State Key Laboratory of Natural and Biomimetic Drugs and School of Pharmaceutical Sciences, Peking University, Xue Yuan Road No. 38, Beijing 100191, China.
J Org Chem. 2012 Jun 15;77(12):5255-70. doi: 10.1021/jo3002084. Epub 2012 May 30.
Catalytic or stoichiometric amounts of Lewis acids were found to be very effective α-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 α-stereoselectivities were achieved by the addition of 0.2 equiv of BF(3)·OEt(2). On the other hand, the β-selectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thioglucoside donor toward glycosylations were reversed completely to the α-selectivities by the use of 1 equiv of SnCl(4), 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 SnCl(4) as additive.
路易斯酸的催化或化学计量用量被发现是通过预活化方案立体选择性糖基化各种 2,3-O-碳酸酯保护的葡萄糖和半乳糖硫代糖苷供体的非常有效的α-定向添加剂。4,6-二-O-乙酰基-2,3-O-碳酸酯保护的硫代糖苷供体在糖基偶联反应中的立体选择性很差，通过添加 0.2 当量的 BF(3)·OEt(2)，可以大大提高其立体选择性，并获得优异的α-立体选择性。另一方面，使用 1 当量的 SnCl(4)，4,6-二-O-苄基-2,3-O-碳酸酯保护的硫代葡萄糖苷供体对糖基化的β选择性完全反转到α选择性，使立体选择性可控。此外，使用 SnCl(4)作为添加剂也可以改善 4,6-二-O-苄基-2,3-O-碳酸酯保护的硫代半乳糖苷供体在糖基化反应中的差立体选择性。
