Sydnones azomethine imine cyclizations

The mesoionic compounds known as sydnones serve as cyclic azomethine imines. Thus, sydnone (167), isolable after preparation from die corresponding nitrosamine, underwent cyclization as an azomethine imine at 20-35 C (Scheme 52).86 Photolysis of sydnones also results in cyclization but through nitrile imine intermediates vide infra). 

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