Alkaline degradation of model compounds related to beech xylan

To address the chemical behavior of beech xylan (O-acetyl-4-O-methyl- glucuronoxylan) under alkaline conditions, three model compounds, 2-O-methylxylobiose (1), aldobiouronic acid (4-O-methyl-α-D- glucopyranosyluronic acid-(1 → 2)-xylose, 2), and aldotriouronic acid (4-O-methyl-α-D-glucopyranosyluronic acid-(1 → 2)-β-D- xylopyranosyl-(1 → 4)-D-xylose, 3), were subjected to strong alkaline conditions equal to those used for the industrial production of viscose (18% NaOH, 43°C). Kinetics of the degradation of the model compounds were monitored by capillary electrophoresis in combination with pre-column derivatization. It was demonstrated that substitution at O-2 of the reducing xylose unit strongly retarded the alkaline degradation reactions (1 and 2). By isotopic labeling experiments and isolation of degradation products it was shown that under the pertinent conditions deprotonation at C-2 occurs, followed by epimerization to the respective lyxo derivative. Aldotriouronic acid 3 was degraded to 2 as an intermediate according to classical peeling pathways. Genuine degradation reactions and epimerization processes were distinguished.