Hydroxylamines, manganese dioxide

Neither of these systems has been prepared in the fully conjugated state. The tetrahydro-isoxazolo[4,3-e][l,2,4]triazine (145) is prepared by cyclization of the benzylidene triazinone (144) with hydroxylamine (Equation (18)) <82iJC(B)ii5>. Manganese dioxide oxidation of the isoxazole (146) forms the isoxazolo[4,5-e]triazine derivative (147) (Equation (19)) <86JHC1795>. 

Because nitroso compounds cannot be prepared by the reduction of nitro compounds, the oxidation of hydroxylamines is often the best way for their preparation. Nitroso compounds are obtained by treatment of hydroxylamines with silver oxide [373], silver carbonate [37S], sodium dichromate [645], potassium dichromate [657], manganese dioxide [ 76], and diethyl azodicarboxylate [978] (equation 487). 

Preparation. Singhla reports an improved procedure for the preparation based on a patent by H.-J. Teuber. Hydroxylamine disulfonate, HON(SOaNa)2, is prepared from sodium nitrite, sodium bicarbonate, and sulfur dioxide and then oxidized with potassium permanganate with pH-control. Manganese dioxide is removed by centrifugation followed by filtration. Fremy s salt separates from the filtrate after addition of KC1 and is recrystallized from 1 N KOH. Material prepared in this way can be stored in the cold for several months. 

Isoxazoles.—Isoxazolin-5-ones (513) exist in the gaseous phase in the form shown.Dehydrogenation of isoxazolines (514 R R = alkyl or Ar) with active y-manganese dioxide yields the corresponding isoxazoles quantitatively. The oxazinone (515) undergoes ring contraction on treatment with hydroxylamine hydrochloride to give the isoxazole derivative (516). The condensed isoxazoles... 

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