1-phenylpyrazolo[3,4-d]pyrimidines as adenosine antagonists: The effects of substituents at C4 and C6

Forty-two 1-phenyl-pyrazolo[3,4-d]pyrimidines substituted at C6 with thioethers containing distal amide substituents and substituted at C4 with thiol, thiomethyl or amino were synthesized and tested for adenosine A(1) and A(2a) receptor binding. Compared with a thiol at C4, both S-methylation and conversion to an amino resulted in increased affinity at both receptors with the C4 amino compounds having the highest affinity. The C-4 region of the receptor consists of an alkyl pocket containing a hydrogen-bonding site. The study established that for high affinity at both the A(1) and A(2a) adenosine receptors the distal amide should be separated from the C6 thiol by only one carbon. In this study, 2'-(4-amino-1-phenylpyrazolo[3,4-d]pyrimidin-6-ylthio)- N-ethyl-ethanamide (4b) had the highest affinity at the A(1) receptor with a K-i of 12.1 nM while 2'-(4-amino-1-phenylpyrazolo[3,4-d]pyrimidin-6-ylthio)ethanamide (4a) had the highest affinity at the A(2a) receptor with a K-i of 44.9 nM. (C) 1997, Elsevier Science Ltd.