Xylitol, oxidation

The reactivity of xylitol in the polyol dehydrogenase reactions has been extensively studied for its relations to pentosuria. Xylitol was found to be oxidized in two ways (a) to L-xylulose by a highly specific NADP-requiring dehydrogenase and (b) to D-xylulose by a NAD-linked enzyme with lesser substrate specificity ... 

The enantiomorph, also a syrup, was synthesized from 3,6-anhydro-4,5-O-isopropylidene-D-mannitol18 by periodate oxidation to the corresponding D-arabinose derivative, followed by reduction in the presence of Raney nickel.17 2,5-Anhydro-D-xylitol, 2,5-anhydro-D-ribitol, and 2,5-anhydro-D-lyxitol (1,4-anhydro-D-arabinitol)19,20 were... 

Absorption of mannitol (209), sorbitol (210), and xylitol (4) from the intestinal tract is relatively slow, compared to that of glucose. In humans, approximately 65% of orally administered mannitol is absorbed in the dose range of 40—100 g. About one-third of the absorbed mannitol is excreted in the urine. The remainder is oxidized to carbon dioxide (211). 

Xylitol 1 1 1.6 in water Oxidizing agents Laxative decreases dental plaque and decay cooling sensation... 

Ascorbic acid is readily oxidized to dehydro-ascorbic acid (Box 18-D Fig. 20-2, step e), which may be hydrolyzed to L-bisoxogulonic acid (step/). The latter, after decarboxylation and reduction, is converted to L-xylulose (steps g and h), a compound that can also be formed by a standard oxidation and decarboxylation sequence on L-gulonic acid (step z). Reduction of xylulose to xylitol and oxidation of the latter with NAD+ (steps j and k) produces D-xylulose, which can... 

We finish this Section with enzymic conversions that are difficult to classify elsewhere Takasweet, a commercial variety of immobilized glucose-iso-merase, converts 6-O-methyl-D-fructose and 6-deoxy-D-fructose into the gluco isomers in not very satisfactory yield.34 A mixture of catalase (75 U/mmol) and glucose oxidase (80 U/mmol) oxidizes xylitol to L-xylose in 50% yield, on the 100- mol scale.106 The enzyme cyclodextrin a-(l—>4)-glucosyltransferase (1000 U immobilized on silica gel-glutaraldehyde) allows107 preparation of cyclomaltohexaose (0.3 g), cyclomaltoheptaose... 

Incubation of D-xylose with an aqueous solution of bovine lens protein gave both xylitol and xylonic acid. Studies of the reaction under a variety of conditions suggest that both the reduction and oxidation reactions are protein (possibly enzyme) catalyzed and appear to be unique to lens... 

Methylene-xylitol has been obtained also by periodate oxidation of 3,5-methylene-gluco-j/uZo-heptitol (XVI) and reduction of the 2,4-methylene-xyfo-trihydroxyglutardialdehyde thus produced.68 Benzyli-denation of 2,4-methylene-xylitol affords a mixed acetal, 2,4-methylene-3,5-benzylidene-D,L-xylitol, the structure of which was established by conversion of its tosyl ester into 1-desoxy-compounds.28... 

Detection of side products generated by Rubisco is accomplished by various means. XuBP (30) and pyruvate (36) are both conveniently detected spectrophotometrically by coupling to NADH oxidation with appropriate enzymes. Alternatively, our chromatographic procedure (27) gives a complete profile of all RuBP-derived products. Resolution of these compounds is enhanced by inclusion of 10 mM sodium borate, which complexes vic-diols, in elution buffers. Since our initial report of the separation of borohydride-reduced misprotonation products (27), we have observed that borate also effects complete separation of unreduced RuBP and XuBP (37). Thus, the analysis is simplified by circumventing the necessity to deduce the amounts of misprotonation-derived bisphosphate based on ratios of ribitol-, arabinitol-, and xylitol-1,5-bisphosphates. 

Xylitol has been derived from the product of photo-oxidation of cyclopentadiene [204], (Z)-(4RS)-4,5-epoxypent-2-enal (Scheme 13.105). Chemoselective reduction of the formyl group gives cis-hydroxyepoxypentene 448, which is directly acetylated into 449. Treatment of 449 with tetrabutylammonium acetate in AC2O opens the epoxide with formation of 450. De-O-acetylation gives 451, the epoxidation of which with p-nitroperbenzoic acid generates a 3 7 mixture of epoxides 452 and 453, isolated as peracetates. The major epoxide 453 is hydrolyzed into xylitol via the orthoester 454. 

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