Heterocyclic Chemistry at a Glance, Second Edition. John A. Joule and Keith Mills.
© 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd.
The diazines – pyridazine, pyrimidine and pyrazine – show many similarities to pyridines with respect to their reacti-
vities, but to an exaggerated degree. In particular, the presence of two nitrogens, both as imines, results in increased
electron defi ciency at carbon and hence an increased susceptibility to nucleophilic addition but an increased resistance
to electrophilic attack.
As is also the case for the purines (Chapter 13), there has been emphasis on the study of amino- and oxy-pyrimidines
(uracil, thymine and cytosine) due to their importance as constituents of the nucleic acids DNA and RNA (see pages
163–164), where they occur in nucleotide polymers. These three pyrimidines, and their nucleosides, can be isolated from
some natural sources, but they are readily available in quantity via fermentation (with or without further chemical modi-
fi cation) or direct chemical synthesis – uracil in particular, as would be expected for this simple compound, is very cheap.
It should be noted that 2- and 4-amino-pyrimidines exist as the amino tautomers; 2- and 4-oxy-pyrimidines exist as
carbonyl tautomers. These tautomeric preferences are essential for their role in DNA, where hydrogen bonding holds
the two chains of the double helix together (see page 164).