Tandem Cycloaddition Chemistry of Nitroalkenes: Probing the Remarkable Stereochemical Influence of the Lewis Acid.
Tandem Cycloaddition Chemistry of Nitroalkenes: Probing the Remarkable Stereochemical Influence of the Lewis Acid.
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硝基烯烃的串联环加成化学：探讨路易斯酸的显着立体化学影响。
DOI：
10.1021/jo9820869
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发表时间：
1999
期刊：
影响因子：
--
通讯作者：
M. Seierstad
中科院分区：
文献类型：
--
作者：
S. Denmark;M. Seierstad
研究了几种刘易斯酸对硝基烯烃1与手性非外消旋丙烯基醚8 [4 + 2]环加成反应的影响。所有检测的刘易斯酸都有利于ul相对非对映选择性（“exo”方法）; TiCl（4）、TiBr（3）（Oi-Pr）、SnCl（4）和ATPh是最具选择性的。在钛基刘易斯酸中，发现增加卤化物与醇盐的比率增加ul（相对）选择性的程度，如从氯化物切换到溴化物。内部非对映选择性也取决于刘易斯酸;大多数异丙醇钛卤化物（溴化物和氯化物）和SnCl（4）对（1，3-lk）方法具有高度选择性，选择性随卤化物含量增加而增加。两个铝基刘易斯酸（MAPh和ATPh）的选择性相反的内部非对映选择的意义。仅在TiCl（2）（Oi-Pr）（2）中观察到的高lk（相对）非对映选择性被认为是由内型方法的库仑稳定化或乙烯基醚与刘易斯酸的预络合引起的。在exo流形中看到的内部非对映选择性的开关被认为是由刘易斯酸-硝基烯烃络合物的空间性质的微妙变化引起的。
The influence of several Lewis acids on the stereochemical course of the [4 + 2] cycloaddition of nitroalkene 1 and chiral, nonracemic propenyl ether 8 has been examined. All of the Lewis acids examined favored ul relative diastereoselection ("exo" approach); TiCl(4), TiBr(3)(Oi-Pr), SnCl(4), and ATPh were the most selective. Within the titanium-based Lewis acids, it was found that increasing the halide-to-alkoxide ratio increased the degree of ul (relative) selectivity, as did switching from chloride to bromide. The internal diastereoselectivity was also dependent on the Lewis acid; most titanium isopropoxide-halides (bromide and chloride) and SnCl(4) were highly selective for (1,3-lk) approach, with the selectivity increasing with increasing halide content. Two aluminum-based Lewis acids (MAPh and ATPh) were selective for the opposite sense of internal diastereoselection. The high lk (relative) diastereoselectivity observed only with TiCl(2)(Oi-Pr)(2) is proposed to arise either from Coulombic stabilization of an endo approach or precomplexation of the vinyl ether to the Lewis acid. The switch in internal diastereoselectivity seen in the exo manifold is thought to arise from subtle changes in the steric nature of the Lewis acid-nitroalkene complex.