n.
Any of a class of heterocyclic compounds with a ring structure of four carbon and two nitrogen atoms.
The simplest member, pyrimidine itself (C 4 H 4 N 2 ), is not common, but derivatives with the structure are. Examples include thiamine (vitamin B 1 ), several sulfa drugs , barbiturates , and three of the bases in nucleic acids ( cytosine , thymine , and uracil ).