Dihydropyrans from 1,4-Cycloaddition of Enamines to Arylmethylenepyrazolones. Polar Character of the Thermal Rearrangement of the Resulting Cycloadducts

Abstract

The inverse electron demand hetero-Diels-Alder reaction of arylmethylenepyrazolones with enamines results in the selective formation of 3,4-dihydro-2H-pyran derivatives. These cycloadducts under thermodynamic conditions, are transformed via the zwitterion, which can be captured in the presence of tetracyanoethylene (TCE), as the more stable Michael-type adducts. The zwitterion could also be the intermediate for the cyclic adducts.