Remote substituent effect favoring the formation of syn-adducts in the chelation controlled radical reactions of γ-benzyloxy-α-methylenecarboxylic acid esters
作者:Hajime Nagano、Hisako Ohkouchi、Tomoko Yajima
DOI:10.1016/s0040-4020(03)00421-6
日期:2003.5
The chelation controlled radical reactions of ethyl γ-benzyloxy-α-methylenecarboxylates bearing a bulky γ-substituent, such as CHMe2, CHPh2, c-C6H11 and CH(Ph)OTBDMS, with alkyl iodides gave the syn-adducts with high diastereoselectivities. However, the diastereoselectivity for the substrates bearing a γ-substituent CH(i-Pr)OTBDMS depended critically on the configuration of the substituent; the substrate
带有大量γ-取代基的γ-苄氧基-α-亚甲基羧酸乙酯(如CHMe 2,CHPh 2,c -C 6 H 11和CH(Ph)OTBDMS)与烷基碘的螯合控制的自由基反应生成了顺式加合物非对映选择性高。然而,对带有γ-取代基CH(i - Pr)OTBDMS的底物的非对映选择性主要取决于取代基的构型。具有抗γ-苄氧基基团的OTBDMS基团的底物显示出较差的非对映选择性,但其非对映异构体仅产生顺式加合物。高同步-选择性是指H原子转移到带有Z-几何形状的带有乙氧基的急剧折叠的七元螯合中间体中的自由基中心的外表面。整体最小能量构象异构体的自由基中心原子的角扑动产生了局部最小构象异构体,并且向新形成的结构的自由基中心的外表面的H原子转移给出了反加合物。非对映体选择性差是由于两个构象异构体之间的能量差很小,因此合成的顺式和反式加合物的量几乎相等。