C-alkylated spiro[benzofuran-3(2H),4'-1'-methyl-piperidine-7-ols] as potent opioids: a conformation-activity study

Among a series of C-alkylated analogs of the weak mu opioid ligand spiro[benzofuran-3(2H),4'-1'-methylpiperidine-7-ol] (1), the 2-methyl, 2-ethyl, and cis 3'-methyl analogs, namely compounds (+/-)2, (+/-)-3, and (+/-)-4, showed much enhanced mu-affinities, with (+/-)-4 being almost as potent as (-)-morphine; while the trans 3'-methyl analog (+/-)-5 remained a weak mu-binder. Energy calculations and nmr data indicated that compounds 2-4 favor phenyl-axial conformations, while compounds 1 and 5 favor phenyl-equatorial conformations.