Lignin model glycosides: Preparation and optical resolution

Synthetic protocols for the preparation of the 3-hydroxy-3-(4-hydroxy-3-methoxyphenyl)-2(2-methoxyphenoxy)propyl beta-D-glucopyranosides and corresponding xylopyranosides have been developed. Glycosylation of racemic 1-(4-benzyloxy-3-methoxyphenyl)-3-hydroxy-2-(2-methyoxyphenoxy)propanone with the per-benzoylated pyranosyl bromides of D-glucose and D-xylose affords diastereomeric mixtures of the beta-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 beta-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 H-1 NMR spectroscopy.