Bravo Fernando, Viso Antonio, Alcázar Eva, Molas Pineda, Bo Carles, Castillón Sergio
Departament de Química Analítica i Química Orgànica, Universitat Rovira i Virgili, Imperial Tàrraco 1, 43005 Tarragona, Spain.
J Org Chem. 2003 Feb 7;68(3):686-91. doi: 10.1021/jo026319e.
We studied the geometric and electronic structure of several cations (9 and 18-23) that are usually proposed as intermediates for glycosylation reactions using DFT methods. For ions 9 and 18-23 the geometries obtained corresponded to an open ion (6b type) irrespective of the exocyclic heteroatom (S, Se, I) and the size of the cycle (furanoses as well as pyranoses). We studied how substituents bonded to the exocyclic heteroatom (R in SR and SeR) affect the structure of ions and found that the geometry approached that of episulfonium and episelenonium ions (6a type) when the substituents were electron donors. The tert-butyl group and the 2,4,6-trimethyl- and 2,4,6-trimethoxyphenyl and trimethylsilyl groups produced the strongest of these effects. The presence of an electron-withdrawing group (CO-CF(3)) bonded to the hydroxymethyl group produced a similar but weaker effect. We analyzed this behavior in terms of molecular orbital interactions.
我们使用密度泛函理论（DFT）方法研究了几种通常被认为是糖基化反应中间体的阳离子（9和18 - 23）的几何结构和电子结构。对于离子9以及18 - 23，无论环外杂原子（S、Se、I）以及环的大小（呋喃糖和吡喃糖）如何，所得到的几何结构都对应于一种开环离子（6b型）。我们研究了与环外杂原子相连的取代基（SR和SeR中的R）如何影响离子的结构，发现当取代基为给电子体时，其几何结构接近环硫鎓离子和环硒鎓离子（6a型）。叔丁基、2,4,6 - 三甲基苯基、2,4,6 - 三甲氧基苯基以及三甲基硅基产生了最强的这些效应。与羟甲基相连的吸电子基团（CO - CF(3)）的存在产生了类似但较弱的效应。我们从分子轨道相互作用的角度分析了这种行为。
