Studies on quantitative relationships between reactivity and stereo structure of lignin

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By the improvement of ozonation method, the yield of tetronic acids (erythronic and threonic acids) from β-O-4 type lignin model compond, guaiacylglycerol-β-guaiacyl ether, reached 70 %. Determination of β-O-4 rythro/threo ratio calculated based on the yields of two tetronic acids was also quite satisfactory. By this improvement, ozonation method was established as a quantitative method for the stereo structure of lignin.First, this method was applied to the enantiomer analysis of β-O-4 structure. The results confirmed that erythro type of β-O-4 structure is present as racemic forms. There was some difficulty to evaluate the threo type but the total enantiomer excess of side-chain β carbon is, if present, less than 5 %, indicating that β-O-4 type is optically inactive.Relationships between the erythro/threo ratio of β-O-4 and reactivity during delignification reactions was also investigated. With the progress of alkali-delignification, both the yield of tetronic acids and erythro/threo ratio in the residual lignin decreased significantly. The decrease in tetronic acids confirmed that β-O-4 structure in the residual lignin is well cleaved by alkali. The decrease in erythro/threo ratio was attributed to stereo preferential β-O-4 cleavage and to the original structural feature of lignin "hindered" from the reaction. On the other hand, when delignified by sodium chlorite, delignification was not related to the erythro/threo ratio of β-O-4 in residual lignin nor to the yield of tetronic acids, indicating that chlorite delignification is not dependent on the side-chain structure. Although the oxidation of lignin by chlorite was not affected by the stereo structure of β-O-4 linkage, the occurrence of erythro-preferential degradation was found during the permanganate oxidation of milled wood lignin (MWL).As summarized above, the ozonation method contributed to the analysis of relationships between stereo structure and reactivity of lignin.