Influence of the stereochemistry of sugars on the selectivity of formation of carbohydrate-derived cyclopentadienyl and indenyl ligands
作者:Richard La?、Sandrine Martin、Renaud Touret
DOI:10.1039/b517146f
日期:——
Interaction of lithium cyclopentadienide with a suitable partially protected α-D-allofuranose triflate, 4, epimer at C3 of glucose, gives as a major product, besides the expected glucose-cyclopentadiene, 5, a glucose-disubstituted cyclopentadiene, 6. This unprecedented behaviour, which does not occur with α-D-glucofuranose and other sugars, is tentatively explained by a complexation of LiCp and the oxygen atoms of the isopropylidene function of one molecule of 4 and one of 5, giving a termolecular structure as the result of a template effect. The results of other experiments, such as the use of MgCp2 in place of LiCp or the complexation of oxygen atoms by lithium triflate, which changed the selectivity of the reaction largely in favour of the monosubstitution product 5, support this hypothesis. When lithium indenide is reacted with 4, glucose-monosubstituted and glucose-disubstituted indenes, 8 and 9, respectively, are formed, and 9 is obtained with almost total diastereoselectivity. This result can also be rationalised by a stereoselective complexation of lithium, as shown by separate experiments and by molecular mechanics calculations. Methyltricarbonyl molybdenum(II) complexes have been synthesised and characterised from glucose-monosubstituted 5 and glucose-disubstituted 6 cyclopentadienes.
环戊二烯锂与适当的部分受保护的 α-D-allofuranose triflate(4,葡萄糖 C3 的缩合物)相互作用,除了得到预期的葡萄糖-环戊二烯(5)外,还得到葡萄糖-二取代环戊二烯(6)作为主要产物。α-D-呋喃葡萄糖和其他糖类不会出现这种前所未有的现象,初步解释是 LiCp 与 4 和 5 分子中一个分子的异亚丙基官能团的氧原子发生了络合反应,从而产生了作为模板效应结果的三分子结构。其他实验的结果,如使用 MgCp2 代替 LiCp 或用三酸锂络合氧原子,在很大程度上改变了反应的选择性,有利于单取代产物 5,也支持了这一假设。当茚化锂与 4 反应时,分别生成葡萄糖一取代茚和葡萄糖二取代茚,即 8 和 9,并以几乎完全的非对映选择性得到 9。这一结果也可以通过锂的立体选择性络合得到合理解释,正如单独的实验和分子力学计算所显示的那样。以葡萄糖一取代的 5 环戊二烯和葡萄糖二取代的 6 环戊二烯为原料,合成了甲基三羰基钼(II)配合物,并对其进行了表征。