3- Pyrazolin-5-ones, rearrangement

The photorearrangement of pyrazoles to imidazoles is probably analogous, proceeding via iminoylazirines (82AHC(30)239) indazoles similarly rearrange to benzimidazoles (67HCA2244). 3-Pyrazolin-5-ones (56) are photochemically converted into imidazolones (57) and open-chain products (58) (70AHC(ll)l). The 1,2- and 1,4-disubstituted imidazoles are interconverted photochemically. 

Irradiation of 4-hydroxy- and 4-alkoxy-3-pyrazolin-5-one derivatives (163 R = OH, OR) leads to ring cleavage with the formation of /3-diamides (165) 69TL271). The methylene blue sensitized rearrangement of the same pyrazolinone (R = H) to the oxindole (166) also... 

Pyrazblin-5-one, 3-alkyl-(l,2,4-thiadiazol-5-yl)-reactions, 6, 483 2-Pyrazolin-5-one, 3-amino-tautomerism, 5, 215 2-Pyrazolin-5-one, 4,4-diazido-rearrangement, 5, 720 2-Pyrazolin-5-one, 3-hydroxy-tautomerism, 5, 215 2-Pyrazolin-5-one, 3-methyl-1 -phenyl-reactions, 5, 252... 

Cyclohexadiene 45 was converted to 46 by what has proven to be a general method for preparation of the cyclohexa-2,4-dien-l-one ring system.2 Fragmentation of the aziridinyl imine in 46 at 110 °C gave an intermediate diazoalkane which underwent an intramolecular 1,3-dipolar cycloaddition to give the pyrazoline 47. At 140 °C, pyrazoline 47 expelled N2 and rearranged to the tricyclic ketone 48. The development of this and related bicyclizations29 illustrated a practical synthetic equivalence of an intramolecular diene-carbene 4-1-1 cycloaddition in the cyclohexa-2,4-dien-l-one series. 

The formation of pyrazoline derivatives 175 (but not dihydrotetrazolotria-zepines 173, as assumed earlier [53]) is brought about by the condensation of chalcones with diaminotetrazole 172 [128,129]. The structure of the compounds 175 is convincingly verified using X-rays. The mechanism of their formation implies a Dimrothe rearrangement for either the initial diamine or for one of the cyclocondensation intermediates (Scheme 4.51). 

PyrazoUne-5-ones. Treatment of 2-bromo-l,3-diphenyl-l,3-propanedione (1) with methylhydrazine in ethanol at room temperature gives l-methyl-3,4-diphenyl-A -pyrazoline-5-one (2) in 56% yield. It is probably formed by a halohydrin rearrangement as indicated. 

Pyrazottne-5-ones Treatment of 5-hydroxy-3,5-diphenyl-A2-pyrazoline-4-one (l)withmethanolicpotassiumhydroxidegives4-hydroxy-3,4-diphcnyl-A -pyrazoline-5-one (2) in 76-80% yield. A pinacol rearrangement is probably involved. Compound (2)... 

The rearrangement of 4-dimethylamino-2,3-dimethyl-l-phenyl-3-pyrazolin-5-one to a 2-pyrazolin-5-one has already been mentioned (eq. 169). 4-Benzylideneamino-2,3-dimethyl-l-phenyl-3-pyrazolin-5-one reacts with dimethyl sulfate in aqueous media to give the 4-methyl-amino analog.940 Presumably the Schiff base is hydrolyzed and the resulting amino group is methylated. 4-Dimethylamino-3-pyrazolin-5-ones react with phosphorus oxychloride by conversion to the metho-chloride of a pyrazole.1000... 

The N-l acyl substituted 2-pyrazolin-5-ones react as do the other classes of 2-pyrazolin-5-ones since the hydrogen atoms at C-4 are active.519,1296 The jV-acyl substituents are readily removed by hydrolysis284,355 or treatment with aniline.6 Pyridine and acetic acid cause a rearrangement of iV-acetyl groups to the oxygen atom.1598... 

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... 

It is known that 4-benzal-l-phenyl-A -pyrazolin-5-one reacts both like an heterodiene and an alkene in a Diels-Alder reaction towards 2,3-dimethylbutadiene <88T5229>. The 1,3-dipolar cycloaddition of nitrones to the N//-tautomer of l-phenyl-3-methyl-3-pyrazolin-5-one yields a nonisolated cycloadduct which rearranges to a 4-substituted derivative of the starting material (4-substituent = CH(Ar)— N(Ph)—OH) <94JCR(S)154>. 

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. 

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