抄録
Ellagitannins are a kind of natural hydrolyzable tannins. Despite of the wide-ranging structural diversity, the structure can basically be expressed as D-glucose possessing hexahydroxydiphenoyl (HHDP) bridge. Ellagitannins have been considered to be biosynthesized via β-pentagalloylglucose (PGG) by oxidative coupling of gallates. However, detailed mechanism of the coupling reaction and the regioselectivity in the HHDP-bridge formation has not been elucidated. We have developed a new method for synthesizing HHDP group via intramolecular coupling of 4-O-benzylgallates induced by CuC1_2-n-BuNH_2, and have applied this reaction to several total syntheses of ellagitannins. We here describe a chemical simulation of the oxidative coupling of β-PGG. The aim of this study was to examine the coupling-prone positions. As an analog of β-PGG, penta-0-(4-0-benzylgalloy1)-β-D-glucopyranose was designed for the chemical simulation. The 4-O-benzylgallates on the substrate were coupled by CuCl_2-BuNH_2. As a result, the couplings were mainly induced in two positions; one was of the 1- and 6-O-gallates, and the other was of the 4- and 6-O-gallates. The coupling between the 1- and 6-positions was the first chemical formation of the HHDP-bridge accompanying inversion of glucopyranose ring into the axial-rich conformation. Additionally, application of the results allowed total syntheses of davidiin and tellimagrandin II in four steps from D-glucose.
