Xylose selective methylation

In other work,540 it was found that, with one-quarter of an equivalent of sodium borohydride, 96 was regio- and stereo-selectively reduced in good yield to a >4 1 mixture of 28 and 48. This result contrasts with that of earlier work,529 in which it was reported that, on reduction of 96 with an excess of sodium borohydride, followed by a Ruff degradation of the mixture to the pentoses, D-arabinose (104) was the preponderant product, with a smaller proportion of L-xylose (23). This finding suggests that the reduction was stereoselective in the direction opposite to that actually observed in the later work. The methyl ester (115) of 96 has been selectively reduced with sodium borohydride, to afford methyl L-xi/Zo-2-hexulosonate as the major product.540... 

Physical or chemical modification of a substrate may additionally selectively affect transformation or uptake Keil and Kirchman (1992) compared the degradation of Rubisco uniformly labeled with 3H amino acids produced via in vitro translation to Rubisco that was reductively methylated with 3H-methane. Although both Rubisco preparations were hydrolyzed to lower molecular weights at approximately the same rate, little of the methylated protein was assimilated or respired. The presence of one substrate may also inhibit uptake of another, as has been demonstrated for anaerobic rumen bacteria. Transport and metabolism of the monosaccharides xylose and arabinose were strongly reduced in Ruminococcus albus in the presence of cellobiose (a disaccharide of glucose), likely because of repression of pentose utilization in the presence of the disaccharide. Glucose, in contrast, competitively inhibited xylose transport and showed noncompetitive inhibition of arabinose transport, likely because of inactivation of arabinose permease (Thurston et al., 1994). 

Despite the obvious possibilities for disengaging the allylic methyl group of 7 by retro-S reaction, Kim preferred to disjoin this molecule by a (Z)-selective Wittig reaction to obtain aldehyde 9 and ylide 8. Since further analysis of 9 also indicated that it could potentially be reached from the readily available 5-deoxy-D-xylose derivative 10, this was the strategy eventually settled upon. 

In the presence of the H-Mordenite with a Si/Al ratio of 11 as catalyst, a close parallelism is observed with the results obtained for the dehydration of fructose, except that toluene is the co-solvent instead of methyl isobutyl ketone. The transformation of xylose into furfural is easily achieved at 170 °C with a selectivity as high as 90 to 95% as far as the conversion is kept at a low extent, 30 to 40%.[33] At those high temperatures, ion-exchange resins cannot compete with zeolites. 

L-Arabinose reacts differently than o-xylose with acetone and gives a pyrano-side instead of a furanoside. Thus an alternative route to 18 was sought that can be applied both to o-xylose and L-arabinose [59]. It starts with the selective silylation of HO-C(5) [60], then acetonide formation protects alcohols moieties at C(l) and C(2). Subsequent benzylation of HO-C(3), hydrolysis of the silyl ether, and iodination provides 18 from o-xylose and 19 from L-arabinose (Scheme 7). Zinc reduction of 19 generates enal 20, but not the reduction of 18 [61]. Thus 18 and 19 are converted first into their methyl furanosides 22. The latter are reduced with Zn into... 

The selective tritylation of pentoses has been studied, and providing that the reaction is conducted at below 25 C and with only one equivalent of trityl chloride, the 5-0-tritylate is the major product. However, even under these conditions lyxose afforded substantial amounts of diethers, and the 5-monoether was isolated in only 32% yield. The monotritylation of D-xylose has been studied in detail at 50 °C in the presence of either pyridine or AgOAc-HMPT, and four of the monotrityl ethers were isolated as tetra-O-acetyl derivatives. The ratio of 1-, 3-, 4-, and 5-substitution was 36 0 25 100 (in pyridine) and 0 49 57 100 (AgOAc-HMPT). The products were also prepared unequivocally by the tritylation of the appropriate D-xylose tetra-acetate. The selective tritylation of methyl a- and j3-L-rhamnosides has also been studied the a-anomer giving the 3-, 4-, and 2-trityl ethers in 57, 3, and 1% yields respectively. The j3-anomer afforded the 3- and 4-trityl ethers in 34 and 17% yields respectively. Pyridinium perchlorate has been used for the selective de-O-tritylation of otherwise peracetylated methyl gluco- and manno-pyranosides. ... 

Methyl 4,6-O-methylene-o-D-mannopyranoside was the only product isolated from the LiBr-catalised transacetalation of the unprotected methyl glycoside with dimethojqmiethane. Cyclopentylidene derivatives of pentoses have been prepared in moderate yields by treatment of the free sugars with cyclopentanone in the presence of copper (II) sulphate and sulphuric acid. D-Xylose formed the diacetal (11) (also used in Scheme 3 below), whereas from D-ribose the 2,3-monoacetal (12) was obtained. A novel, selective synthesis of (5)-configurated 4,6-pyruvate acetals of methyl D-hexopyranosides is illustrated in Scheme 1. It relies on transacetalation from the dimethyl acetal of 3,4-dimethoxybenzophenone to give, after acetylation, preferentially the intermediate (13) with an axial aryl substituent which, on oxidation, suffers rapid degradation to a carboxylic acid group. ... 

Sindhu et al. (2014) compared the ability of three bacterial strains (Bacillus firmus Nil 0830, B. sphaericus Nil 0838, and Paracoccus denitrificans) to accumulate P(3HB) using a rice straw hydrolysate as carbon source. This mUd acid pretreated hydrolysate contained 23 g of xylose and 4.9 g of acetic acid, together with much lower concentrations of glucose, arabinose, formic acid, furfural, and hydroxyl-methyl-furfural. They selected B. firmus Nil 0830 as this strain produced higher amounts of P(3HB) using the hydrolysate without any detoxifying step (Sindhu et al. 2014). 

The Kishi-Nozaki reaction [71] is the key step in the preparation of methyl C-isomaltoside (Scheme 40) [72]. Coupling of vinyl iodide 150 with the xylose derivative 151, readily available in six steps from L-xylose, afforded the corresponding allylic alcohol with a diastereomeric ratio of 15 1 in favor of 152. Finally, selective hydrogenation of the double bond and oxidation of the primary alcohol led to the C-disaccharide 153, which could be converted to its methyl glycoside 21. A useful dideuterated derivative of C-isomaltoside 154 for solution conformation studies was also accessible from this synthetic approach via the m-deuteration of the alkene intermediate 152 with Pt on AI2O3 and D2. 

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