Replacing one carbon with a nitrogen yields pyridine . Top of page Annulenes of other Ring Sizes Just as one or more carbons in a benzene can be replaced by heteroatoms, so can the carbons of non-benzenoid systems, such as the cyclopentadiene anion and the tropylium cation be similarly replaced to give heteroaromatic systems of ring size other than six. Replacement of a carbon of furan with nitrogen generates oxazole or isoxazole depending at which position the nitrogen is inserted. Similarly, replacement of a thiophene carbon atom by nitrogen generates thiazole or isothiazole . They are said to be condensed or fused.
Systems containing a number of heteroatoms are also common.
These include benzimidazole , benzoxazole and benzthiazole , and their positional isomers benzpyrazole , benzisoxazole and benzisothiazole .