Retro-aldol-type fragmentation of reducing sugars preferentially occurring in polyether at high temperature: Role of the ether oxygen as a base catalyst

Abstract

The pyrolysis behavior of reducing monosaccharides was compared in the presence and absence of tetraethyleneglycol dimethylether (TEGDE), a polyether (N2/150–250 °C). The pyrolytic pathways changed drastically in TEGDE. Glucose started to decompose at >160 °C under the neat conditions, and polysaccharides, anhydrosugars (levoglucosan and 1, 6-anhydroglucofuranose), a colored substance and char were the major products. However, glucose was completely stabilized against decomposition in TEGDE and instead converted into fragmentation products including formaldehyde, glycolaldehyde, glyceraldehyde, 1, 3-dihydroxyacetone, erythrose and erythrulose at higher temperatures. The total yield of the fragmentation products reached a 74.9 wt% at 250 °C. An aldose–ketose isomerization and retro-aldol fragmentation including a six-membered cyclic transition state were suggested as the principle mechanisms. Several other polyethers gave similar results. This unique property of polyether can be explained by the basicity of the ether oxygen which acts as a proton acceptor for the hydroxyl groups in the sugar. This H-bonding between the polyether and glucose may prevent inter- and intramolecular H-bonding (H-donation to the oxygen atoms) of glucose, which results in stabilization against transglycosylation and dehydration reactions. Such inter- and intramolecular H-bonding (H-donation) may also be involved in the thermal decomposition of the melt sugar as an activation (acid catalysis) mechanism.