Stereoselective Formation of β-O-4 Structures Mimicking Softwood Lignin Biosynthesis: Effects of Solvent and the Structures of Quinone Methide Lignin Models.

Abstract

p-Quinone methide (QM) is formed as an intermediate during lignin biosynthesis. The aromatization of the QM by the attack of a nucleophile at the α-position of its side chain generates a phenolic hydroxy group in a growing polymer and creates stereoisomeric forms in the side chain. A series of β-O-4-aryl ether QMs was reacted with water at 25 °C to replicate the formation of p-hydroxyphenyl (H) and guaiacyl (G) β-O-4 structures in plant cell walls. Water addition occurred in 3-methoxy-substituted QMs (G-type QMs) with half-lives ( t1/2) between 13 and 15 min, at pH 7, in 50% water solution (dioxane-water, 1:1). The rate increased as the water concentration increased to 99% ( t1/2, 1.2-1.4 min). Similar solvent effects were observed for more reactive nonsubstituted QMs (H-type QMs with t1/2 of <1 min). Consequently, t1/2 of the H-type QMs was shorter than that of the G-type QMs under every solvent condition. Upon increasing the water concentration, the variation in the erythro/ threo ratios of the four dimeric β-O-4 products increased. Interestingly, the effect of pH on the stereopreference, which was observed in 50% water solution, was small and became imperceptible as the water concentration increased to 99%, suggesting that the effect of the solvent, as well as the effect of the pH, plays an important role in understanding the reaction conditions in cell walls during lignin biosynthesis. The threo isomer was preferentially formed in the four dimeric β-O-4 structures, which is inconsistent with the structural features of compression wood lignin rich in H-units. However, the erythro-selective formation was attained in an H-type QM at every pH studied (pH 3.5-7) by introducing a biphenyl structure into the β-etherified ring moiety.