Single and double diastereoselection in azomethine ylide cycloaddition reactions with unsaturated chiral bicyclic lactams
Single and double diastereoselection in azomethine ylide cycloaddition reactions with unsaturated chiral bicyclic lactams
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DOI：
10.1021/jo9600870
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发表时间：
1996-05-17
影响因子：
3.6
通讯作者：
Meyers, AI
中科院分区：
文献类型：
--
作者：
Fray, AH;Meyers, AI
双非对映选择性数据进行了分析，以提供深入了解的结构特征，影响π面选择性的手性和非手性甲亚胺叶立德手性，不饱和双环内酰胺的1，3-偶极环加成。确定了三个主要的空间贡献的差异之间的竞争性环加成过渡态的稳定性（三角洲三角洲G（双刃剑））。第一组主要的空间相互作用涉及偶极与左半球（R（2））上的取代基和双环内酰胺凹面之间的相互作用。这有效地阻碍了图1所示的非扩展过渡态中的α和β途径。第二个主要的空间相互作用是由双环内酰胺上的R（1）角取代基与偶极的π系统之间的非键相互作用（i）提供的，如图3和4所示。这种相互作用被证明是非常显着的，当角取代基从苯基或甲基改变为氢时，导致手性和非手性偶极子的π面攻击逆转。现在观察到的高非对映选择性开辟了一条路线，高度取代的手性，非外消旋吡咯烷。
Double diastereoselectivity data were analyzed to provide insight into the structural features that influence pi-facial selectivity in 1,3-dipolar cycloadditions of chiral and achiral azomethine ylides to chiral, unsaturated bicyclic lactams. Three major steric contributions to the differences in stability (Delta Delta G(double dagger)) between competing cycloaddition transition states were identified. The first major set of steric interactions involve that between the dipoles and the substituents on the left hemisphere (R(2)) and concave faces of the bicyclic lactams. This effectively hindered both alpha- and beta-approaches in the nonextended transition states shown in Figure 1. The second major steric interaction was provided by the nonbonded interactions (i) between the R(1) angular substituent on the bicyclic lactam and the pi-system of the dipole as shown in Figures 3 and 4. This interaction was shown to be very significant, causing reversal in pi-facial attack of chiral and achiral dipoles when the angular substituent is changed from phenyl or methyl to hydrogen. The high diastereoselectivity observed now opens a route to highly substituted chiral, nonracemic pyrrolidines.