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[摘要]：The alpha/beta-selectivity of super-armed rhamnosyl donors have been investigated in glycosylation reactions. The solvent was found to have a minor influence, whereas temperature was crucial for the diastereoselectivity. At very low temperature, a modest beta-selectivity could be obtained, and increasing temperature gave excellent a-selectivity. The donors were highly reactive, and activation was observed at temperatures as low as -107 degrees C. Different promoter systems and leaving groups were investigated, and only activation with a heterogeneous catalyst increased the amount of the beta-anomer significantly. By introducing an electron-withdrawing nonparticipating group, benzyl sulfonyl, on 2-O, an increase in beta-product was observed. |
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