Synthesis and evaluation of adenosine derivatives as A, A, A and A adenosine receptor ligands containing boron clusters as phenyl isosteres and selective A agonists.

A series of adenosine and 2'-deoxyadenosine pairs modified with a 1,12-dicarba-closo-dodecaborane cluster or alternatively with a phenyl group at the same position was synthesized, and their affinity was determined at A, A, A and A adenosine receptors (ARs). While AR affinity differences were noted, a general tendency to preferentially bind A AR over other ARs was observed for most tested ligands. In particular, 5'-ethylcarbamoyl-N-(3-phenylpropyl)adenosine (18), N-(3-phenylpropyl)-2-chloroadenosine (24) and N-(3-phenylpropyl)adenosine (40) showed nanomolar A affinity (K 4.5, 6.4 and 7.5 nM, respectively). Among the boron cluster-containing compounds, the highest A affinity (K 206 nM) was for adenosine derivative 41 modified at C2. In the matched molecular pairs, analogs bearing boron clusters were found to show lower binding affinity for adenosine receptors than the corresponding phenyl analogs. Nevertheless, interestingly, several boron cluster modified adenosine ligands showed significantly higher A receptor selectivity than the corresponding phenyl analogs: 7vs. 8, 15vs. 16, 17vs. 18.