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[摘要]：Catalytic or stoichiometric amounts of Lewis acids were found to be very effective a-directing additives in the stereoselective glycosylations of diverse 2,3-O-carbonate-protected glucose and galactose thioglycoside donors by preactivation protocol. The poor stereoselectivities of 4,6-di-O-acetyl-2,3-O-carbonate protected thioglycoside donors in glycosyl coupling reactions were greatly improved, and excellent alpha-stereoselectivities were achieved by the addition of 0.2 equiv of BF3 center dot OEt2. On the other hand, the beta-selectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thioglucoside donor toward glycosylations were reversed completely to the alpha-selectivities by the use of 1 equiv of SnCl4, making the stereoselectivity controllable. Furthermore, the poor stereoselectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thiogalactoside donor in glycosylations were also improved by using SnCl4 as additive. |
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