Xylopyranosyl bromide

To a solution of silver triiluoromethanesulfonate (7.71 g, 30 mmol) in dry dichloromethane (100 mL) at —40°C in the dark is added dropwise a solution of 9-fluorenylmethoxycarbonyl serine benzyl ester 11 (8.45 g, 20 mmol), 2,3,4-tri-O-benzoyl-a-D-xylopyranosyl bromide 12 (11.14 g, 21.2 mmol) and tetramethyl urea (3.65 g, 31.4 mmol) in dichloromethane (100 mL). After 18 h of stirring at room temperature, the precipitate is filtered off and washed with dichloromethane (200 mL). The organic solution is washed with water (200 mL), 1% KHCOj solution (twice 200 mL) and water, dried with Na SO, and concentrated in vacuo. The crude product is recrystallized from ethyl acetate-n-hexane. (If the reaction was not complete, chromatography on silica gel 60 in toluene/ethanol 9 1 is recommended). Yield 15 g (87%) mp 136°C, [ot]D -27.8° (c 1.3, CHC13), Rf 0.64 (toluene/ethanol 26 1). 

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-... 

Triacetyl-0-D-arabinopyranosyl bromide Triace tyl-fl-n-arabinopyranosyl bromide Triacetyl-a-ir-rhamnopyranoayl bromide Triacetyl-a-D-ribosyl bromide Triace tyl-a-n-ribosyl bromide Triacetyl-a-D-xylopyranosyl bromide Triacetyl-a-n-xylopyranosyl bromide Tetraacetyl-of-D-galactopyranosyl bromide Tetraacetyl-a-D-glucopyranosyl bromide Tetraacetyl- -D-glucopyranosyl fluoride 2,3 5,6-diisopropylidene-a-D-mannofuranosyl chloride Tetraacetyl-a-D-mannopyranosyl bromide Heptaacetyl-a-cellobiosyl bromide Heptaacetyl-a-gentiobiosyl bromide Heptaacetyl-a-lactosyl bromide Heptaacetyl-a-maltosyl bromide... 

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... 

Whistler, R L, van Es, T, Solvolysis of methyl 5-thio-D-xylopyranosides and 2,3,4-tri-0-acetyl-5-thio-a-D-xylopyranosyl bromide, J. Org. Chem., 28, 2303-2304, 1963. 

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... 

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. 

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