Purines 129Conversion of oxygen into leaving groups
As discussed previously, nucleophilic displacement of leaving groups is a very important reaction of purines.
Oxopurines are the prime sources for preparation of the substrates for these reactions, being readily converted into
chloro-compounds by replacement of the oxygen, including even the fully oxygenated purine, uric acid. Alternatively,
an oxygen can be activated by conversion into an acyloxy or sulfonyloxy derivative.
Replacement by sulfur
Replacement of oxygen by sulfur can be achieved directly using a phosphorus sulfi de, or by nucleophilic substitution
after conversion into a leaving group. The thiopurines are of interest in their own right but are also a potential precur-
sor to sulfones, which are highly reactive towards nucleophilic displacement (page 127).
Aminopurines
Alkylation
As discussed previously, alkylation under neutral conditions results in reaction at nuclear nitrogen and the product
can then be converted into a side-chain alkylamino purine by means of the Dimroth rearrangement (see page 127).
An alternative direct method for mono-alkylation of an amino group is by reductive alkylation using an aldehyde and
a reducing agent.
Diazotisation
The reaction of purine 2- and 6-amino groups with nitrous acid is similar to that of 2-aminopyridines, in that the
diazonium salts are relatively unstable when compared with phenyl diazonium salts but can, nevertheless, be utilised
for the introduction of groups such as halide. 8-Diazonium salts are considerably more stable due to mesomeric inter-
action with the electron-rich ring. Diazotisation can also be carried out under basic conditions when acid-sensitive
groups such as (deoxy)ribosides are present.