Synthesis and Rearrangement Reactions of Ester-Linked Lignin-Carbohydrate Model Compounds

Abstract

A series of ester-linked lignin-carbohydrate model compounds has been synthesized, which represent possible ester linkages of lignin to hemicellulose in wood through the carboxylic acid group of 4-O-methyl-α-D-glucopyranosiduronic acid moieties. The threo and erythro isomers of these compounds were prepared through an appropriate combination of protective and stereoselective transformations. Spectroscopic characterization of these esters revealed that the uronosyl group migrated between the primary (γ) and benzylic (α) positions of the lignin side chain in both acidic and neutral conditions. It has been determined that the migration equilibrium favors γ- over α-esters, with the reaction possibly proceeding through a six-membered ring intermediate. That uronosyl migrations occur favoring the γ-position suggests that the classical view of lignin-xylan ester linkages in wood needs to be reevaluated.