Structure-activity studies of fentanyl.

The preparation of analogues of fentanyl with para substituents in the anilino aromatic ring, anilino nitrogen separated from phenyl by methylene or bimethylene, and phenacyl replacing propionyl as the N-acyl substituent is reported. Although all para substituents examined depressed antinociceptive potency in rats, most analogues of this kind were more effective than morphine and the p-F, I, and Me derivatives were only a few-fold less active than fentanyl. Separation of anilino nitrogen from phenyl lowered potency with N-phenethyl analogues retaining reasonable levels of activity (greater than morphine). All the phenacyl analogues were of low potency or inactive. Diagnostic details of the mass spectra of analogues likely to be encountered as "designer drugs' are appended.