Determination of the Conformation and Isomeric Composition of Lignin Model Quinone Methides By NMR

Abstract

Proton and ¹³C NMR of representative guaiacyl and syringyl β-aryl ether quinone methides have shown that guaiacyl quinone methides, generated from the corresponding benzyl bromides, exist as isomeric pairs in approximately a 70:30 ratio, the major isomer having the 3-methoxyl group syn with respect to the side chain. The ring protons at the 2- and 6-positions in the syn-and the anti-isomera respectively are markedly deshielded by 3teric compression effects; concomitant shielding of these ring carbons is observed in the C-13 NMR. Nuclear Overhauser enhancement experiments define the major solution conformation of these species, the conformation being consistent with the observed predominance of threo-products resulting from nucleophilic addition reactions.