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Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring. It is isomeric with two other forms of diazine.
Additional recommended knowledge
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.
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 of carbonyls with amines 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 .
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pyrimidine". A list of authors is available in Wikipedia.|