Everything about Pyrimidines totally explained
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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.
Nucleotides
three
nucleobases found in
nucleic acids, namely
cytosine,
thymine, and
uracil, 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.
Chemical properties
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.
Organic synthesis
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 .
A novel method is by reaction of certain
amides with
carbonitriles under electrophilic activation of the amide with 2-chloro-pyridine and
trifluoromethanesulfonic anhydride :
» Further Information
Get more info on 'Pyrimidines'.
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