Conjugation of Vescalagin with Glucose and Phenylpropanoid: Reactions Related to the Insolubilization of Oak Wood Ellagitannins

Vescalagin is an oak wood ellagitannin, which is biosynthesized in sapwood and insolubilized during the conversion of sapwood to heartwood. The insolubilization of this tannin was attributed to the conjugation of cell wall components, such as cellulose and lignin; however, the chemical mechanism is unclear. In this study, we examined the reactions of vescalagin with glucose and sinapylaldehyde as model experiments for the conjugation of vescalagin with cellulose and lignin, respectively. Heating vescalagin with glucose and sinapylaldehyde afforded conjugation products, the structures of which were determined by spectroscopic methods. The condensation of vescalagin with glycerol proceeded at room temperature. These results suggest that the insolubilization of vescalagin is caused by the formation of covalent bonds at the C-1 benzylic carbon of vescalagin with alcohols and α,β-conjugated carbonyl compounds.