3-Alkyl-1 -phenyl-2-pyrazolin-5-ones

The addition of (chlorocarbonyl)phenyl ketene 126 to 3-alkyl-1-phenyl-2-pyrazolin-5-ones 141 resulted in 4-hydroxy-3-alkyl-l,... 

A mechanism has been proposed to rationalize the results shown in Figure 23. The relative proportion of the A -pyrazolines obtained by the reduction of pyrazolium salts depends on steric and electronic effects. When all the substituents are alkyl groups, the hydride ion attacks the less hindered carbon atom for example when = Bu only C-5 is attacked. The smaller deuterohydride ion is less sensitive to steric effects and consequently the reaction is less selective (73BSF288). Phenyl substituents, both on the nitrogen atom and on the carbon atoms, direct the hydride attack selectively to one carbon atom and the isolated A -pyrazoline has the C—C double bond conjugated with the phenyl (328 R or R = Ph). Open-chain compounds are always formed during the reduction of pyrazolium salts, becoming predominant in the reduction of amino substituted pyrazoliums. 

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... 

The alkylation of 4-nitro-2-pyrazolin-5-ones at C-4 with diazomethane478 has already been mentioned. This hydrogen atom is also replaceable with bromine.1642 Reduction of the nitro group with stannous chloride forms the corresponding amine.478,480 Picrolonic acid condenses with itself to form a bis(2-pyrazolin-5-one) connected by a double bond at the 4,4 -position, with elimination of two moles of nitrous acid.690 Condensation also occurs in the presence of phenyl-hydrazine, but 4,4 -bis[3-methyl-l-(4-nitrophenyl)-2-pyrazolin-5-one] is formed.690... 

The alkylation of 4-amino-3-pyrazolin-5-ones has already been discussed to a considerable extent in connection with the preparation of such compounds having two substituents on the extranuclear nitrogen atom (see p. 76). The most straightforward alkylation is that with alkyl halides.101,431 Alkylation by means of formation of the Schiff base with aldehydes or ketones followed by catalytic reduction has frequently been used.111,131,1303 Treatment of 4-amino-2,3-dimethyl-l-phenyl-3-pyrazolin-5-one with formaldehyde and sodium bisulfite in 5 ... 

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