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| - Study of Interhalogens/Silver Trifluoromethanesulfonate asPromoter Systems for High-Yielding Sialylations
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| - We have studied interhalogen/silver trifluoromethanesulfonate (IX/AgOTf) promoted glycosylationsand found differences in the sensitivity of the formed oxocarbenium ions (e.g. from compoundswith or without participating groups) toward halide nucleophiles. These differences can be explainedusing the HSAB theory. By applying this theory on sialylations, we increased the yield for a modelreaction from a highly unpredictable 35-46% using ICl to 74% using IBr. We have also showedthat the most prominent role of the silver ions is lowering the concentration of the halide nucleophilerather than activating the interhalogen compound and, by increasing the amount of AgOTf from1 to 1.5 equiv (with respect to IBr), the yield in the model reaction improved from 74% to 89%. Acomparison of two different anomeric leaving groups showed that glycal formation can be minimizedusing a thiophenyl donor instead of xanthate. By combining these observations, we were able toincrease the yield of the model reaction to 97%.
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