5- Oximino-2-pyrazolin-4-ones

Knorr 809 first prepared 4-oximino-2-pyrazolin-5-ones by treatment of 2-pyrazolin-5-ones with nitrous acid (eq. 208) and this has remained the standard method of preparation.25,413,446 818 1004,1549 If an excess of nitrous acid is used, the nitroso substituent introduced is oxidized further to a nitro substituent.992 Amyl nitrite9 and nitrogen trioxide355,406 have also been used as nitrosating agents, though not extensively. 

It has been claimed that 3-pyrazolidinones can be converted to 4-oximino-2-pyrazolin-5-ones by treatment with nitrous acid.816,1550 However, later workers911,1342,1491 report that this reaction gives... 

Ridi1185 has reported that the reaction of 2,3-dimethyl-l-phenyl-4-nitroso-3-pyrazolin-5-one with phenylhydrazine gives 4-phenylazo-3-methyl-l-phenyl-2-pyrazolin-5-one. This may occur by replacement of the methylphenylhydrazine moiety in the 3-pyrazolin-5-one by phenylhydrazine. The 4-oximino-2-pyrazolin-4-one could then react with phenylhydrazine to give the reported product. 

Although the compounds obtained by introduction of a nitroso group at C-4 in 2-pyrazolin-5-ones are considered as 4-nitroso-2-pyrazolin-5-ones for purposes of classification in this discussion, they actually exist as the oximino isomers (eq. 208) and are usually so... 

The standard /3-ketoester-hydrazine reaction for preparation of 2-pyrazolin-5-ones has been used for direct synthesis of the 4-oximino derivatives by starting with an a-oximino-jS-ketoester.269,1125,1534,1536 In a modification of this by Biilow and Bozenhardt258 a hydrazone of the a-oximino-j8-ketoester was used. Ponzio and Ruggeri1125 have used a-oximino-j8-hydrazonohydroxamic acids and hydrazines, and other oximino compounds, as illustrated in eq. 209. 

Freri478,480 has reported a very interesting synthesis of 4-oximino-3-methyl-2-pyrazolin-5-one. This consisted in the treatment of citra-conic acid hydrazide with nitrous acid (eq. 211). The physical data which... 

Only one oximinobis(2-pyrazolin-5-one) is known.459 This is 2,3-bis(4-oximino-3-phenyl-5-oxo-2-pyrazolin-l-yl)naphthalene prepared by nitrosation with nitrous acid. 

Two 4-nitro-5-imino-2-pyrazolines are listed in Table XII. One of these was prepared by permanganate oxidation of the 4-oximino compound1010 and the other by direct nitration.1656 These compounds were believed to be aminopyrazoles rather than 5-imino-2-pyrazolines. 

These are listed in Table XLVIII. The nitrosation of 3,5-pyrazoIi-dinediones has already been mentioned (Section 2). A second method of preparation of the 4-nitroso-3,5-pyrazolidinediones (which are usually considered to have the tautomeric oximino form) has been used by Michaelis and Kirstein.981 This involves treatment of 3-chloro-3-pyrazolin-5-ones with nitrous acid (eq. 260). A third method of syn-... 

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