Nonenymic Sugar Transformation promoted by Metal Complexes

近年来，随着糖蛋白、糖脂、核酸和抗生素中所含碳水化合物的生物学功能的阐明，人们越来越关注稀有的天然糖。因此，需要开发一种金属配合物促进糖的转化和糖的立体选择性摄取的方法。在阐明糖-过渡金属相互作用的重要研究部分中，合成了一种前所未有的双核镍- (II)配合物，该配合物含有N，N'-二甲基-乙二胺和d -甘露糖（DMna）形成的两种N-糖苷，并通过x射线晶体学确定了其结构。金属中心为双核镍配合物，偶联甘露糖残基。桥接糖环以不寻常的呋喃糖形式存在。当D-葡萄糖代替D-甘露糖作为起始糖时，尽管产率很低，但令人惊讶的是得到了镍（II）配合物。基于这一认识，我们选择了N，N，N'-三甲基乙二胺（tmen）作为二胺组分，它在分子中有一个反应点（仲氨基）。我们发现醛糖在[Ni(H_2O)_2(tmen)_2]Cl_2存在下，在C-2处迅速外映，在这两种C-2外映体中，镍（II）离子只与甘露糖型外映体形成立体选择性配合物。EXAFS研究表明，甘露糖型外旋体立体选择性络合的关键在于呋喃苷残基的桥接部分，该部分似乎对镍（II）离子具有很强的亲和力。13c核磁共振。对Ni^<2+>-二胺-[1-^3C]- d -葡萄糖体系的研究表明，该反应涉及碳骨架的一种新的立体定向重排，即通过C-1-C-2醛糖片段的反转来交换C-1和C-2原子。EXAFS研究表明，反应性配合物是单核的。根据这些观察结果，我们提出了Ni2+-二胺-醛糖体系外聚化的合理机制。

Much attention has recently been focused on rare naturally occurring sugars, with the elucidation of the biological functions of carbohydrates contained in glycoprotains, glycolipids, nucleic acids, and antibiotics. It is thus desirable to develop a methods whereby metal complexes promote transformations of sugars and stereoselective uptake of sugars. During a significant part of study to elucidate sugar-transition-metal interactions, an unprecedented binuclear nickel- (II) complex containing two types of N-glycosides formed from N,N'-dimethyl- ethylenediamine and D-mannose(DMna) was synthesized, and its structure was determined by X-ray crystallography. The metal center is a binuclear nickel complex with a bridbing mannose sugar residue. The bridging sugar ring takes the unusual furanose form. When D-glucose was used as a starting sugar instead of D- mannose, a nickel(II) complex was surprisingly obtained although its yield was very low. Prompted by this knowledge, we chose N,N,N'-trimethylethylenediamine (tmen) as the diamine component, which has one reaction point (secondary amino group) in the molecule. We have found that aldoses are rapidly epimerized at C- 2 in the presence of [Ni(H_2O)_2(tmen)_2]Cl_2 and, of the two C-2 epimers, nickel(II) ions form complexes with only the mannose-type epimers stereoselectively. The EXAFS study suggested that the Key to the stereoselective complexation of mannose type epimers lies in the bridging part of the furanoside residue which seems to have a strong affinity for-nickel(II) ions. The 13 C.NMR. study-of the Ni^<2+>- diamine-[1-^3C]-D-glucose system reveals that a novel stereospecific rearrangement of the carbon skleton, the exchange of C-1 and C-2 atoms by inversion of the C-1-C-2 aldose fragment, is involved in this reaction. The EXAFS study indicated that the reactive complex is mono nuvclear. From these observations obtained we proposed a plausible mechanism for the epimerization in the Ni2+- diamine-aldose system.