Synthesis of 2-chloro-N6-substituted-4'-thioadenosine-5'-N, N-dialkyluronamides as potent and selective A3 adenosine receptor antagonists

The highly selective A(3) receptor agonist, 4'-thio-Cl-IB-MECA was successfully converted into selective A(3) receptor antagonists by appending a second N-alkyl group on the 5'-uronamide position. This result indicates that the hydrogen bonding ability of the 5'-uronamide is essential for the conformational change required for the receptor activation. Among compounds tested, a N(6)-(3-bromobenzyl) derivative with 5'-dimethyluronamide exhibited the highest binding affinity (K(i) = 9.32 nM) at the human A(3) AR with very low binding affinities to other AR subtypes.