Autocatalytic oxidation of primary hydroxyl functions in glucans with nitrogen oxides

The selective oxidation of the primary hydroxyl groups in the glucans cellulose, amylose and pullulan with nitrogen oxides has been studied. The polymers were dissolved in 85% phosphoric acid and sodium nitrate was used as the stoichiometric oxidant. A catalytic amount of sodium nitrite was added to reduce the induction time. With this reaction system, where the oxidising nitrogen oxides are formed in situ, the primary hydroxyl groups could be completely oxidised (> 95%) to carboxylic acids. Undesired ketones due to secondary hydroxyl group oxidation were subsequently reduced with sodium borohydride. Especially for the α-glucans, this oxidation-reduction sequence of secondary hydroxyl functions apparently gave epimerisation. Degradation of the polymers was slow provided the oxidation was performed at 4 °C. Thus, pullulan with 〈Mw〉 ≈ 170 kg/mol yielded a polyuronate with 〈Mw〉 ≈ 100 kg/mol. A study of this reaction system with β-cyclodextrin as the substrate clearly showed that the reaction was autocatalytic.