Pyrazolone


Pyrazolone is 5-membered heterocycle containing 2 adjacent nitrogen atoms. It can be viewed as a derivative of pyrazole possessing an additional carbonyl group. Compounds containing this functional group are useful commercially.

Structure and synthesis

Pyrazolone can exist in 3 isomers: 3-pyrazolone, 4-pyrazolone, and 5-pyrazolone. These isomers can interconvert via lactam–lactim and imine–enamine tautomerism; these conversion often display photochromism. For pyrazolone derivatives, the 5-pyrazolone form 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.
Many pyrazolones are produced by functionalization of preformed pyrazolones.

Applications

Pharmaceuticals

Pyrazolones are amongst the oldest synthetic pharmaceuticals, starting with the introduction of antipyrine in 1880s. The compounds generally act as analgesics and include dipyrone, aminopyrine, ampyrone, famprofazone, morazone, nifenazone, piperylon and propyphenazone, aminophenazone. Of these dipyrone is perhaps the most widely used. Edaravone is useful for prevention and/or therapy of arterial wall injury. Eltrombopag is used to address low blood platelet count.

Dyes

Pyrazolone groups are present in several important dyes. They are commonly used in combination with azo groups to give a sub-family of azo dyes; sometimes referred to as azopyrazolones. Acid Yellow 17, Acid Yellow 23, Pigment Yellow 13, and Pigment Red 38 are produced on the multi-ton scale annually.

Ligands

Pyrazolones have been studied as ligands.