Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235, USA.
J Am Chem Soc. 2010 Dec 8;132(48):17111-7. doi: 10.1021/ja1039133. Epub 2010 Nov 16.
The long-standing question of what is the nature of the cyclization of squalene to form tetracyclic and pentacyclic triterpenes has been addressed computationally. Using the DFT method with an intrinsic reaction coordinate calculation, we find that the first three rings of protonated squalene were formed without the intermediacy of mono- or bicyclic carbocations. The cyclization, calculated in the gas phase, proceeds in a highly asynchronous, concerted reaction to yield the tricyclic, tertiary carbocation with a 5-membered C ring. The fourth double bond of squalene is not properly oriented for the ring expansion of the C ring in concert with the formation of the 5-membered ring.
用 DFT 方法和内禀反应坐标计算，我们发现质子化鲨烯的前三环是在没有单环或双环碳正离子中间体的情况下形成的。在气相中计算的环化反应以高度异步的协同反应进行，生成具有 5 元 C 环的三环、叔碳正离子。鲨烯的第四个双键没有正确定向，无法与 5 元环的形成协同进行 C 环的环扩张。
