Azole

Azoles are a class of five-membered heterocyclic compounds containing a nitrogen atom and at least one other non-carbon atom (i.e. nitrogen, sulfur, or oxygen) as part of the ring. Their names originate from the Hantzsch–Widman nomenclature. The parent compounds are aromatic and have two double bonds; there are successively reduced analogs (azolines and azolidines) with fewer. One, and only one, lone pair of electrons from each heteroatom in the ring is part of the aromatic bonding in an azole. Names of azoles maintain the prefix upon reduction (e.g., pyrazoline, pyrazolidine). The numbering of ring atoms in azoles starts with the heteroatom that is not part of a double bond, and then proceeds towards the other heteroatom.

Imidazole and other five-membered aromatic heterocyclic systems with two nitrogens are extremely common in nature and form the core of many biomolecules, such as histidine.

Imidazole

Pyrazole

1,2,3-Triazole

1,2,4-Triazole

Tetrazole

Pentazole

Oxazole

Isoxazole

1,2,3-oxadiazole
(unstable)

Oxadiazole
(1,2,4-Oxadiazole)

Furazan
(1,2,5-oxadiazole)

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