Various purine-based nucleoside analogues have demonstrated unexpected affinity for nonpurinergic G protein-coupled receptors (GPCRs), such as opioid and serotonin receptors. In this work, we synthesized a small library of new 7-deazaadenosine and pyrazolo[3,4-d]pyrimidine riboside analogues, featuring dual C7 and N6 modifications and assessed their affinity for various GPCRs. During the course of the synthesis of 7-ethynyl pyrazolo[3,4-d]pyrimidine ribosides, we observed the formation of an unprecedented tricyclic nucleobase, formed via a 6-endo-dig ring closure. The synthesis of this tricyclic nucleoside was optimized, and the substrate scope for such cyclization was further explored because it might avail further exploration in the nucleoside field. From displacement experiments on a panel of GPCRs and transporters, combining C7 and N6 modifications afforded noncytotoxic nucleosides with micromolar and submicromolar affinity for different GPCRs, such as the 5-hydroxytryptamine (5-HT)2B, κ-opioid (KOR), and σ1/2 receptor. These results corroborate that the novel nucleoside analogues reported here are potentially useful starting points for the further development of modulators of GPCRs and transmembrane proteins.
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