Tri-O-acetyl-a-D-xylopyranosyl bromide

Figure 7. NMR spectrum of tri-O-acetyl-a-D-xylopyranosyl bromide in chloroformed...

Hydrolysis studies of (5S)-tetra-0-acetyl-5-bromo-/ -D-xylopyranose (28) showed it to be much less susceptible than is tri-O-acetyl-a-D-xylopyranosyl bromide, presumably because the acetoxyl group at C-l inhibits the partici-... 

Further reactions have been performed with 2,4-diethoxypyri-midine as the basic component and 2,3,4-tri-O-acetyl-L-arabino-pyranosyl bromide,7 2,3,4-tri-O-acetyl-a-D-xylopyranosyl bromide,7 2,3,4,6-tetra-O-acetyl-a-D-galactopyranosyl bromide,7 and 2,3,4-tri-O-acetyl-D-ribopyranosyl bromide8 as halogenoses the products... 

The neighboring-group concept of Winstein has been used to explain the difference in reactivity exhibited by anomeric pairs of halides. This suggestion is by no means improbable, and would explain the facts that tri-0-benzoyl-/3-D-ribopyranosyl bromide is 19 times more reactive at 20° in 1 9 dioxane-methanol than is the a anomer, and that the rate for the trans-l3 chloride shows an 85-fold increase over that of the cis-a chloride. However, in view of the properties of the series XXVII to XXX, steric effects from the whole molecule may be equally important, although the disadvantage of this concept is that hindrance as assessed from models is difficult to define precisely. It is of interest to compare the rates of methanolysis of tri-O-benzoyl-a-D-xylopyranosyl bromide and of tri-O-acetyl-a-D-xylopyranosyl bromide [10 fc ° = 51(sec ) in 1 9 dioxane-methanol and = 139(sec 0 in 100% methanol, respectively]. Although the... 

Tri-O-acetyl-a-D-xylopyranosyl bromide (138) and N-tosyl-L-serine methyl ester (139) were condensed in the presence of Drierite and silver oxide, and then the 0-acetyl and methyl ester groups were removed by treatment with sodium hydroxide, and the N-tosyl group by means of sodium in liquid ammonia, to give 140. Synthesis of this compound has also been described by other workers " and the a-o an-omer by Brendel and Davidson. ... 

The reaction of methyl 3-0-benzyl-jJ-D-xylopyranoside with 2,3,4-tri-O-acetyl-a-D-xylopyranosyl bromide in acetonitrile in the presence of mercury(ll) cyanide yields crystalline methyl 3-0-benzyl-2,4-di-0-(2,3,4-tri-0-acetyl-j3-D-xylopyranosyl)- 3-D-xylopyranoside. Debenzylation and deacetylation gave... 

Condensation of tri-O-acetyl-a-D-xylopyranosyl bromide with benzyl 3,4-di-0-benzyl-j3-D-xylopyranoside afforded a synthesis of 2-0- 3-D-xylopyranosyI-D-xylose, and condensation with the three diacetates of methyl /3-D-xylopyrano-side led to the isomeric (1- 2),(1- 3), and (l->4)linked xylobiosides. The main products were the 3-bonded disaccharides, but small proportions of a-compounds were also obtained. A penicillium ]3-D-xylosidase has been used to transfer a xylose unit from one phenyl jS-D-xylopyranoside to another to produce (1" 4) and (1- 3)linked phenyl j3-xylobiosides some corresponding triosides and higher glycosides were also obtained. ... 

A high yield of 1,4-jS-linked product was obtained when methyl 2,3-anhydro-j3-D-ribopyranoside was condensed with 3,4-di-0-acetyl-2-0-benzyl-a-D-xylopyranosyl bromide, and use of tri-O-acetyl-a-D-xylopyranosyl bromide afforded the related product which, on base-catalysed hydrolysis, gave the methyl jS-xylobioside. Several 2-0-glycosyl-D-arabinoses have been prepared by glycosylation of benzyl 3,4-0-isopropylidene-j8-D-arabinopyranoside, and from benzyl 2-0-j3-D-glucopyranosyl-jS-D-arabinoside the aldobiuronic acid was produced. ... 

The stepwise synthesis of methyl 4-(9-[3,4-di-0-( 3-D-xylopyranosyl)-j3-D-xylopyranosyl]-/3-D-xylopyranoside (17), a methyl xylotetraoside related to branched xylans, has been achieved by stereoselective condensation of 2,4-di-(9-acetyl-2-O-benzyl-a-D-xylopyranosyl bromide with methyl 2,3-anhydro- -D-ribopyranoside and reaction of the deacetylated product with 2,3,4-tri-C>-acetyl-a-D-xylopyranosyl bromide. 

Acetylated and Benzoylated Derivatives of Sugars.—1,2,3,5-Tetra-O-acetyl-jS-D-ribofuranose, 3-0-acetyl-jS-L-arabinofuranose 1,2,5-orthobenzoate, (Z)-l-O-acetyl-2,3 4,5-di-0-isopropylidene-D-e/ y/Aw- and -D-/Areo-pent-l-enitol, 2,3,4-tri-O-benzoyl-jS-D-xylopyranosyl chloride, 2,3,4-tri-0-acetyl-j9-D-xyIopyranosyI azide, 1,3,4-tri-0-acetyl-2-deoxy-2-fluoro-a-D-xylopyranose, 1,2,3,4-tetra-O-acetyl-a-D-Iyxopyranose, 2,3,4,6-tetra-0-acetyl-a -D-glucopyranosyl bromide, 3,4,6-tri-0-acetyl-l,2-0-(l-cyanoethyIidene)-a-D-glucopyranose, 3,4,6-tri-O-acetyl-)S-D-mannopyranose l,2-(methyi orthoacetate), 2,3-di-0-acetyI-l,6-anhydro-jS-D-galactopyranose, 2,3,4,6-tetra-0-acetyl-j8-D-galactopyranosyl cyanide, 1,2,3,4,6-penta-O-acetyl-a-D-idopyranose, 1,4,5-tri-0-acetyl-2,3-0-isopropyIidene-)S-D-fructopyranose, and 2,3,4-tri-O-acetyl-a-L-rhamnopyrano-syl and 2,3,4,6-tetra-O-acetyl-a-D-mannopyranosyl chlorides. ... 

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