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An easy and versatile approach for the regioselective De-O-benzylation of protected sugars based on the I_2/Et_3SiH combined system

机译:基于I_2 / Et_3SiH组合系统的简便且通用的受保护糖区域选择性De-O-苄基化方法

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摘要

The use of cheap and easy to handle reagents, such as I_2 and Et_3SiH, at low temperature allows the regioselective removal of benzyl protecting groups from highly O-benzylated carbohydrates. The observed regioselectivity is dependent on the nature of the precursor, the least accessible carbinol often being liberated. A mechanistic investigation reveals that in situ generated HI is the promoter of the process, whereas the regioselectivity appears to be mainly controlled by steric effects. However, the presence of an electron withdrawing acyl protecting group can switch the regioselectivity to favour deprotection of the carbinol position farthest from the ester group. The protocol is experimentally simple and provides straightforward access in useful yields to a wide range of partially protected mono- and disaccharide building blocks that are valuable for the synthesis of either biologically useful oligosaccharides or highly functionalised chiral compounds. Partially protected sugars thus obtained can also be coupled in situ with a glycosyl donor, as illustrated by the one-pot synthesis of a Lewis X mimic from fully protected precursors.
机译:在低温下使用便宜且易于处理的试剂,例如I_2和Et_3SiH,可以从高度O-苄基化的碳水化合物中区域选择性地除去苄基保护基。观察到的区域选择性取决于前体的性质,最难获得的甲醇经常被释放。机理研究表明,原位产生的HI是该过程的促进剂,而区域选择性似乎主要受空间效应控制。然而,吸电子酰基保护基的存在可以切换区域选择性,以促进离酯基最远的甲醇位置的脱保护。该协议在实验上很简单,并且可以以有用的收率直接访问范围广泛的部分受保护的单糖和二糖结构单元,这些单元对于合成生物学上有用的寡糖或高度官能化的手性化合物都具有重要意义。如此获得的部分保护的糖也可以与糖基供体原位偶联,如由完全保护的前体的路易斯X模拟物的一锅法合成所说明的。

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