Hydrazone-azomethine imine isomerization

The thermal hydrazone-azomethine imine isomerization can be easily performed under microwave irradiation in the absence of solvent. The subsequent 1,3-dipolar cydoadditions with electron-defident dipolarophiles occur in only a few minutes to afford the corresponding cycloadducts. The use of pyrazolyl hydrazones 205 leads to valuable compounds, such as bipyrazoles 213, in good yields and this provides a new approach to the preparation of these heterocyclic derivatives [116] (Scheme 9.67). Reactions undertaken with dassical heating under comparable reaction conditions (time and temperature) lead to cydoadduct yields that are considerably lower and, indeed, several dipolarophiles do not react at all. 

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