Strategies and Methods for the Total Synthesis of Ellagitannins

Ellagitannins are a structurally and biologically diverse class of hydrolysable tannins. The structure typically consists of galloyl and hexahydroxydiphenoyl group(s), which are esterified to glucose. A number of biological and pharmacological activities have been reported in this class of natural products. The focus of this review is progress in strategies and methodologies for the total synthesis of ellagitannins. The first half of this review describes the previously achieved total syntheses along with the synthetic problems and solutions including formation of the HHDP-bridge, preparation of dehydrodigalloyl ether, control of anomeric isomers, and protecting groups in the syntheses. The latter half describes the three recent total syntheses of ellagitannins including details of the first synthesis of an axial- rich ellagitannin, of which glucose moiety is in a 1C4 or a twist-boat conformation that has more axially oriented C-O bonds.