N-Heterocyclic carbene-catalyzed 1,3-dipolar cycloaddition reactions: a facile synthesis of 3,5-di- and 3,4,5-trisubstituted isoxazoles
作者:Shravankumar Kankala、Ravinder Vadde、Chandra Sekhar Vasam
DOI:10.1039/c1ob06072d
日期:——
A first example of organo-N-heterocyclic carbene (NHC) catalyzed click-type fast 1,3-dipolar cycloaddition of nitrile oxides with alkynes was developed for the regioselective synthesis of 3,5-di- and 3,4,5-trisubstituted isoxazoles. Triethylamine (Et3N) was employed as an effective base to generate both nitrile oxide and the organo-NHC catalystin situ. This catalytic approach was used to attach a variety of substituents, including other biologically active fragments, onto the isoxazole ring to selectively design multinucleus structures. Further, we have also optimized the conditions for Cu(I)-free Sonogashira cross-coupling to obtain internal alkynes in high yields, which were subsequently used in cycloaddition. A catalytic cycle is proposed and the remarkable regiocontrol in the formation of isoxazoles was ascribed to a beneficial zwitterion intermediate developed by the interaction of the strongly nucleophilic organo-NHC catalyst with alkyne followed by nitrile oxide.
一种以有机N-杂环卡宾(NHC)催化的“点击”型快速1,3-偶极环加成反应的第一个例子被开发出来,用于区域选择性合成3,5-二取代和3,4,5-三取代的异噁唑。三乙胺(Et3N)作为有效的碱被使用,原位生成氰氧化物和有机NHC催化剂。该催化方法用于将多种取代基(包括其他生物活性片段)连接到异噁唑环上,以选择性设计多核结构。此外,我们还优化了无铜(I)的索诺加希拉交叉偶联反应的条件,以获得高产率的内部炔烃,随后用于环加成反应。提出了一个催化循环,异噁唑形成中的显著区域控制归因于强亲核性有机NHC催化剂与炔烃相互作用后形成的有益的两性离子中间体,以及与氰氧化物的后续反应。