Stereoselective synthesis of cyclic ethers using vinylogous sulfonates as radical acceptors: effect of E/Z geometry and temperature on diastereoselectivity.
Stereoselective synthesis of cyclic ethers using vinylogous sulfonates as radical acceptors: effect of E/Z geometry and temperature on diastereoselectivity.
复制标题
使用插烯磺酸盐作为自由基受体立体选择性合成环醚：E/Z 几何形状和温度对非对映选择性的影响。
DOI：
10.1021/jo991970b
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
Manangan,T
中科院分区：
文献类型：
--
作者：
Evans,PA;Manangan,T
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-乙烯基双磺酸酯1a − g，得到环醚2/3a− g，具有良好到优异的非对映选择性，有利于顺式异构体2。这项研究表明，立体控制的水平，在6-外自由基环化，并可能归因于自由基中间体的类型。因此，对于环化1 e获得的适度选择性可能是酰基自由基几何形状（sp2）和与烷基（1 kcal/mol）和乙烯基（2.9 kcal/mol）自由基相比的高转化势垒（29 kcal/mol）的函数。这与酰基自由基环化具有比相应的烷基和乙烯基自由基更早的过渡态是一致的。使用Z-乙烯基磺酸酯（≥34：1; R = Ph）可显著提高适度的非对映选择性，以分别促进反式异构体I和III的动力学捕获（图1）。研究了在非还原性烯丙基化条件下的5-外烷基环化反应，得到8，总收率91%。这种转化为原位同源化提供了一种方便的方法，并应适用于靶向合成。
Treatment of theE-vinylogous sulfonates1a−gwith tris(trimethylsilyl)silane and triethylborane, in the presence of air, furnished the cyclic ethers2/3a−gwith good to excellent diastereoselectivity favoring thecis-isomer2. This study demonstrated the level of stereocontrol in a 6-exo radical cyclization and may be attributed to the type of radical intermediate. Hence, the modest selectivity obtained for the cyclization of1emay be a 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 theZ-vinylogous sulfonate (≥34:1; R = Ph) to promote kinetic trapping of thes-transrotamerIandIII, respectively (Figure 1). The 5-exo alkyl radical cyclization reaction under nonreductive Keck-allylation conditions was also examined, in which8was formed in 91% overall yield. This transformation provides a convenient method for in situ homologation and should be applicable to target directed synthesis.