Ring transformations of 2,3-dihydroisoxazoles via azomethine ylides—formation of annulated 5- and 7-membered N-heterocycles

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.