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.

New synthetic approach to pyrazolo[1,5‐a]pyrazine derivatives from 2,2‐dichlorovinylacetophenones

“…After developing the first synthesis of 2,2dichlorovinylacephenones [31,32] these inexpensive and readily available compounds were found to be excellent starting materials to prepare pyrazolines [31,33], imidazopyrazolones [34], and pyrazolooxazines [35]. Direct reaction of 2,2-dichlorovinylacephenones with 2-hydroxyethylhydrazine produces 2-pyrazoline intermediates that contain vicinal hydroxyethyl and dichloromethyl groups.…”

A novel synthetic approach to Pyrazolooxazines. One‐pot synthesis of Pyrazolo[5,1‐c][1,4]oxazine derivatives from 2,2‐dichlorovinylacetophenones

“…For some years we have been working in developing new synthetic approaches to six‐ and five‐membered heterocyclic compounds starting from chloral derivatives. For example, imidazo‐pyrazolones [22], oxadiazolines [23], triazoles [24], pyrimidines [25], isoxazolines [26], pyrazolines [27], thiazolines [28], oxazolines [29], etc. In connection with this line of research we report here a new, general and efficient method for preparing still almost unknown pyrazolooxazines based on 2,2‐dichlorovinylacetophenones, which are readily available chloral derivatives [30] capable to react efficiently with commercial 2‐hydroxyethylhydrazine.…”