Acid hydrolysis kinetics and identification of erythro and threo α-ethyl ether derivatives of non-phenolic arylglycerol-β-syringyl ether lignin model compounds

Based on their hydrolysis behavior, NMR spectra of the erythro and threo forms of 3-(3,4-dimethoxy-phenyl)-2-(2,6-dimethoxyphenoxy)-3-ethoxypropan-1-ol (named VSE, i.e., α-ethyl ether derivative of veratrylglycerol-β-syringylether) were successfully assigned. A mixture of erythro and threo VSEs was treated at 90°C in pH 1.2 nitric acid solution. In order to assign them, the molar ratios of erythro and threo forms of 1-(3,4-dimethoxyphenyl)-2-(2,6-dimethoxy-phenoxy)propane-1,3-diol (named VS, i.e., veratrylglycerol-β-syringyl) ether given by each VSE were kinetically determined during the treatment. It was found that the inversion ratio at which the first VSE isomer gave erythro-VS was 0.73. Therefore, this isomer was identified as threo-VSE. In addition, from calculations using the least-squares method, the inversion ratio at which the other VSE isomer gave threo-VS was found to be >0.73. These values mean that threo-VSE yields 73% erythro- and 27% threo-VS, while erythro-VSE yields mainly threo-VS.