Pyrazolone

Pyrazolone is 5-membered heterocycle containing 2 adjacent nitrogen atoms. It can be viewed as a derivative of pyrazole possessing an additional keto (=O) group. Pyrazolone is 5-membered heterocycle containing 2 adjacent nitrogen atoms. It can be viewed as a derivative of pyrazole possessing an additional keto (=O) group. 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 synthesis of pyrazolones was first 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 1880's. The compounds generally act as analgesics and include dipyrone (Metamizole), aminopyrine, ampyrone, famprofazone, morazone, nifenazone, piperylon and propyphenazone. Of these dipyrone is perhaps the most widely used. 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 (tartrazine, orange B, mordant red 19, yellow 2G). Pyrazolones have been studied as ligands.