A modified version of the Bischler indole synthesis has been developed in which the key step is the NâH insertion reaction of rhodium carbene intermediates derived from α-diazo-β-ketoesters with anilines. Thus N-methylanilines 1 react with diazoketoesters 2 in the presence of dirhodium(II) acetate to give (N-arylamino)ketones 3, cyclisation of which using boron trifluorideâethyl acetate or acidic ion exchange resin gives the indoles 4. In order to extend this method to the synthesis of N-unsubstituted indoles, a new protecting group strategy for indoles was developed. In this, anilines are reacted with α,β-unsaturated-esters or -sulfones to give the conjugate addition products 6 and 9,
cyclisation of which gives indoles 8 and 11. The N-(2-ethoxycarbonylethyl)- and -(2-sulfonylethyl)- protecting groups are readily removed from indoles 8 and 11 by treatment with base.
一种改进的Bischler
吲哚合成方法已被开发,其中关键步骤是
铑卡宾中间体从α-重氮-β-
酮酯与
苯胺的N-H插入反应。因此,
N-甲基苯胺1与重
氮酮酯2在二
铑(II)
醋酸盐存在下反应生成(N-芳
氨基)酮3,使用
三氟化硼-
乙酸乙酯或酸性离子交换
树脂进行环化反应得到
吲哚4。为了将这种方法扩展到N-未取代的
吲哚的合成,开发了一种新的
吲哚保护基团策略。在此过程中,
苯胺与α,β-不饱和酯或砜反应生成共轭加成产物6和9,其环化反应生成
吲哚8和11。N-(2-乙氧羰基乙基)- 和 -(2-砜乙基)- 保护基团通过用碱处理从
吲哚8和11中容易地去除。