conditions. Thus, the A 1 adenosine receptors agonist selodenoson is
obtained ( 59 ).^9
NN NNHOOHOO54NN NNHO 2 COHC 2 H 5 OCO OCOC 2 H 555- SO 2 Cl
2. C 2 H 5 OH
NN NNOHC 2 H 5 OCO OCOC 2 H 556O OPOCl 3N
N NNClC 2 H 5 OCO OCOC 2 H 5O ONN NNNHC 2 H 5 OCO OCOC 2 H 5O O57NH 258CH 3 CH 2 NH 2N
N NNNHHO OHHN O59Yet another modification leading to an adenosine agonist involves con-
version of one of the amino groups on the fused pyrimidine ring of adeno-
sine to a pyrazole. The synthesis begins with the conversion of guanosine
to its 5^0 acetate by reaction with acetic anhydride. The hydroxyl at the
4 position is replaced by chlorine in the usual manner by treatment with
phosphorus oxychloride to afford 61. In a variation on the Sandmeyer reac-
tion, this last intermediate is allowed to react with amyl nitrite in the pre-
sence of methylene iodide in an aprotic solvent. The low concentration of
nitrite ion from decomposition of its amyll ester serves to diazotize the
amine; the diazonium intermediate then captures iodine from the other
reagent to form the iodo derivative ( 62 ). Reaction of 62 with ammonia
interestingly proceeds selectively at the 4 position on the purine to
afford 63. Treatment of this product with hydrazine, presumably under
more strenuous conditions, displaces iodine to form 64. Condensation of
64 with carbethoxymalonaldehyde ( 65 ) then affords the pyrazole ring,
196 BICYCLIC FUSED HETEROCYCLES