Xylose 5,5-dimethyl

The action of 1% methanolic hydrogen chloride on 1,2-O-iso-propylidene -3,5-di - O - p-tolylsulfonyl-D-xylofuranose (65) leads,81 after boiling for three hours under reflux, to 2,5-anhydro-3-0-p-tolylsulfonyl-D-xylose dimethyl acetal (66). The structure of 66 was demonstrated by conversion into the disulfonate 67 this was prepared independently from a 2,5-anhydro-D-xylose dialkyl dithio-acetal72 (48) by p-toluenesulfonylation followed by exchange of the acetal group in methanol in the presence of mercury salts. 

Tri-O-benzoyl-D-xylose Dimethyl Acetal, M. L. Wolfrom and Walter von Bebenburg,/. Amer. Chem. Soc., 81, 5705-5706 (1959). 

A soln. of 3-azido-3-deoxy-l,2-0-isopropylidene-5-0-tosyl-a-D-xylofuranose in methanolic 3%-HCl refluxed 10 hrs. -> crude 2,5-anhydro-3-azido-3-deoxy-D-xylose dimethyl acetal. Y 94%. H. Ohrui, H. Kuzuhara, and S. Emoto, Agr. Biol. Chem. 34, 375 (1970). 

Di-Me acetal, 3-tosyl 2,5-Anhydro-3-0-tosyl-D-xylose dimethyl acetal Mp 76-78°. [a]g +53.7 (c, 1.9 in CHCI3). 

The synthesis and some reactions of 4,5-epithio-2,3-di-0-methanesulphonyl-L-xylose dimethyl acetal (41) are summarised in Scheme 9, and Scheme 10 shows the preparation of 3-0-acetyl-5,6-epithio-l,2-0-isopropylidene-a-L-galactofuranose (43) from the D-altrose compound (42) - itself available in ten steps from D-glucose - and its conversion to 5-thio-L-fucose (44), an inhibitor of a-fucosidases. ... 

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... 

The mode of union of these units in the complex methylated tragacanthic acid is obvious from their identification but the order of combination is still obsciu%. The 2,3,4-trimethyl-D-xylose (XXXII) and 2,3,4-trimethyl-L-fucose (XLII) must arise from terminal residues in the complex, while the 3,4-dimethyl-D-xylose (XLIII) must be involved in union... 

Apparently related to the two preceding reactions is the action of 2% methanolic hydrogen chloride on l,2-0-isopropylidene-5-seleno-a-D-xylofuranose (72) (obtained from the benzylseleno derivative, 71), which affords83 a mixture of 2,5-anhydro-5-seleno-D-xylose (and -D-lyxose) dimethyl acetal (73). Similarly, the action of the same reagent... 

Anhydro-D-xylose 3,4-di-O-acetyl-, dimethyl acetal amorph. amorph. 

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-,... 

Fio. 2.—Curves for Light-absorption in the Disohe Test (A) 2-Desoxy-D-xylose (B) D-Xylal (C) w-Methoxylevulinaldehyde Dimethyl Acetal. 

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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