Experimental and theoretical approaches into the C- and D-ring problems of sterol biosynthesis. Hydride shift versus C–C bond migration due to cation conformational changes controlled by the counteranion
作者:Mugio Nishizawa、Arpita Yadav、Yoshihiro Iwamoto、Hiroshi Imagawa
DOI:10.1016/j.tet.2004.07.064
日期:2004.10
spirocyclic ether 21 was formed as the sole product via a tert-cationic intermediate 16 through 1,2-hydride shift. However, the treatment with TiCl4 afforded six-membered ring products 22, 23, 24, 25, 26, and 27 via the ring expansion into the unstable six-membered ring secondary cation 17. Occurrence of both α and β chloride 23 and 24 is distinctive evidence of the existence of secondary cation 17, ruling
从实验和理论的角度,通过使用模型化合物,研究了固醇生物合成的C环和D环问题,即一种酶如何克服马尔可夫尼科夫壁的问题。当用BF 3 ·Et 2 O,SnCl 4,TiF 4,Sc(OTf)3,FeCl 3或TfOH处理模型二醇20时,螺环醚21经由叔阳离子中间体16至1形成为唯一产物。,2-氢化物转移。然而,用TiCl 4处理4,得到六元环的产品22,23,24,25,26,和27通过环膨胀进入不稳定六元环二次阳离子17。α和β氯化物23和24的出现是存在次级阳离子17的独特证据,排除了协同机制的想法。裸阳离子15的分子力学计算阐明了两个可能的构象异构体,平行的15 - I(五元环和阳离子平面),有利于氢化物移位生成16和垂直的15 - II,从而导致C-C键迁移至17。在抗衡阴离子(例如[TiCl 4 OH] -,[TiF 4 OH] -,[BF 3 OH] -和[OTf] -)存在下对阳