Synthesis of 4-hydroxylamino-1-azabuta-1,3-dienes and their cyclization to 2-substituted pyrazole 1-oxides

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Abstract
2-Aromatic or 2-aliphatic substituted pyrazole 1-oxides with substituents at the 3-, 4- or 5-position have been prepared from β-dicarbonyl compounds or ketones. The β-dicarbonyl compounds were treated with p-toluidine to give 1,5-di-p-tolyl-1,5-diazapenta-1,3-dienium salts in which the p-tolylimino groups can be converted into alkylimino groups by treatment with an aliphatic amine. Subsequent deprotonation and treatment with O-tert-butyldimethylsilylhydroxylamine produced 4-arylamino- or 4-alkylamino-1-tert-butyldimethylsilyloxy-1-azabuta-1,3-diene. The tautomeric structure, the configuration and the conformation of the dienes were elucidated by 1H and 13C NMR spectroscopy. Oxidation of the alkylamino-1-tert-butyldimethylsilyloxy-1-azabuta-1,3-dienes with copper(II) ions led to cyclization with formation of pyrazole 1-oxides. These could also be prepared by oxidation of 3-amino oximes obtained from ketones through amino-alkylation at the α-position and treatment with hydroxyammonium chloride. If 1,5-diaryl-1,5-diazapenta-1,3-diene was treated with O-(p-tolylsulfonyl)hydroxylamine spontaneous cyclization occurred to give 1-(p-tolyl)pyrazole.

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