5-Imino-2-pyrazolin-3-carboxylic acids

The decarboxylation of l,3-diphenyl-5-imino-2-pyrazolin-3-carboxylic acid gives l,3-diphenyl-5-imino-2-pyrazoline.506 Druey and Schmidt391 have taken advantage of the selective replacement of an oxo group by chlorine over that of an imino group to prepare 5-imino-2-pyrazolines (eq. 95). 

Cusmano340 341,345 has converted 5-alkyl- and 5-arylisoxazole-3-carboxylic acids into 5-imino-2-pyrazolines by treatment with phenyl hydrazine. Musante has also used this procedure.1043-1045 This reaction goes by way of /J-ketonitrile intermediates and gives very poor yields.86... 

For purposes of classification the 4-aminopyrazoles are considered to be 4-imino-2-pyrazolines and analogs of 2-pyrazolin-4-ones. These compounds are listed in Table XL. Such compounds can be prepared by direct cyclization using ethyl diazoacetate and ethyl cyanoacetate.92 This is the same as eq. 243, except that the malonic ester is replaced by ethyl cyanoacetate. Purines can be hydrolyzed to 4-imino-2-pyrazolines by using strong acid.1210 1846 By far the most frequently used preparation is reduction of appropriately substituted pyrazoles, such as 4-nitro,368,812,819,1015,1019,1049 4-nitroso1165 or 4-aryl-azo.671 974,995 The hydrolysis of the carbethoxy 4-imino-2-pyrazolines derived from ethyl cyanoacetate and ethyl diazoacetate forms 4-imino-2-pyrazolin-3-carboxylic acid which is readily decarboxylated to the parent compound.92... 

One of the most widely used methods for the preparation of 5-imino-3-pyrazolidinones is from 5-oxo-3-pyrazolin-3-carboxylic acids or their derivatives by use of the Hofmann167,697 or Curtius167,1387,1395 rearrangements (eq. 264). A similar synthesis is from the analogous... 

Bromo-5-imino-3-pyrazolidinones are synthesized by Curtius rearrangement of the azides of 4-bromo-5-oxo-3-pyrazolin-3-carboxylic acids.697 4-Nitroso-5-imino-3-pyrazolidinones are believed to exist as the oximino tautomers.337 They are usually prepared by direct nitro-sation with nitrous acid.644-1339 1649 A Curtius rearrangement of 4-oximino-5-oxo-2-pyrazolin-3-carboxylic acid azide has also been used.337... 

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