Stereoselective Synthesis of Cyclic Ethers Using Vinylogous Sulfonates as Radical Acceptors: Effect of <i>E</i>/<i>Z</i> Geometry and Temperature on Diastereoselectivity
作者:P. Andrew Evans、Thara Manangan
DOI:10.1021/jo991970b
日期:2000.7.1
function of the acyl radical geometry (sp2) and high inversion barrier (29 kcal/mol) as compared to the alkyl (1 kcal/mol) and vinyl (2.9 kcal/mol) radicals. This is consistent with the acyl radical cyclization having an earlier transition state than the corresponding alkyl and vinyl radicals. The modest diastereoselectivity can be improved dramatically using the Z-vinylogous sulfonate (> or =34:1; R =
在空气存在下,用三(三甲基甲硅烷基)硅烷和三乙基硼烷处理E-乙烯基磺酸盐1a-g,使环醚2 / 3a-g具有良好的至优异的非对映选择性,有利于顺式异构体2。 6-exo自由基环化中立体控制的水平,可能归因于自由基中间体的类型。因此,与烷基(1 kcal / mol)和乙烯基(2.9 kcal / mol)相比,环化1e所获得的适度选择性可能是酰基自由基几何形状(sp2)和高反型垒(29 kcal / mol)的函数mol)自由基。这与具有比相应的烷基和乙烯基自由基更早的过渡态的酰基自由基环化相一致。使用Z-乙烯基磺酸盐(>或= 34:1; R = Ph)分别促进S-反式旋转异构体I和III的动力学捕获(图1)。还研究了在非还原性Keck-烯丙基化条件下的5-exo烷基自由基环化反应,其中形成了8种,总产率为91%。这种转化为原位同源提供了一种方便的方法,应该适用于靶标定向合成。