Hydrolysis of lignin with dioxane-water XIX.: Reaction of β-0-4 lignin model compounds in the presence of carbohydrates

Three lignin model compounds of beta-O-4 type, guaiacylglycerol-beta-guaiacyl ether 1, guaiacylglycerol-beta-(4-methylguaiacyl) ether 2 and veratrylglycerol-beta-guaiacyl ether 3 were heated with dioxane-water mixture (1: 1, v/v) at 180 degrees C for 20 min in the absence of and in the presence of holocellulose. The reaction mixtures were analyzed with high performance liquid chromatography (HPLC). Compound 1 gave coniferyl alcohol 4, guaiacol 5, 1,2-diguaiacyl-propane-1,3-diol 7, dehydrodiconiferyl alcohol 8, DL-pinoresinol 9 and a trilignol 10 in the absence of holocellulose. Compound 2 gave 4, 4-methylguaiacol 6, 8, 9 and an unidentified compound 11 under the same reaction conditions. In contrast, the nonphenolic beta-O-4 model compound 3 did not undergo both degradation and radical recombination on the treatment with the dioxane-water mixture under the same reaction conditions. After the treatment, the starting material was recovered almost quantitatively. Consequently, the reaction products 7, 8, 9 and 10 must be produced by recombination among the radical species derived from a homolytic cleavage of the phenolic beta-O-4 linkage in 1 and 2. These results imply that the cleavage of phenolic beta-O-4 linkages proceeds through the formation of quinone-methide intermediate on the treatment with dioxane-water at elevated temperature. When these model compounds were treated in the presence of holocellulose under the same reaction conditions, none of the secondary products 8, 9 and 10 were detected in the reaction mixtures, except 7 in only a trace amount. These results indicate that the carbohydrates act as hydrogen atom donor, reducing the radical species before the occurrence of radical recombinations.