies for the synthesis of ellagitannin. We adopted Route B, thus the synthesis was commenced with construction of valoneoyl moiety. Enantioselective construction of the axially chiral biaryl function was achieved by the Pd-catalyzed intramolecular biaryl coupling reaction of 10 and the atroposelective lactone opening reaction of 11. As a result, the key intermediate 15 was obtained in an enantiomerically pure form. Carboxylic acid 15 was condensed with sugars 16 and 19 to afford methylated rugosin B (18) and isorugosin B (21) respectively. All attempts for the cleavage of the phenolic methyl ethers were unsuccessful. The practical method to construct the biaryl ether moiety protected by the benzyl groups, which is essential for the total synthesis of ellagitannins, will be also discussed.
