One-Pot Synthesis of Polycyclic 4,5-Dihydropyridazine-3(2H)-ones by Inverse Electron-Demand Diels-Alder (IEDDA) Reactions from Alkenes

In the classical Inverse Electron-Demand Diels-Alder (IEDDA) reactions between alkenes and tetrazines, 4,5-dihydropyridazines are formed. 4,5-Dihydropyridazines are rapidly converted to the more energetically stable 1,4-dihydropyridazines by 1,3-prototropic isomerization. In this study, instead of 1,4-dihydropyridazines, 4,5-dihydropyridazine-3(2H)-ones were obtained as a result of IEDDA reactions between tetrazines with leaving groups at the 3,6-positions, and norbornene and barrelene-derived polycyclic alkenes in the presence of moisture in air or solvent. To show that this new method works not only on strained polycyclic alkenes but also on monocyclic and linear alkenes, the corresponding 4,5-dihydropyridazine-3(2H)-ones were obtained in high yields from the reactions performed with styrene and cyclopentene as well. The chemical structures of the polycyclic 4,5-dihydropyridazine-3(2H)-ones were determined by NMR and HRMS analyses. In addition, the exact structures of the polycyclic 4,5-dihydropyridazine-3(2H)-ones were also experimentally proven by converting them to pyridazine-3(2H)-ones known in the literature.