Synthesis of β-Substituted α-Amino Acids via Lewis Acid Promoted Enantioselective Radical Conjugate Additions
作者:Liwen He、G. S. C. Srikanth、Steven L. Castle
DOI:10.1021/jo051334f
日期:2005.9.1
Lewis acid promoted radical conjugate additions to β-substituted α,β-unsaturated α-nitro esters and amides were investigated. With achiral Lewis acids, there was competition between the desired radical conjugate addition and undesired alkene reduction mediated by Bu3SnH. Zinc Lewis acids provided the greatest amounts of addition products with both substrate classes. Studies with Bu3SnD indicated that
研究了路易斯酸促进的自由基共轭物加成至β-取代的α,β-不饱和α-硝基酯和酰胺的过程。对于非手性路易斯酸,在所需的自由基共轭物加成与由Bu 3 SnH介导的不期望的烯烃还原之间存在竞争。路易斯酸锌在两种底物类别中提供了最大量的加成产物。与卜3研究SnD表明,在受控的后处理条件下,α-硝基酯产物的酸性α-立体中心不会消旋。相应的α-硝基酰胺在色谱过程中显着消旋,但是可以通过对粗加合物进行后续转化来最大程度地减少此问题。铟介导的硝基还原,然后酰化所得胺,可得到良好的β-取代α-氨基酸衍生物高产率,且消旋程度最低。尝试在该过程的立体选择性变体中使用手性路易斯酸表明,Kanaemasa的DBFOX / Ph配体(14a)是唯一有效的。使用酰胺底物可获得中等至良好的ee和较低的dr。确定syn和syn的绝对构型加合物7b的反异构体表明,氢原子提取步骤比自由基共轭加成步骤具有更高的立体选择性。提出了底物与手性路易斯酸结合的模型。