Cyclization of Hydroxyenol Ethers into Spiroacetals. Evidence for the position of the transition state and its implication on the stereoelectronic effects in acetal formation
作者:Normand Pothier、Solo Goldstein、Pierre Deslongchamps
DOI:10.1002/hlca.19920750217
日期:1992.3.18
controlled cyclizations of the same four hydroxyenol ethers produced, along with the more stable spiroacetals mentioned above, the less stable spiroacetals 23, 25, and 28. These results show that the kinetically controlled cyclization takes place via an early transition state which produces a mixture of the less stable and the more stable isomers. These results are explained by an early transition state
据报道,在热力学和动力学控制的条件下,羟基烯醇醚18-21具有酸催化的环化作用。18、19和21的热力学控制环化仅产生更稳定的相应螺缩醛22和27。热力学控制的化合物20的环化反应产生了不可模仿的螺缩醛24和26的1:1混合物。另一方面,与上述提到的更稳定的螺缩醛一起生产的相同的四种羟基烯醇醚的动力学控制环化反应,生成的不稳定的螺缩醛23、25和28也更不稳定。。这些结果表明,动力学控制的环化是通过早期过渡态发生的,该过渡态产生了较不稳定和较稳定的异构体的混合物。这些结果由遵循立体电子控制原理的早期过渡态解释,同时遵循反周平面孤对假设(π系统中亲核分子的Bürgi - Dunitz攻角)。