Microscopic understanding of chemical reactions occurring in lignocellulose constituents and their related compounds
Lignocellulose is composed mainly of cellulose, hemicelluloses, and lignin. Cellulose and hemicelluloses, as they are called wood polysaccharides, are carbohydrate polymers consisting of β-1,4-linked D-glucose, D-mannose, D-xylose monomer units. On the other hand, lignin is an aromatic polymer, where the C6-C3 units are linked via ether and C-C bonds. Considering the huge amount of these organic compounds piled on the earth, detailed understanding of their chemical properties such as chemical structure, reactivity, stability, etc, is highly important to make deep insights into various natural phenomena. From the view point of applied science mechanical understanding of various lignocellulose-related chemical reactions will of great help when their chemical modification and conversion into chemicals are considered. Here we introduce several research topics in our laboratory.
Chemical glycosylation is usually accounted for as a nucleophilic substitution of a glycosyl donor that carries a leaving group X at its 1-position with an acceptor (mostly alcohol) to produce a glycosidic linkage, as shown in Figure 3. This glycosylation is one of the most important reactions in the field of carbohydrate chemistry and plays pivotal roles in synthesis of various carbohydrate-based functional molecules. As easily understood from the fact that wood is composed of polysaccharides that have many glycosidic linkages, wood chemistry is also closely related to chemical glycosylation.
D-Xylose is a monosaccharide that can be readily produced from woody materials, especially from hardwoods. Expanding the use of xylose thus leads to promotion of wood use in human society. To this end we investigate fundamental mechanisms of chemical glycosylation of xylosyl donors, which underlies production of various xylose-based useful molecules.
A major issue to be handled in chemical xylosylation (also in general chemical glycosylation) is controlling of the stereochemical selectivity (α/β-selectivity). As shown in Figure 3, glycosylation gives a product mixture consisting of α- and β-isomers, but the target product is usually only one of the isomers. The production of the undesired isomer therefore not only leads to decease in the yield of the target glycoside, but also makes succeeding purification process demanding. Why is such controlling of the stereoselectivity difficult? One of the major reasons is complexity of the mechanisms underlying glycosylation. As show in Figure 3B, a glycosyl donor usually forms an equilibrium mixture consisting of α/β covalent donors (covalent intermediates, CIs) and various ion pairs such as contact ion pairs (CIPs) and solvent separated ion pairs (SSIPs). These nucleophiles, CIs, CIPs, and SSIPs, undergo SN1 and SN2-type reactions, which results in the formation of both α- and β-products. The controlling of the stereoselectivity eventually requires understanding of the properties of these species, but elucidation of such properties, especially detailed structures and reactivity of the ion pairs are not usually easy, because of their extremely short life time in reaction solution.
Our research group is making basic investigations on molecular mechanisms
of chemical glycosylation, especially xylosylation, combining synthetic
and computational approaches. Quantum chemical calculations facilitate
evaluation of very unstable intermediates and even transition states that
cannot be detected in most of experimental approaches. Our research group
has clarified the energies and the structures of the ion pairs and the
transition states connecting them to the covalent intermediates. More detailed
information is expected to be obtained by further research that follows
these computational results from the experimental side.
● Hosoya, T.; Kosma, P.; Rosenau, T. Effects of a 4,6-diacetal protecting group on the stability of ion pairs from D-glucopyranosyl and D-mannopyranosyl triflates. Carbohydr. Res. 2015, 411, 64-69.
● Hosoya, T.; Kosma, P.; Rosenau, T. Contact ion pairs and solvent-separated ion pairs from D-mannopyranosyl and D-glucopyranosyl triflates. Carbohydr. Res. 2015, 401, 127-131.
● Hosoya, T.; Takano, T.; Kosma, P.; Rosenau, T. Theoretical foundation for the presence of oxacarbenium ions in glycoside synthesis. J. Org. Chem. 2014, 79, 7889-7894.