Structure of oxidized hydrolysis lignin

AbstractAbstractThe main goal of this study was the elucidation of structures of products of oxidation of industrial hydrolysis lignin (HL) obtained in the system H2O2-H2SO4-H2O. It was found that the obtained oxidized hydrolysis lignin (OHL) contained structural fragments of muconic acid dilactone. The latter appeared as a result of the oxidation of aromatic ring of HL into unstable structures of 3,4-dihydroxyadipic acid, which are spontaneously cyclized into the corresponding fragments of muconic acid dilactone in acidic oxidative medium. Consequent reaction of OHL with aqueous sodium hydroxide (NaOH) led to the formation of sodium salt of OHL (Na-OHL), in which the muconic acid dilactone units of OHL opened into fragments of disodium salts of 3,4-dihydroxyadipic acid. The Na-OHL was transformed into the series of novel derivatives of lignins. Thus, the reaction of Na-OHL with thionyl chloride proceeded as a substitution of hydroxyl groups into chlorine atoms in the structure of OHL, that afforded chloro-derivative of OHL. The latter was transformed into amino-amide and ester-ether derivatives of OHL by nucleophilic substitution of chlorine atoms with amines and alcohols correspondingly.Keywords: 3,4-Dihydroxyadipic acidindustrial hydrolysis ligninmuconic acid lactonesNMR of ligninsoxidation AcknowledgmentsSpectral studies were performed at Center for Magnetic Resonance, Center for Chemical Analysis and Materials Research, and Research Center for X-ray Diffraction Studies of Saint Petersburg State University, Saint Petersburg, Russia.Declaration of interest statementThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.Supporting informationIt contains NMR 1H and 13C spectra of compounds in solutions and solid state, IR spectra of compounds, X-ray data for compound 3.Additional informationFundingThis work was supported by Russian grant no. МК-1014.2021.1.3.