(3R,4S,beta-S)-13-fluoro ohmefentanyl is a mu-selective opioid with an in vivo analgesic/psychoactive potency measured as being almost 18,000 times that of morphine. Its ED50 in mice is about 800 nanograms/kg. As Professor Q notes, "there is a good chance that this compound could be made more powerful still, by addition of an alpha-methyl substituent. Although unlikely to increase affinity as such, it would greatly "harden" the agent against metabolic attack, since, as is the case with fentanyl, I would assume N-dealkylation of the piperidine to be the predominant pathway of liver microsomal breakdown. Even if ED50 doesn't go any lower, one would expect duration of action to increase by a factor of three or so...."
Just For Chemists
Stereoisomers of cis-fluoro-ohmefentanyl (PDF)