Pyrazolin-5-ones condensation with ketones

Condensation with Carbonyl Groups The hydrogen atom at the 4-position is sufficiently activated for 5-hydroxypyrazoles (pyrazolin-5-ones) to undergo condensation with ketones in acid media to yield 4-alkylidenepyrazolin-5-ones (61) in alkali, dipyrazolinonylmethane (62) derivatives are formed.644,645... 

Aldehydes and ketones react with azolinones. The reaction between aldehydes and 2-phenyl-5-oxazolinone (131 Y = H), formed in situ from PhC0NHCH2C02H and AC2O, gives azlactones (131 Y = RCH). Similar reactions are given by 4-thiazolidinones, e.g. (132) gives (133) (79AHC(25)83), and 4-imidazolinones. In pyrazolin-5-ones the 4-position is sufficiently activated for condensation to occur with ketones in acidic media (Scheme 8) (66AH06)347). 

If a,j8-unsaturated ketones are condensed with 2-pyrazolin-5-ones the reaction is a Michael addition (eq. 30)686 721 resulting in alkylation. 

Presumably the linking group is >NCH—N<. Bisamides of l-(amino-phenyl)-2-pyrazolin-5-ones and of 3-amino-2-pyrazolin-5-ones have been used. l-(3-Aminophenyl)-3-methyl-2-pyrazolin-5-one reacts with many aldehydes and ketones to form polymers.964 Probably condensation occurs at C-4 and the amino group. The product of reaction of p-cresol with formaldehyde has been found to react with 1-(4-hydroxy -phenyl)-3-methyl-2-pyTazolin-5-one to give a polymer that must have the aromatic and pyrazolinone rings connected by ether linkages.1269... 

The reactions undergone by 5-imino-3-pyrazolidinones are in general very similar to those of 2-pyrazolin-5-ones. The hydrogen atoms at C-4 are active and this position is the reactive center in the nucleus of these molecules. Condensation with aldehydes and ketones occurs with formation of benzylidene644,1083 or 4,4 -benzylidenebis... 

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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