Picenadol is an effective analgesic with similar efficacy to pethidine (meperidine). It has been investigated for some applications such as obstetrics and dentistry, but never commercialised.
It is unusual in that one enantiomer is a pure μ-opioidagonist, while the other is an antagonist. The (3R,4R) isomer is the agonist, while (3S,4S) is antagonist. This means that the racemic mix of the two enantiomers is a mixed agonist-antagonist, with relatively low abuse potential, and little of the κ-opioid activity that tends to cause problems with other opioid mixed agonist-antagonists such as pentazocine.
^US Patent 4081450 - 1,3,4-Trisubstituted-4-arylpiperidines and their preparation
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^Martinelli, M. J.; Peterson, B. C. (1990). "A concise, stereoselective synthesis of picenadol". Tetrahedron Letters. 31 (38): 5401. doi:10.1016/S0040-4039(00)97857-2.