126 Purines
Regiospecifi c 7-alkylations can be achieved via the quaternisation of a 9-riboside followed by hydrolytic removal of
the sugar unit.
In the context of synthesis of nucleosides and analogous compounds, the N-ribosylation (and N-deoxyribosylation) of
purines has been much studied. Such ‘alkylations’ must address not only regioselectivity (N-7/N-9) on the purine, but
also the formation of the required epimer at the linking C-1 of the sugar. Each case must be taken on its merits because
there are no conditions that are universally effective. These alkylations often employ halo-ribosides in conjunction
with a purine derivative of mercury, silicon or sodium, and stereoselective displacements of halide can sometimes be
achieved.
Another useful method, which gives very high stereoselectivity, involves anchimeric assistance by a 2-benzoate.
N-9 Arylation of purine can be achieved with copper catalysis.
Oxidation
Chemical oxidation of purines usually gives N-oxides but C-8-oxidation is an important biological process medi-
ated, for example, by the oxo-transferase enzyme, xanthine oxidase, which converts hypoxanthine and xanthine into
uric acid (see page 159).
Nucleophilic substitution
Purines are susceptible to nucleophilic substitution, which is of particular relevance in the displacement of a halide,
and this is probably the most widely used reaction for manipulation of purines. A wide range of nitrogen, oxygen,
sulfur and carbon nucleophiles can be employed, and the reaction occurs via an addition–elimination mechanism
as in similar displacements in pyridines. Many different leaving groups have been used on purines, and the interplay