Abstract

Hydrolysable gallotannins are naturally occurring polygalloyl esters of polyols (e.g., glucose) and are widespread in the plant kingdom. A totally new synthesis is described here for polygalloyl (3,4,5-trihydroxybenzoyl) esters of polyols as structural models for gallotannins, so that the structure-function relationships of this group of phytochemicals can be studied. The new synthesis consists of two steps, condensation of trimethoxybenzoyl chloride with a polyol and demethylation with anhydrous aluminium chloride/thiol. Compared to methods already reported, the new synthesis shortens the total reaction time by up to 30 times and gives high yields. Six polygalloyl esters of polyols were synthesised: di-galloyl-ethylene glycol (DGE), tri-galloyl-glycerol (TGG), tetra-galloyl-meso-erythritol (TGE), penta-galloyl-adonitol (PGA), ß-penta-galloyl-D-glucose (PGG) and hexagalloyl- dulcitol (HGD). Amongst them, TGE, PGA and HGD are prepared for the first time. The polyphenols and their permethylated derivatives were characterised using a combination of NMR and mass spectroscopic techniques. The effects of galloylation on the NMR properties of polyols are also discussed.