A divergent synthesis of dihydropyridazinones, N ‐substituted dihydropyrazoles, and O ‐substituted pyrazoles

Dihydropyridazinones 4a , 4b , N ‐substituted dihydropyrazoles 5b , 5c , 5d , and O ‐substituted pyrazoles 6a , 6b , 6c , 6d have been synthesized starting from spirocyclopropanepyrazole derivative 2 . Treatment of 2 with α‐chloro esters, e.g., methyl chloroacetate, ethyl chloroacetate, isopropyl chloroacetate, and tert ‐butyl chloroacetate, in potassium carbonate/sodium iodide system caused ring opening and subsequent C ‐ or N ‐attack nucleophilic substitution to give the corresponding dihydropyridazinones 4a , 4b and N ‐substituted dihydropyrazoles 5b , 5c , 5d . On the other hand, in the absence of sodium iodide, O ‐substituted pyrazoles 6a , 6b , 6c , 6d were obtained from 2 via an O ‐attack nucleophilic substitution . J. Heterocyclic Chem., 2011.