Of D-xylose

Refer to the Fischer projection of d (+) xylose in Figure 25 2 (Section 25 4) and give struc tural formulas for... 

Xyhtol is synthesized by reduction of D-xylose catalyticahy (40), electrolyticahy (41), and by sodium amalgam (42). D-Xylose is obtained by hydrolysis of xylan and other hemiceUulosic substances obtained from such sources as wood, com cobs (43), almond shells, hazelnuts, or oHve waste (44). Isolation of xylose is not necessary xyhtol results from hydrogenation of the solution obtained by acid hydrolysis of cottonseed hulls (45). 

Carrel, H.L., et al. X-ray structure of D-xylose isomerase from Streptomyces nibiginosus at 4 A resolution. 

Enzymic syntheses are considered next. Xylitol is a substrate for sheep-liver L-iditol dehydrogenase, a NAD-linked enzyme. 1-Deoxy-D-xylitol, prepared by Raney nickel reduction of D-xylose diethyl dithioacetal in a 27% overall yield from D-xylose, was also reported31 to be a substrate, although with a higher Km and lower Vmax. The product was assumed to be l-deoxy-D-f/ireo-pentulose because of the appearance of a yellowish fluorescent spot when a chromatogram was sprayed with acidic 3,5-aminobenzoic acid, resembling that formed from 1 -deoxyfructose. There was no more-rigorous characterization. 

The configuration of the penturonic acid LV was not however conclusively proved to be that of D-xylose and consequently this experiment is not determinative in regard to the configuration, as distinct from constitution, of 3,6-anhydro-D-glucose. 

S. Lima, M. Pillinger, and A. A. Valente, Dehydration of D-xylose into furfural catalysed by solid acids derived from the layered zeolite Nu-6(1), Catal. Com-mun., 9 (2008) 2144-2148. 

Fischer and Armstrong17 suggested the application of the benzaldehyde method for the decomposition of D-xylose phenylosazone, and a slight modification of this method180 was later employed in obtaining D-xylosone for the synthesis of D-isoascorbic acid,12-181-182 and L-xylosone182 for the... 

Fermentation of D-xylose by Escherichia coli at pH 5.5 (in bicarbonate buffer) gives more than one mole of lactic acid per mole because of fixation of carbon dioxide by the two-carbon fragment,198(a> an observation that may have an important bearing on theories of photosynthetic fixation of carbon dioxide.198[Pg.223]

The dithioacetal of D-xylose generates a primary tosylate 48 that can undergo a 1,5-elimination under basic conditions giving the corresponding 2,5-anhydro-pentose dithioacetal 49. Hydrolysis of the dithioacetal and NaBH4 reduction furnishes the corresponding 2,5-anhydropentitol 5083 (Scheme 19). 

The D-xyloketose was prepared by autoclaving at 120° a 10% solution of D-xylose for 45 minutes in the presence of 0.2 M phosphate buffer, pH 6.8. Most of the unreacted D-xylose was separated by crystallization, leaving a sirup consisting chiefly of D-xyloketose. 

Relatively pure xylan isolated from the holocellulose of aspen (Populus) wood is said to contain 85% of xylose residues.78 One of the characteristic properties of xylan is its ease of hydrolysis. Because it hydrolyzes much more readily than cellulose, mild acid treatment may be employed to bring about preferential hydrolysis of xylan from plant material. Xylose is ordinarily prepared in the laboratory by direct sulfuric acid hydrolysis of the native xylan in ground corn cobs.74 Hydrolysis in hydrochloric acid proceeds rapidly, but decomposition to furfural also occurs to some extent.76 A commercial method for the production of D-xylose from cottonseed hulls76 and straw77 and from corn cobs17 78 has been described. 

Pure xylan is not employed in industry. but crude xylan or pentosans are of industrial importance. Xylan has been proposed as a textile size but is not employed as yet for this purpose.130 Perhaps the largest use of pentosans is in their conversion to furfural, which has many applications and serves as the source of other furan derivatives. At the present time, large quantities of furfural are used in the extractive purification of petroleum products, and recently a large plant has been constructed to convert furfural by a series of reactions to adipic acid and hexamethylene-diamine, basic ingredients in the synthesis of nylon. In commercial furfural manufacture, rough ground corn cobs are subjected to steam distillation in the presence of hydrochloric acid. As mentioned above, direct preferential hydrolysis of the pentosan in cobs or other pentosan-bearing products could be used for the commercial manufacture of D-xylose. 

D-xylopyranose units. This structural concept is substantiated by estimation of the formic acid obtained when the xylan is oxidized by periodate ions. On hydrolysis of the fully oxidized xylan there is obtained a small amount of D-xylose which presumably occupied the branch points in the polysaccharide and consequently was protected from periodate oxidation by possessing no adjacent free hydroxyl groups. 

