Synthesis and structure-activity relationships of 1-substituted 4-(1,2-diphenylethyl)piperazine derivatives having narcotic agonist and antagonist activity.

Racemates and enantiomers of 1-substituted 4-[2-(3-hydroxyphenyl)-1-phenylethyl]piperazine derivatives (3-18) were synthesized, and their analgesic and other pharmacological activities and structure-activity relationships were investigated. The S-(+) enantiomers of 2a, 5, 7, 9, 10, and 15-18 had a stronger analgesic activity than their R-(-) enantiomers; analgesic activity of the strongest one [(S)-(+)-10] was 105 times as potent as that of morphine. The S-(+) enantiomers of these compounds had the opposite configuration to that of morphine with respect to its (C-9) asymmetric center but the same configuration to that of the tyrosine residue of Met5-enkephalin. The R-(-) enantiomers of 16 and 18 showed narcotic antagonist activity, but the S-(+) enantiomers did not. (R)-(-)-18 had analgesic and narcotic antagonist activities comparable to pentazocine but showed no significant physical dependence liability. From these results, it is suggested that these compounds show an uncommon enantioselectivity in comparison with morphine and its surrogates, and belong to a new series of compounds having a potent analgesic activity.