Pyrazolone

It can be viewed as a derivative of pyrazole possessing an additional carbonyl (C=O) group.

For pyrazolone derivatives, the 3-pyrazolone isomer can be stabilized with N-alkyl or N-aryl substituents.

The first synthesis of pyrazolones was reported in 1883 by Ludwig Knorr, via a condensation reaction between ethyl acetoacetate and phenylhydrazine.

Pyrazolones are amongst the oldest synthetic pharmaceuticals, starting with the introduction of antipyrine (phenazone) in 1880s.

[3][4] The compounds generally act as analgesics and include dipyrone (Metamizole), aminophenazone, ampyrone, famprofazone, morazone, nifenazone, piperylon and propyphenazone.