Structure-Activity Studies of Fentanyl

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Abstract
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 (> 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.