2-Dialkynyl derivatives of (N)-methanocarba nucleosides: ‘Clickable’ A3 adenosine receptor-selective agonists Original Research Article

We modified a series of (N)-methanocarba nucleoside 5′-uronamides to contain dialkyne groups on an extended adenine C2 substituent, as synthetic intermediates leading to potent and selective A3 adenosine receptor (AR) agonists. The proximal alkyne was intended to promote receptor recognition, and the distal alkyne reacted with azides to form triazole derivatives (click cycloaddition). Click chemistry was utilized to couple an octadiynyl A3AR agonist to azido-containing fluorescent, chemically reactive, biotinylated, and other moieties with retention of selective binding to the A3AR. A bifunctional thiol-reactive crosslinking reagent was introduced. The most potent and selective novel compound was a 1-adamantyl derivative (Ki 6.5 nM), although some of the click products had Ki values in the range of 200–400 nM. Other potent, selective derivatives (Ki at A3AR in nM) were intended as possible receptor affinity labels: 3-nitro-4-fluorophenyl (10.6), α-bromophenacyl (9.6), thiol-reactive isothiazolone (102), and arylisothiocyanate (37.5) derivatives. The maximal functional effects in inhibition of forskolin-stimulated cAMP were measured, indicating that this class of click adducts varied from partial to full A3AR agonist compared to other widely used agonists. Thus, this strategy provides a general chemical approach to linking potent and selective A3AR agonists to reporter groups of diverse structure and to carrier moieties.