RAPID PROTON NMR METHOD FOR DETERMINATION OF THREO - ERYTHRO RATIOS IN LIGNIN MODEL COMPOUNDS AND EXAMINATION OF REDUCTION STEREOCHEMISTRY

Threo:erythro ratios of beta-aryl ether lignin model compounds are rapidly obtained by integration of alpha-OH proton signals at ca. 4.5 ppm in proton NMR spectra in acetone-d6 solvent containing traces of water. The excellent separation of these sharp signals (ca. 0.11 ppm), which is unaffected by etherification of the phenol, and the additional dispersion of the beta-proton signals make NMR in this solvent ideal for homonuclear correlation spectroscopy. Sodium borohydride reduction of precursor ketones shows a substantial solvent effect: for example, in ethanol, isopropyl alcohol or tertbutyl alcohol the erythro isomer is mildly favored (ca. 40:60 threo:erythro), whereas in 50% aqueous alcohols the stereochemistry reverses to a marked threo preference (ca. 80:20 threo:erythro).