4-Substituted 2,3-Dihydroisoxazoles as Masked Azomethine Ylides: Access to Pyrrole Derivatives by 1,5- and 1,7-Dipolar Electrocyclizations of Enynyl Derivatives
作者:Wolfgang Friebolin、Wolfgang Eberbach
DOI:10.1002/1522-2675(20011219)84:12<3822::aid-hlca3822>3.0.co;2-o
日期:2001.12.19
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.