2013 Volume 71 Issue 10 Pages 1051-1060
Ellagitannins are a biologically and structurally diverse class of natural products. The structure typically consists of galloyl and hexahydroxydiphenoyl (HHDP) group(s) esterified to glucose. The pyranose ring of the glucose moiety can even be in the axial-rich conformation when the HHDP group bridges the discontiguous hydroxy groups. We here described first total syntheses of two “axial-rich ellagitannins,”(−)-corilagin and (+)-davidiin, which have the HHDP bridge between 3- and 6-oxygens and between 1- and 6-oxygnens, respectively. In the synthesis of corilagin, the 3,6-bridge was constructed through a temporary ring-opening strategy. With this strategy, three synthetic method of the HHDP group were investigated. The total synthesis was achieved using originally developed method that is the CuCl2-n-BuNH2 mediated coupling of 4-O-benzylated gallates. The 1,6-bridge in davidiin was also constructed by this method with conformational locked strategy relying on the steric repulsion of bulky silyl groups.