Synthesis of a Novel Series of Tricyclic Dihydrofuran Derivatives: Discovery of 8,9-Dihydrofuro[3,2-c]pyrazolo[1,5-a]pyridines as Melatonin Receptor (MT1/MT2) Ligands

Novel tricyclic dihydrofuran derivatives were designed, synthesized, and evaluated as melatonin receptor (MT1/MT2) ligands based on the previously reported 1,6-dihydro-2H-indeno[5,4-b]furan 1a. By screening the central tricyclic cores, we identified 8,9-dihydrofuro[3,2-c]pyrazolo[1,5-a]pyridine as a potent scaffold with a high ligand-lipophilicity efficiency (LLE) value. Subsequent optimization of the side chains led to identification of the potent MT1/MT2 agonist 4d (MT1, K i = 0.062 nM; MT2, K i = 0.420 nM) with good oral absorption and blood–brain barrier (BBB) penetration in rats. The oral administration of compound 4d exhibited a sleep-promoting action in freely moving cats at 0.1 mg/kg.