Relative reactivities in the O-methylation of glucomannans: the influence of stereochemistry at C-2 and the solvent effect

• A set of O-methyl glucomannans (GM) in DS range 0.02–2.89 were prepared in DMSO with Li-dimsyl/methyl iodide.
• Relative reactivities of stereoisomeric glucose and mannose in glucomannan were studied.
• Up to DS 2, the order of the six OH groups in GM is G-k2 > M-k3 > M-k2 > G-k3 > M-k6 > G-k6.
• Enhanced reactivity of O-3 in both glucose and mannose in the early stage of the reaction is explained by a solvent effect.

The main hemicellulose in softwood, glucomannan (GM), structurally resembles cellulose but has quite different physical and chemical properties. In addition to branching and original acetylation, the only other difference between these two β-1,4-linked glycans is the configuration at C-2 in approximately 80% of the sugar residues. In contrast to glucose, the 2-OH in mannose has an axial orientation. The influence of this stereochemistry on the relative reactivities of glucosyl compared to mannosyl units in methylation reactions are studied in this work.Glucomannan isolated from spruce (SGM) and commercially available konjac glucomannan (KGM) was methylated in DMSO/Li-dimsyl/MeI and water/NaOH/MeI system, respectively. In the early stage of the reaction, the glucose part of the SGM achieved slightly higher DS values than the mannose residues, but the overall relative rate constants were close to 1:1. The order of reactivities in glucose was k2 > k3 > k6 and k3 > k2 > k6 for mannose (in DMSO/Li-dimsyl/MeI). The rate constants did not remain constant, but k3 decreased when k2 increased for both epimeric sugars. In water/NaOH/MeI, the methylation of the primary 6-OH was much more pronounced with an order of reactivity of O-6 > O-2 > O-3 for mannose and O-2 > O-6 > O-3 for glucose. The results are discussed with respect to the OH-acidity and the stereoelectronic, sterical, and solvent effects.

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