Xylose, reduction

For D-xylose-grown yeast, extracts catalyze oxidation of NADPH with D-xylose.828 Purified enzyme catalyzes xylitol D-xylose, reduction of D-glyceraldehyde, D-erythrose, L-arabinose, D-ribose, D-galactose, and D-glucose, and oxidation of galactitol, L-arabinitol, D-glucitol, ribitol, erythritol, and glycerol.818-811... 

S. cerevisiae has been transformed with the P. stipitis genes XYLl and XYL2 coding for XR and XDH, respectively [46,47,141 ]. The choice of P. stipitis as the donor organism was based on its capability to utilize NADH in the xylose reduction step. Attempts to ferment xylose to ethanol with these recombinant S. cerevisiae producing XR/XDH have resulted in low ethanol yield and considerable xylitol by-product formation. This has been ascribed to the unfavorable thermodynamic properties of the reactions [140] and the fact that the first reaction preferably consumes NADPH, whereas the second reaction exclusively produces NADH. When less NADH is consumed in the XR reaction, then less NAD" is available for the XDH reaction. If the amount of NAD+ is insufficient, xylitol is produced and excreted [133]. 

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). 

Reduction. Mono- and oligosaccharides can be reduced to polyols (polyhydroxy alcohols) termed alditols (glycitols) (1) (see Sugar alcohols). Common examples of compounds in this class ate D-glucitol (sorbitol) [50-70-4] made by reduction of D-glucose and xyhtol [87-99-0] made from D-xylose. Glycerol [56-87-5] is also an alditol. Reduction of D-fmctose produces a mixture of D-glucitol and D-mannitol [69-65-8],... 

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 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). 

Alternatively, pyranosyl radicals can be generated through the reduction of 3,4,6-tri-O-benzyl glucal epoxide with Cp2TiCl2 and manganese metal.136 With the conformationally restricted 1-phenylseleno-D-xylose derivatives 151 and 152 (4Ci conformation) their reaction with Bu3SnCH2 CH=CH2 in the presence of AIBN (Scheme 51) affords the corresponding a-C-pyranosyl derivatives (153) preferentially.137... 

Aside from the multifaceted chemical conversions, there are sources to develop into industrially viable microbial conversions. 1,2,4-Butanetriol, for example, used as an intermediate chemical for alkyd resins and rocket fuels, is currently prepared commercially from malic acid by high-pressure hydrogenation or hydride reduction of its methyl ester. In a novel environmentally benign approach to this chemical, wood-derived D-xylose is microbially oxidized to D-xylonic acid, followed by a multistep conversion to the product effected by a biocatalyst specially engineered by inserting Pseudomonas putida plasmids into E. coli ... 

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. 

Numerous investigations concerning the mechanism of these reductive fermentations have been reported. Basically it is important that pentoses (u-arabinose and D-xylose) yield the same products of fermenta-... 

The effect of the base used for the condensation of xylose (7) with pentane-2,4-dione (1) was carefully examined. The best results were obtained using NaOH since 54 was obtained in 45 min at 50°C in 97% yield. Reduction of 54 gave 60, an activator of glycosaminoglycans biosynthesis, launched on the market in cosmetic skincare products as Pro-Xylane by L Oreal in 2006 [116, 117] (Fig. 10). 

It was our intention to synthesize 52 by the reductive amination of the free sugar 53 in actual fact, any such successful cyclization of 53 would yield the enantiomer of 52. In the explorative stages it was cheaper to work with 53, the precursor of which is o-xylose. 

Furaldehyde has also been reported as a source of reductic acid,189 and this has been verified.54 The yields are, however, less than 0.5%. D-Xylose also affords small proportions of reductic acid,54,94 but the yields are, apparently, much higher from the other pentoses when they are treated in concentrated sulfuric acid.52... 

A similar result was found with D-xylose-J-14C and 2-furaldehyde-a-14C. When treated under identical conditions, each formed reductic-14C acid having 60% of the activity at C-2 and 40% at C-l. 

D-Xylose was found to yield 2-furaldehyde almost exclusively, but D-lyxose, D-ribose, and L-arabinose produce another, as yet unidentified, compound absorbing at 289 nm, which is the maximum absorption wavelength for reductic acid. D-Glucose, D-fructose, and sucrose give almost identical yields (—85%) of 5-(hydroxymethyl)-2-fural-dehyde, but D-galactose and D-mannose give much lower yields thereof. 

Another, more efficient route43 to DL-lyxose and DL-xylose consisted in reduction of 24 with lithium aluminum hydride to the trans-alkene 29a, cis-hydroxylation of 29b, and acid hydrolysis of the products obtained (30 and 31, respectively). In an alternative approach to DL-ri-bose and DL-arabinose, l-ethoxy-l-penten-3-yn-5-ol (32) was used24 as the substrate. Reduction of 32a with lithium aluminum hydride afL... 

DL-Dihydrostreptose and its ribo isomer were similarly obtained. Birch reduction of 2-methyl-3-furoic acid, followed by addition of methanol, bromination, and dehydrobromination, gave 402 as a mixture of the isomers. Hydroxylation of 402 with osmium tetraoxide-so-dium chlorate, and subsequent treatment with acetone-sulfuric acid afforded three isomeric acetals (403-405). The structures of these compounds were assigned on the basis of their H-n.m.r. spectra. In addition, the relationship between 403 and 404 was established by hydrolysis and reglycosidation. The methyl esters 403-405 were quantitatively reduced to the corresponding alcohols. The mixture of alcohols obtained from 403 and 404 was converted into crystalline 5-deoxy-3-C-(hydroxymethyl)-l,2-0-isopropylidene-a-DL-ribofuran-ose (406), which was compared directly with a sample prepared from D-xylose. Methyl 5-deox y-3-C-(hydroxy methyl)-2,3-O-isopropy lidene-/3-DL-lyxofuranoside (407), obtained by reduction of 405 with lithium aluminum hydride, was hydrolyzed with dilute hydrochloric acid, to give a,/3-DL-dihydrostreptose.2,ifi... 

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