title: Relative reactivities in the O-methylation of glucomannans: the influence of stereochemistry at C-2 and the solvent effect.
abstract: 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.
authors.affiliation: Department of Fiber and Polymer Technology, Royal Institute of Technology, KTH, Teknikringen 56-58, SE-100 44 Stockholm, Sweden.
authors.forename: Yujia
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Department of Fiber and Polymer Technology, Royal Institute of Technology, KTH, Teknikringen 56-58, SE-100 44 Stockholm, Sweden; Wallenberg Wood Science Center, The Royal Institute of Technology and Chalmers University of Technology, SE-100 44 Stockholm, Sweden.
authors.forename: Jiebing
authors.initials: J
authors.lastname: Li
authors.affiliation: Department of Fiber and Polymer Technology, Royal Institute of Technology, KTH, Teknikringen 56-58, SE-100 44 Stockholm, Sweden; Wallenberg Wood Science Center, The Royal Institute of Technology and Chalmers University of Technology, SE-100 44 Stockholm, Sweden.
authors.forename: Mikael E
authors.initials: ME
authors.lastname: Lindström
authors.affiliation: Department of Fiber and Polymer Technology, Royal Institute of Technology, KTH, Teknikringen 56-58, SE-100 44 Stockholm, Sweden; Department of Food Chemistry, Technische Universität Braunschweig, Schleinitzstraße 20, D-38106 Braunschweig, Germany. Electronic address: p.mischnick@tu-braunschweig.de.
authors.forename: Petra
authors.initials: P
authors.lastname: Mischnick
mesh_terms: D008351:Mannans / Q000737:chemistry*; D008358:Mannose / Q000737:chemistry; D008745:Methylation; D010100:Oxygen / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D008351:Mannans; D012997:Solvents; C022901:(1-6)-alpha-glucomannan; D008358:Mannose; D010100:Oxygen
keywords: Partial methylation; Relative reactivities; Solvent effect; Spruce glucomannan