3- Pyrazolin-5-ones nitrosation

The vast majority of reactions undergone by 3-pyrazolin-5-ones occur at the 4-position. These pyrazolinones react by direct substitution, very much as do activated benzene rings. Such reactions as halogenation, nitrosation, sulfonation, acylation, the Mannich reaction and many others give 4-substituted-3-pyrazolin-5-ones. 

Nitrosation of 3-pyrazolin-5-ones is quite easy, commonly occurring in the 4-position (eq. 118). The usual reagent is nitrous acid,145,781,809,860,984,992,1174 prepared in situ by the use of sodium... 

Pyrazolin-5-ones react with concentrated nitric acid809,880,888, 992.1001 or an excess of nitrous acid984,992,1001 to form 4-nitro-3-pyxazolin-5-ones. Presumably the excess of nitrous acid first nitrosates and this product is oxidized to the nitro compound. 

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. 

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. 

The 4-nitroso-3-pyrazolin-5-ones are always prepared by nitrosation of 3-pyrazolin-5-ones with nitrous acid.99,103,145,647,781,807 809,854,888, 972,978.984 Only one 3-nitroso-3-pyrazolin-5-one has been reported.972 This was prepared by nitrosation of 3-pyrazolin-5-one having a 4-methyl substituent and no substituent at C-3. 

Alkylation of 4-methyl-3-pyrazolidinone with butyl ethylenesulfonate gave alkylation at N-l with addition of nitrogen at the /3-position of the a,/3-unsaturated system.1209 Earlier workers have claimed that nitrosation of 3-pyrazolidinones leads to either substitution at C-4446 or oxidation to 2-pyrazolin-5-ones followed by substitution at q 4 sis, 1550 However, later workers uniformly report that nitrosation occurs at N-l (eq. 247).911-1342,1491 Treatment of 3-pyrazolidinones... 

In the presence of sulfuric acid 1-nitroso-3-pyrazolidinones lose nitrous acid by elimination, giving 2-pyrazolin-5-ones which are then nitrosated at C-4.335,1028 The amino group in 4-amino-3-pyrazol-idinones is diazotizable and the resulting diazonium salt undergoes coupling.12... 

Owing to the active hydrogen atoms at C-4 in 3,5-pyrazolidinediones various substitutions at C-4 can be accomplished. Reaction with acid chlorides in the presence of aluminum chloride gives 4-acyl-3,5-pyrazolidinediones.919,976 However, if two groups are already present at C-4, the enolic form of the dione is acylated on the oxygen atom to give 3-acyloxy-2-pyrazolin-5-ones.998 Treatment of 3,5-pyrazolidinediones with nitrous acid gives nitrosation at C-4.976 The nitroso... 

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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