3- Pyrazolin-5-ones dimerization

Acyl-3-pyrazolin-5-ones dimerize by an aldol condensation.901... 

Pyrazolones show a great variety of reactions with carbonyl compounds (B-76MI40402). For instance, antipyrine is 4-hydroxymethylated by formaldehyde and it also undergoes the Mannich reaction. Tautomerizable 2-pyrazolin-5-ones react with aldehydes to yield compound (324) and with acetone to form 4-isopropylidene derivatives or dimers (Scheme 8 Section 4.02.1.4.10). 

Cycloadditions are in general an effective way of constructing cyclobutane rings. A wide variety of heterocyclic systems dimerize in this way. 1,3-Diacetylindole, for example, affords the head-to-tail dimer 242 on irradiation in ethanol.185 Ethyl 2-ethoxy-l,2-dihydroquinoline-l-carboxy-late is similarly converted in diethyl ether into the trans head-to-head dimer.186 Notable among many analogous photodimerizations are those reported in 1,4-dihydropyridines,187 in furo[3,2-b]pyridin-2(4//)-ones,188 in 8-methyl-s-triazolo[4,3-a]pyridine,189 and in 2H-2-benzazepine-1,3-diones.190 The [ 2 + 2] dimerization of amidopyrine is the first reported example of a photocycloaddition in a 4-pyrazolin-3-one.191... 

Isoxazolin-5-ones and pyrazolin-5 ones have been shown to give the anhydro dimers (140a and b, respectively), when treated with a base such as piperidine.191,259... 

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