Enantioselective Friedel–Crafts type addition of indoles to nitro-olefins using a chiral hydrogen-bonding catalyst – synthesis of optically active tetrahydro-β-carbolines
作者:Wei Zhuang、Rita G. Hazell、Karl Anker Jørgensen
DOI:10.1039/b505220c
日期:——
The enantioselective FriedelâCrafts addition of indoles to nitro-olefins using chiral hydrogen-bonding bis-sulfonamides as the catalysts has been developed. The reactions, in the presence of only 2 mol% catalyst, generally proceed in high yields and with enantioselectivities up to 64% ee, and the enantiomeric excess can be improved to >98% ee by recrystallization. Various synthetic transformations of the FriedelâCrafts adducts are demonstrated: the nitro group can easily be reduced to the corresponding amine and the product obtained can undergo a stereocontrolled PictetâSpengler cyclization to give, for example, enantiopure tetrahydro-β-carbolines. The X-ray structure of the chiral bis-sulfonamides has been determined and based on these structures the mechanism for the stereoselectivity in the reaction is discussed.
已经开发出使用手性氢键双磺酰胺作为催化剂,将吲哚对硝基烯烃进行费德勒-克拉斯反应的对应选择性加成。在仅存在2 mol%催化剂的情况下,反应通常以高收率进行,对应选择性可达64% ee,并且通过重结晶可以将对应过剩提高至>98% ee。费德勒-克拉斯加成物的各种合成转化得到了证实:硝基很容易被还原为相应的胺,并且所得产物可以经历立体控制的皮克特-斯彭格勒环化反应,从而得到例如对应纯的四氢-β-咔啉。手性双磺酰胺的X射线结构已经确定,并且基于这些结构讨论了反应中的立体选择性机制。