Discovery of New Human A(2A) Adenosine Receptor Agonists: Design, Synthesis, and Binding Mode of Truncated 2-Hexynyl-4'-thioadenosine.

The truncated C2- and C8-substituted 4′-thioadenosine derivatives 4a−d were synthesized from d-mannose, using palladium-catalyzed cross-coupling reactions as key steps. In this study, an A3 adenosine receptor (AR) antagonist, truncated 4′-thioadenosine derivative 3, was successfully converted into a potent A2A AR agonist 4a (Ki = 7.19 ± 0.6 nM) by appending a 2-hexynyl group at the C2-position of a derivative of 3 that was N6-substituted. However, C8-substitution greatly reduced binding affinity at the human A2A AR. All synthesized compounds 4a−d maintained their affinity at the human A3 AR, but 4a was found to be a competitive A3 AR antagonist/A2A AR agonist in cyclic AMP assays. This study indicates that the truncated C2-substituted 4′-thioadenosine derivatives 4a and 4b can serve as novel templates for the development of new A2A AR ligands.