4-Substituted 2,3-Dihydroisoxazoles as Masked Azomethine Ylides: Access to Pyrrole Derivatives by 1,5- and 1,7-Dipolar Electrocyclizations of Enynyl Derivatives
The enynyl-substituted 2,3-dihydroisoxazoles ('isoxazolines') 9-14 were prepared by highly (Z)-selective Peterson olefination reaction from the corresponding carbaldehydes 6-8. On short-time thermolysis (280-406degrees/10 s) the TMS derivatives 9-11 give rise to the annulated pyrrolines 18-20, which, in some cases. suffer CH4 elimination affording the pyrroles 15-17 In contrast. thermolysis of the terminal alkyne derivatives 12-14 leads to the bicyclic compounds 21-23. The reaction pathways are discussed on the basis of the formation of conjugated azomethine ylides as key intermediates. which either undergo a 1,5-cyclization to 18-20 or a 1,7-ring-closure affording cycloallene intermediates of type V, which are further transformed into the azepino pyrroles, 21-23.
Ring transformations of 2,3-dihydroisoxazoles via azomethine ylides—formation of annulated 5- and 7-membered N-heterocycles
作者:Wolfgang Friebolin、Wolfgang Eberbach
DOI:10.1016/s0040-4020(01)00322-2
日期:2001.5
On thermal activation the 2,3-dihydroisoxazoles 12–14 are transformed into annulated dihydroazepines 15–17 as main products, besides minor amounts of the corresponding pyrrole derivatives 18–20. In the proposed mechanism the azomethine ylides of type III and VI are involved as intermediates which undergo 1,5- and 1,7-ring closure reactions, respectively.