A. Tri-O-acetyl-D-xylono-l,4-lactone 2. a) Bromine oxidation. A 250-mL, threenecked, round-bottomed reaction flask equipped with a magnetic stirrer, thermometer, and an addition funnel is charged with 30.0 g (0.20 mol) of D-xylose and 80 mL of water. After the clear aqueous solution is cooled with an ice-water bath, 34.0 g 0.23 mol) of potassium carbonate is added in portions, keeping the temperature below 20°C. After the mixture is cooled to below 5°C, 12 mL (0.22 mol) of bromine is added dropwise over 90 min, keeping the temperature of the reaction mixture below 10°C (Note 1). The orange solution is stirred at that temperature for 30 min, then at room temperature overnight. The reaction is quenched by careful addition of 88% formic acid (2.5 mL) to afford a colorless solution (Note 2). The solution is concentrated at 50°C on a rotary evaporator and 20 mL of acetic acid is added. The mixture is Concentrated again at 50°C to remove any residual water (Note 3). 

Singh G, Chaudry KL, Chudler LC, Oneill PJ and Chaudry IH (1991) Measurement of D-Xylose Gut Absorptive-Capacity in Conscious Rats. Am J Physiol 261 pp R1313-R1320. 

Note Data represent the mean S.E. (n = 3). MW, molecular weight P0/w, octanol-to-water partition coefficient CLapp, apparent membrane permeability clearance SI, midgut area of the small intestine NA, not available or applicable. Absorption was evaluated in our laboratory using the closed loop of the rat intestine in situ (urethane anesthesia, 1.125 g/4.5 ml/kg, i.p.) in 60 min for riboflavin and L-camitine and 30 min for the others. For those that are transported by carriers in part (riboflavin and glycerol in both colon and SI, and L-carnitine, 5-fluorouracil, and cephradine in SI), absorption was evaluated at higher concentrations where the contribution of carrier-mediated transport is negligible. Values of P0/w were obtained from a report by Leo et al. [30] except for that of D-xylose, which was determined in our laboratory. a Data by single-pass perfusion experiments. b Unpublished data from our laboratory. 

P-D-Xylop5Tanose Scheme 2.13 Acquisition of D-xylose from xylans in woody materials. 

Diehl, V. Cuny, E. Lichtenthaler, F. W. Conversion of D-xylose into hydrophilically functionalized pyrazoles. Heterocycles, 1998, 48, 1193-1201. 

Although small proportions of other products are formed when D-xylose is exposed to rather high acid concentrations, arabinose, lyxose, and ribose form considerably more of alternative products (generally reductic acid) than of 2-furaldehyde under these conditions. Reductic acid (2,3-dihydroxy-2-cyclopenten-l-one, 47) has been detected as a product after acid exposure of D-xylose or its major dehydration product, 2-furalde-hyde. Further work performed with D-[l- C]xylose and [a- C]2-fural-dehyde showed that reductic acid having identical label distribution was obtained from both starting materials. This indicated that a common primary source was involved, probably 2-furaldehyde, as it is readily formed from D-xylose under acidic conditions. 

D-[l- C]Xylose was subjected to boiling in 4 M aqueous sodium hydroxide. The resulting mixture contained 2,4-dihydroxybutanoic acid, lactic acid, and D-a,j8-xylometasaccharinic acid. The almost uniform distribution of the C label among the carbon atoms of 2,4-dihydroxybutanoic acid indicated that this acid is probably formed by the recombination of completely isomerized, two-carbon fragments. Fragmentation of D-xylose occurred mainly at one of the central bonds, C-2-C-3 or C-3-C-4. 

A solution of D-xylose in boiling 0.63 M sodium hydroxide gave interesting results compared with those from a similar reaction conducted... 

Essentially pure xylo-oligosaccharides in 0.5% concentration were incubated with Xylanase II at 50°C in 0.05M sodium citrate buffer, pH 5.0 for X5 through X7 and pH 6.05 for Xg and X9, and samples after varying periods were submitted to HPLC. With X5, X2 and X3 were the major products, with smaller amounts of X4 and D-xylose produced. Incubation of gave X3 as the major product, followed by X4 and X2. With prolonged incubation the X4 was consumed to form X2 and smaller amounts of D-xylose and X3. Hydrolysis of X7 led to the formation of X4 and X3, with smaller amounts of X5, and X2, and a very small amount of Xg. Tlie course... 

At the same time, in this laboratory, we detected the dimethyl acetals of D-xylose and D-glucose by chromatographic resolutions of the products of methanolsis of labelled free sugcirs, and we have, likewise, concluded that they are not primary products but are formed either concurrently with the furanosides or, more probably, from them 8). Fig. 3 illustrates the variations of the main components of the reaction of D-xylose as determined by radiochemical counting of the chromato-graphically resolved components (the pyranosides were vmresolved under the conditions used), and in Fig. 4 the concentration of the acetal is... 

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