Discovery of a New Nucleoside Template for Human A3 Adenosine Receptor Ligands: <scp>d</scp>-4‘-Thioadenosine Derivatives without 4‘-Hydroxymethyl Group as Highly Potent and Selective Antagonists

Truncated D-4&#39;-thioadenosine derivatives lacking the 4&#39;-hydroxymethylene moiety were synthesized starting from D-mannose, using cyclization to the 4-thiosugar and one-step conversion of the diol to the acetate as key steps. At the human A3 adenosine receptor (AR), N6-substituted purine analogues bound potently and selectively and acted as antagonists in a cyclic AMP functional assay. An N6-(3-chlorobenzyl)purine analogue 9b displayed a Ki value of 1.66 nM at the human A3 AR. Thus, truncated D-4&#39;-thioadenosine is an excellent template for the design of novel A3 AR antagonists to act at both human and murine species.