Synthesis of new 1H-indazoles through Diels-Alder transformations of 4-styrylpyrazoles under microwave irradiation conditions

Microwave irradiation under solvent-free conditions induces 1-acetyl-4-styrylpyrazoles to undergo Diels-Alder cycloaddition reactions with N-methyl- or N-phenylmaleimide to give tetrahydroindazoles in good yields and with high selectivities, With conventional heating, these reactions either do not occur or afford only traces of the cycloadducts. These cycloadducts were then converted into the corresponding 1Hindazoles by dehydrogenation with DDQ in dry 1,2,4-trichlorobenzene under microwave irradiation or classical heating conditions, The structures of all new derivatives and the stereochemistries of the cycloadducts were assigned by NMR spectroscopy. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.Peer Reviewe