Nitrogen Heterocycles

5-Membered rings

With heterocycles containing five atoms, the unsaturated compounds are frequently more stable because of aromaticity.Five-membered rings with a single heteroatom:

Heteroatom	Saturated	Unsaturated
Nitrogen	Azolidine	Pyrrole
Oxygen	Oxolane	Furan
Sulfur	Thiolane	Thiophene
Phosphorus	Phospholane	Phosphole
Silicon	Silolane	Silole
Arsenic	Arsolane	Arsole

The 5-membered ring compounds containing two heteroatoms, at least one of which is nitrogen, are collectively called the azoles. Thiazoles and isothiazoles contain a sulfur and a nitrogen atom in the ring. Dithiolanes have two sulfur atoms.

Heteroatom	Saturated	Unsaturated
Nitrogen	Imidazolidine Pyrazolidine	Imidazole Imidazoline Pyrazole Pyrazoline
Nitrogen/oxygen	Oxazolidine Isoxazolidine	Oxazole Oxazoline Isoxazole Isoxazoline
Nitrogen/sulfur	Thiazolidine Isothiazolidine	Thiazole Thiazoline Isothiazole Isothiazoline
Oxygen	Dioxolane
Oxygen/sulfur	Oxathiolane
Sulfur	Dithiolane

A large group of 5-membered ring compounds with three heteroatoms also exists. One example is dithiazoles that contain two sulfur and a nitrogen atom.

Heteroatom	Saturated	Unsaturated
Nitrogen		Triazoles
Nitrogen/2-sulfur		Dithiazole
2-Nitrogen/oxygen		Furazan Oxadiazole
2-Nitrogen/sulfur		Thiadiazole

Five-member ring compounds with four heteroatoms:

Heteroatom	Saturated	Unsaturated
Nitrogen		Tetrazole

With 5-heteroatoms, the compound may be considered inorganic rather than heterocyclic. Pentazole is the all nitrogen heteroatom unsaturated compound.
Literature