Pyrazolone

Pyrazolone refers to a class of heterocyclic compounds based on the pyrazolone ring, a five-membered ring containing two adjacent nitrogens and a carbonyl group at the third position. The core motif is often described as a 3-pyrazolone or 4-pyrazolone ring; many derivatives exist in which various substituents are attached to N1, N2, C4, and C5.

Chemically, pyrazolones are prepared by cyclization of 1,3-dicarbonyl substrates with hydrazines and can be further derivatized to yield a wide range of compounds. They are studied as analgesics, antipyretics, and anti-inflammatory agents, with several clinically used drugs belonging to this class.

Notable examples include metamizol (dipyrone) and phenazone (antipyrine), historically used for fever and pain relief. Other derivatives show anti-inflammatory activity and have been investigated for various pharmacological effects.

Regulatory status varies by country. Some pyrazolone drugs have been withdrawn from or restricted in several markets due to safety concerns, especially rare but serious agranulocytosis and other hematologic effects. Modern use tends to be limited and tightly regulated, often with monitoring requirements.

Outside medicine, the pyrazolone ring is a synthetic scaffold in medicinal chemistry and may be found in intermediates for organic synthesis.