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.
Purines are of great interest for several reasons, but in particular, together with certain pyrimidines, they are compo-
nents of DNA and RNA, the genetic templates of all life (pages 163–164), and also serve in a range of other biological
roles. A corollary of this central biological role is the signifi cance of purines and their analogues in a number of areas
of medicinal chemistry.
Chemically, there are signifi cant lessons to be learnt from the study of purines since their reactions exemplify the
interplay of the constituent imidazole and pyrimidine rings, just as the reactions of indole show modifi ed pyrrole and
modifi ed benzene chemistry. The pyrimidine contributes an electron-poor and the imidazole an electron-rich unit to
the structure and chemistry. Although purines can theoretically exist as four tautomers, only the 7H- and 9H-forms
are of signifi cance.
In their biological roles, purines (and pyrimidines) occur mainly as oxy- and amino-derivatives, with a ribose or
2-deoxyribose residue attached to N-9. When the ribose residue is a simple sugar, the compounds are referred to as
‘nucleosides’, and when a phosphate is also attached to the sugar, they are called ‘nucleotides’. (Note: The nucleosides
and nucleotides are drawn with the purine above the ribose to save space. In reality it would normally be rotated away
from the ribose.)
Other than their occurrence in the polymers DNA and RNA, their other biological roles are fulfi lled as monomers.
The nucleoside adenosine is an important hormone and neurotransmitter, and is of medical use for the treatment of
some cardiovascular conditions. Adenine/adenosine and guanine/guanosine are constituents of DNA and RNA, while
hypoxanthine/inosine is present in tRNA and is also the biosynthetic precursor for the other purine nucleosides.