Lignin model glycosides: preparation and optical resolution

Abstract

Synthetic protocols for the preparation of the 3-hydroxy-3-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propyl β-D-glucopyranosides and corresponding xylopyranosides have been developed. Glycosylation of racemic 1-(4-benzyloxy-3-methoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)propanone with the per-benzoylated pyranosyl bromides of D-glucose and D-xylose affords diastereomeric mixtures of the β-glycosides in up to 92% yield. Stereoselective reduction of the benzoyl ketone with Zn(BH₄)₂ gives the protected erythro diastereomers (2R,3S and 2S,3R) of 3-hydroxy-3-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propyl β-D-gluco- and -xylo-pyranosides. Reduction with (+)- or (-)-DIP chloride affords the protected threo diastereomers (2S,3S and 2R,3R) without any significant enantioselectivity. Deprotection then gives the desired lignin dimer glycosides. The use of the pyranoside to provide diastereomers leads to the enrichment (> 90%) of several individual enantiomers using silica gel chromatography, and also allows the rapid assessment of enantiomeric purity by ¹H NMR spectroscopy.