Hydroxylamines reaction with 3-pyrazolin-5-ones

A number of syntheses of compounds having nitrogen on a carbon attached to 2-pyrazolin-5-one rings have already been mentioned, but several others are known. The most important of these is the reaction of 4-formyl- or 4-acyl-2-pyrazolin-5-ones with hydroxylamine, hydrazines, semicarbazide and similar chemicals.559 1841 These compounds can also be obtained by direct synthesis from an appropriate /3-ketoester or derivative and a hydrazine (eq. 50).138 512-1100 An interesting syn-... 

The most frequently used method of preparation of compounds having nitrogen attached to a carbon atom which is substituted on the 3-pyrazolin-5-one ring is reaction of hydroxylamine, amines, hydrazines, hydrazides, etc., with 5-oxo-3-pyrazolin-4-carboxaldehydes. 

The formation of pyrylium salts from methyl(ene) ketones and 1,3-diketones, e.g., of 2,4,6-triphenylpyrylium from acetophenone and dibenzoylmethane, has an interesting counterpart in several reactions of pyrylium salts. With nucleophiles like phenylhydrazine,353 hydroxylamine,353 or benzylmagnesium chloride,178-180 2,4,6-triphenylpyrylium forms unstable 2,4-dien-l-ones or 4 -pyrans, which rearrange easily to more stable pyrazolines, isoxazolines, or 2H-pyrans on treatment with strong acids, both types of product split off acetophenone, yielding 1,3,5-triphenylpyrazole, 3,5-diphenyl-isoxazole, and 1,3-diphenylnaphthalene, respectively. These same products can be obtained directly from the above nucleophiles and dibenzoylmethane.63,354-357 The condensation of methyl(ene) ketones with 1,3-diketones yielding pyrylium salts can, therefore, be considered reversible in a certain sense. 

Formation of Pyrazoles from Bis(hydrazones).—Mesoxaldehyde bis(phenylhydrazone) (193), obtained by periodate oxidation of saccharide osazones (192) is readily cyclized in the presence of acids to give l-phenyl-4-phenylazo-pyrazole (195).162 365 Hexulose phenylosazones (192) are also disproportionaled in the presence of acidic salts of carbonyl reagents, such as hydroxylamine hydrochloride, to give l-phenyl-4-phenylazo-pyrazolin-5-one (196). The reaction probably proceeds via mesoxalic acid 1,2-bis(phenylhydrazone) (194).365 The hydroxalkyl derivatives of 196 are produced from dehydroascorbic acid bis(phenylhydrazone) by treatment with base to open the lactone ring and permit the conversion of 197 to 199.351,366 Another type of pyrazole that is formed by dehydrating osazones with acetic anhydride is discussed later under anhydroosazones (see Schemes 45,53). 

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