Synthesis and Crystal Structures of 4,5-Dihydroimidazo[1,5-b]pyrazol-6-ones

Abstract The synthesis of previously unattainable 2,5-disubstituted 4,5-dihydroimidazo[1,5- b ]pyrazol-6-ones has been developed. Electrochemical reductions of readily available 2,2,2-trichloroethylideneacetophenones were followed by reaction with hydrazine, leading to 3-aryl-5-dichloromethyl-2-pyrazolines. These were treated with isocyanates to obtain the corresponding aminocarbonyl derivatives, which were found to be able to form an otherwise almost inaccessible imidazo[1,5- b ]pyrazole ring system via a one-step reaction involving internal condensation followed by hydrogen chloride elimination and aromatization. The molecular­ structures of 2-(4-methylphenyl)-5-tosyl-4,5-dihydro­imidazo[1,5- b ]pyrazol-6-one, 5-dichloromethyl- N -(4-chlorophenyl)-4,5-dihydro-3- p -tolylpyrazole-1-carboxamide, and 5-(4-bromophenyl)-2- p -tolyl-4,5-dihydroimidazo[1,5- b ]pyrazol-6-one were determined by X-ray crystallographic analysis.