The Methyl Ethers of D-Xylose

With the exception of 4-methylxylose all the methyl ethers that are derivable theoretically from D-xylopyranose or D-xylofuranose are known. From the products of hydrolysis of methylated polysaccharides 2-methyl-, 3-methyl-, 2,3-dimethyl-, 2,4-dimethyl-, 3,4-dimethyl- and 2,3,4-tri-methyl-D-xylose have been separated. No D-xylofuranose derivatives have been isolated from a natural source. Unknown at the present time are the 4-methyl-, 4,5-dimethyl-, 2,4,5- and 3,4,5-trimethyl- and 

The synthesis of this sugar (II) has been achieved10 from methyl 

No synthesis of 4-methyl-D-xylose has been reported. The osazone has been obtained from 2,4-dimethyl-D-xylose.14 

5-Methyl-D-xylose (VII) was synthesised by Levene and Raymond13 from l,2-isopropylidene-5-tosyl-D-xylofuranose by conversion into the corresponding 3,5-anhydride (VI) followed by heating with sodium 

Originally obtained as a sirup, the carefully purified 2,3-dimethyl-D-xylose prepared from esparto xylan4(b) and pear cell-wall xylan6 has now been obtained crystalline.6 16  

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The effect of adverse interactions is shown even more strongly in the case of the month and di-methyl ethers of D-arabinose and D-xylose (see... 

Apart from their intrinsic interest the methyl ethers of certain of the pentoses, especially those of L-arabinose and D-xylose, are of great importance in structural studies of the naturally occurring pentosans, and of plant gums and mucilages.1 This also applies to the methyl ethers of L-rhamnose and L-fucose. 

D-Xylose, which is one of the most abundant sugars in plant polysaccharides, is a rare component of bacterial polysaccharides. It is found in the LPS of Type 1 Neisseria gonorrhoeae strain" GC 6. L-Xylose and its 3-methyI ether are components of the LPS of Pseudomonas maltophila strain NCTC 10257, and are j -pyranosidic. The d- and L-sugars, and different methyl ethers of these, have also been found in the LPS of some photosynthetic bacteria."... 

Analyses of the hemicelluloses of woods indicate the presence of a high proportion of D-xylose residues in association with those of a methyl ether of a hexuronic acid.64 The first evidence for the chemical structure of such wood xylans came from the isolation of xylobiose, xylotriose,65 and the aldobiouronic acid 0-(4-0-methyl-a-D-glucosyluronic)-(l —> 2)-D-xylose acid (IX)22 from partial hydrolyzates of aspen wood. The role of such oligosaccharide fragments in wood xylans became more apparent as the result of a study of a xylan from European beechwood,26 in which it was shown... 

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. ... 

Although aqueous solutions of D-xylose and o-glucose contain insignificant proportions of furanose forms, n.m.r. spectroscopy has indicated that 15% of the j8-furanoses are present in the tautomeric equilibria of the corresponding 3-0-methyl ethers these equilibria are not governed by stereochemical factors... 

Xylans are the major hemicelluloses of many plant materials, where they often contribute to the rigidity of plant cell walls. Most xylans are heteropolysaccharides with a homopolymeric backbone chain of 1,4-linked j8-D-xylo-pyranose units. The degree and t pe of substitution of the backbone is dependent on the plant origin of a xylan. In addition to xylose, xylans may contain L-arabinose, D-glucuronic acid or its 4-O-methyl ether, and acetic, p-coumaric, and ferulic acids. 

As with most of the synthetic polysaccharides, complete methylation189 of the polyxylose was difficult. The fully methylated polymer was soluble in petroleum ether (30-60°) containing 6% of chloroform. Hydrolysis of the methylated xylan gave tri-, di-, and mono-O-methyl-D-xyloses, together with D-xylose, in the molar ratio of 31 33 19 5. The tri-O-methyl-D-xylose fraction contained 38% of 2,3,5-tri-O-methyl-D-xylose, the remainder being the 2,3,4-trimethyl ether. The dimethyl ethers included the 2,5-,... 

D-xylose on hydrolysis with dilute nitric acid. Percival and Chanda7 isolated a xylan from the same plant. They found that the methylated xylan produced on hydrolysis 2-methyl-D-xylose, 2,3-dimethyl-D-xylose, 2,4-dimethyl-D-xylose and 2,3,4-trimethyl-D-xylose. From this and from the results of periodate oxidation, Percival and Chanda considered that the polysaccharide contains 1 — 3 and 1 —> 4 linkages with a non-reducing endgroup (yielding the 2,3,4-trimethyl-D-xylose) for every 20-21 D-xylose units. They considered that this xylan was not a mixture of 1 —> 3 and 1 —> 4 linked polysaccharides because careful fractionation of its diacetate and dimethyl ether failed to establish any polymer heterogeneity. Barry, Dillon, Hawkins and O Colla74 confirmed the conclusion of Percival and Chanda. 

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