溴代糖的制法和检验？ nucleophilic substitution of an acetate for a bromide; glycosyl bromide (2,3,4-tri-o-acetyl-6-deoxy-alpha-d-galactopyranosyl-1-bromide ) rocedure the carbohydrate (2,3,4-tri-o-acetyl-6-deoxy-a-d-galactopyranosyl-1-bromide) (17.2 g, 0.05 mol, 1 equiv.) was dissolved in acetic acid (9.5 ml, 0.16 mol, 3 equiv.) and hydrobromic acid (39 ml of a 30 wt.% solution in acetic acid, 0.20 mol, 3.7 equiv.) was added at room temperature (under nitrogen and protected from light). the reaction mixture was left to stir for 1½ hours after which the mixture was cooled to 0 °c and dichloromethane (300 ml) was added followed by slow addition of ice-cold water (100 ml). the two layers were separated and the organic fraction further washed with cold water (50 ml) and cold dilute aqueous sodium hydrogen carbonate solution (2 x 50 ml). drying over sodium sulfate and solvent removal in vacuo (water bath temperature below 30 °c to avoid decomposition) yielded the desired compound as an orange oil (17.0 g, 93 %). author's comments this procedure has been carried out numerous times on a variety of different scales, 0.05 mol being the largest for the fucose derivative. the corresponding galactose derivative has been converted to the bromide in a one-pot procedure (up to 0.2 mol, 97 %). the acetate protection is achieved with acetic anhydride and hydrogen bromide or perchloric acid to yield excluvely the alpha-acetate, and when conversion to the protected species is complete, i.e. all of the sugar has gone into solution, additional hydrobromic acid (3.7 equiv.) can simply be added. this procedure was not chosen for the fucose derivative, as it produced poor yields due to impurities present in the crude fucose employed. purification of the bromide by flash column chromatography does in some cases result in decomposition of these unstable sugars (see example), which should be stored in the freezer (keep cool, away from moisture and light) where they last at least several months with little decomposition (repurification may however be necessary). the corresponding galactose derivative, for example, can be purified by flash column chromatography without decomposition. data (anomeric proton: h-1) reference: syntheticpage 77 (2001) submitted 16th aug 2001 , published 17th aug 2001 original location: http:///pages/77 [此贴子已经被作者于2006-12-18 15:42:01编辑过] 查看更多
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