Absolute Configuration of β- and α-Asymmetric Carbons within β-O-4-Structures in Hardwood Lignin

Journal article, 2014

In this study, we investigated the proportion of erythro-and threo-forms of beta-O-4-ether structures and their enantiomeric compositions in hardwood lignin by applying the ozonation method to birch wood meal. Optical activity was not substantially observed in either the erythronic or threonic acids obtained as the ozonation products of beta-O-4-structures in birch wood meal. The proportions of the four stereoisomeric forms {(alpha S,beta R)-erythro, (alpha R,beta R)-threo, (alpha S,beta S)-threo, and (alpha R,beta S)-erythro forms} were estimated to be 37-38%, 13-14%, 12-13%, and 36-37% based on the yields of erythronic and threonic acids, and on their optical activities. The proportions suggest that the entire components of beta-O-4-ether structures in birch wood lignin have R-and S-configurations at the beta-carbon in approximately the same quantities {(beta R)-beta-O-4-structure: (beta S)-beta-O-4structure = 50-52: 48-50}; i.e., that the beta-ether structures are essentially racemic. This estimation implies that, during lignin biosynthesis, an equal number of enantiomeric forms of beta-O-4-bonded quinone methides were formed by radical coupling reactions.