Xylose bases

W. S. Fyvie, M. Morton, and M. W. Peczuh, Synthesis of 2-iodo-2-deoxy septanosides from a D-xylose-based oxepine Intramolecular cyclization in the absence of a glycosyl acceptor, Carbohydr. Res., 339 (2004) 2363-2370. 

Glucuronoxylan Even if hemicelluloses in various hardwood species differ from each other both quantitatively and qualitatively, the major component is an 0-acetyl-4-0-methylglucuronoi3-D-xylan, sometimes called glucuronoxylan. Often the xylose-based hemicelluloses in both softwoods and hardwoods are termed simply xylans. 

Xylose-based chemicals analogous to some of the glucose derivatives discussed in the next section are commercially available. An example is xylitol, the pentahydric alcohol that is manufactured by hydrogenation of the aldehyde group in xylose. One of the unique reactions of the pentoses known since the nineteenth century is cyclodehydration to yield furfural. Xylose, for example, is dehydrated by acid treatment through one or two intermediates to form the furan-substituted aldehyde in quantitative yield, or 64.0 wt % of the xylose... 

While all of the natural products just referred to were heteropolysaccharides (although several of the hemicelluloses were largely xylose-based), some attention was paid to compounds composed of single hexoses. A minor amount of research was conducted on cellulose and starch chemistry, and callose, an unusual />-(T 3)-linked glucan, found in small amounts in the sieve plates of the phloem of the grape vine, was characterized. Two papers were published on ivory nut mannan and several on fructose-based polymers. 

Xylose fermentation was not tested, but with our thermophilic anaerobic bacterium Thermoanaerobacter BGILI (16), ethanol production could theoretically be increased by more than 39% by including the xylose. Based on the results from the current study, 285 1 ethanol per ton DM could be possible from unwashed or undetoxified wet-exploded wheat straw. Pilot scale studies are now carried out at the Maxifuels pilot plant (DTU, Denmark) to optimize the pretreatment method at higher DM up to 30%. 

Hemicellulose is one of the major components of lignocellulose. Depending on the nature of the raw material, the hemicellulose fraction contains varying levels of xylose-based hemicelluloses, xylans (Table 1). The xylan content is generally high in hardwood (wood from deciduous trees) and in agricultural residues, and somewhat lower in softwood (wood from coniferous trees). 

The n.m.r. spectra of a set of xylose-based oligosaccharides are referred to in Chapter 20. 

Glucose-linked 1,2,3-triazolium ionic liquid 122 was prepared by copper(I)-catalyzed regioselective cycloaddition of a glucose azide with a glucose alkyne followed by quaternization with methyl iodide (Scheme 6.32). This IL was used as reusable ligand and solvent in copper(I)-catalyzed amination of aryl hahdes with aqueous ammonia [71]. Similar chemistry, that is, chcking of P-propargylxyloside with phenyl azide, followed by acetate removal and methylation was employed for the syntheses of D-xylose-based ionic hquids [72]. 

Figure 8.9 External mass transfer resistance—xylose hydrogenation and isomerization to xylitol and by-products on sponge Ni (based on the results of Mikkola et al. [22]).

Traditional fermentation processes can only make use of glucose, leaving other sugars present, notably xylose, untouched. This both increases the waste element and lowers the competitiveness of the process compared to petrochemical-based ethanol. 

The nomenclature adopted in the designation of the analogs of L-ascorbic acid is based for convenience upon the name of the sugar from which the osone was prepared. For example, D-xylose (XVIII)... 

A new synthetic route for functionalized polyhydroxyalkyl-pyrimidines starting from unprotected aldoses and based on montmorillonite K-10 catalysis and solvent-free microwave irradiation conditions, has been reported by Yadav et al,m Thus, reaction of D-glucose and D-xylose with semicarbazide or thiosemicarbazide (186) in the presence of montmorillonite K-10, under microwave irradiation, proceeded via domino cycloisomerization, dehydrazination, and dehydration of the intermediate semi- or thiosemicarbazones (187) to afford l,3-oxazin-2-ones or l,3-oxazine-2-thiones (188) in one single step and in yields between 79% and 85% (Scheme 34). Other mineral catalysts tested, such as silica gel and basic alumina, were far less effective for this transformation and only silica gel was active at all, giving low yields (15-28%) of compounds 188a-d. The l,3-oxazin-2-ones(thiones) thus synthesized were subsequently converted into the target pyrimidines by reaction with aromatic... 

An interesting approach to both enantiomers of biologically active cyclo-phellitol, based on the latent symmetry concept, from the same common starting material D-xylose was proposed by Kireev et. al. Functionalization of this monosaccharide at the C-l provided compound 64, while similar processes initiated from the end (C5) afforded the ent-64 (Fig. 23).36 Proper functionalization of these intermediates led to both enantiomers of cyclophellitol. 

The major directions that broad-scale exploratory research toward carbohydrate-based nonfood products will have to take, are—as far as conceivable today— outlined in the following for the four key sugars of biomass the royal carbohydrate sucrose, D-glucose, D-fructose, and D-xylose. 

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