is a heterocyclic aromatic organic compound
similar to benzene
, containing two nitrogen atoms
at positions 1 and 3 of the six-member ring. It is isomeric
with two other forms of diazine
found in nucleic acids
) are pyrimidine derivatives:
In DNA and RNA, these bases form hydrogen bonds with their complementary purines. Thus the purines adenine (A) and guanine (G) pair up with the pyrimidines thymine (T) and cytosine (C), respectively.
In RNA, the complement of A is U instead of T and the pairs that form are adenine:uracil and guanine:cytosine.
These hydrogen bonding modes are for classical Watson-Crick base pairing. Other hydrogen bonding modes ("wobble pairings") are available in both DNA and RNA, although the additional 2'-hydroxyl group of RNA expands the configurations through which RNA can form hydrogen bonds.
A pyrimidine has many properties in common with pyridine
, as the number of nitrogen atoms in the ring increases the ring pi electrons
become less energetic and electrophilic aromatic substitution
gets more difficult while nucleophilic aromatic substitution
gets easier. An example of the last reaction type is the displacement of the amino
group in 2-aminopyrimidine by chlorine and its reverse. Reduction in resonance stabilization
of pyrimidines may lead to addition and ring cleavage reactions rather than substitutions. One such manifestation is observed in the Dimroth rearrangement
Compared to pyridine, N-alkylation and N-oxidation is more difficult, and pyrimidines are also less basic: The pKa value for protonated pyrimidine is 1.23 compared to 5.30 for pyridine.
Pyrimidines can also be prepared in the laboratory by organic synthesis
. One method is the classic Biginelli reaction
. Many other methods rely on condensation
for instance the synthesis of 2-Thio-6-methyluracil from thiourea
and ethyl acetoacetate
or the synthesis of 4-methylpyrimidine with 4,4-dimethoxy-2-butanone and formamide
A novel method is by reaction of certain amides with carbonitriles under electrophilic activation of the amide with 2-chloro-pyridine and trifluoromethanesulfonic anhydride :