| Summary: | 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.
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