Copper-Catalyzed Oxidative [3 + 2]-Annulation of Quinoxalin-2(1H)-one with Oxime Esters toward Functionalized Pyrazolo[1,5-a]quinoxalin-4(5H)-ones as Opioid Receptor Modulators

Pyrazolo[1,5-a]quinoxalin-4(5H)-one derivatives as novel opioid receptor modulators have been synthesized via copper-catalyzed oxidative [3 + 2]-annulation of quinoxalin-2(1H)-one and oxime-O-acetates. This hydrazine-free C–C and N–N bond formation strategy starts with the generation of C2N1 synthon using oxime acetate, which reacts in a [3 + 2] manner with quinoxalin-2(1H)-one, followed by oxidative aromatization. The synthesized compounds were tested against opioid receptors, of which eight compounds exhibited an antagonistic effect with EC50 < 5 μM at various opioid receptors. Molecular docking studies were performed to identify the binding of active pyrazolo[1,5-a]quinoxalin-4(5H)-one ligands with hKOR protein. Docking results indicated that compounds 3d and 3g participate in hydrogen bonding with the hydroxyl group of T111 of the active site pocket residue.