(E)/(Z)-Isomeric allylic carbamate esters were deprotonated by n-butyllithium/(-)-sparteine in toluene. Trapping experiments with chlorotrimethylsilane afforded the alpha-substitution products, with (R)-configuration, revealing that the pro-S proton is removed predominantly to form the corresponding (S)-lithium (.) (-)-sparteine derivatives; k(s)/k(R) > 15:1 and > 7: 1, respectively. A slow (S) -> (R)-epimerization occurs at - 78 degrees C (T-1/2 > 60 min). The allylic double bond is stable to (Z)-(E) isomerization under these conditions. (c) 2005 Elsevier Ltd. All rights reserved.
(E)/(Z) 异构的邻位复
碳酸酯均由 n-乙基Li 和 (-)- paylaşine 在toluene中脱质子化。通过捕获实验使用
氯 triflate 得到α-取代产物,并发现这些产物具有 (R)构型,这表明 S 端的质子被优先去掉而形成相应的 (S)-Li (.) (−)-`;
等产物;实验结果表明k(s)/k(R) 值分别大于15:1和7:1。在−78°C下,发生 S-R 型转置反应的速度非常慢,且半衰期大于60 min。邻位双键在这些条件下对(Z)-(E)异构稳定。 (c)2005 Elsevier Ltd. All rights reserved.