Conformational flexibility of the N-acyl portion of fentanyl-type analgetics was restricted through the synthesis of novel perhydro-1, 6-naphthyridin-2-one derivatives. Neither the cis- fused derivative (5a), the trans-fused derivative (5b), nor the enamide 8a possessed analgetic activity in the mouse tail-flick assay, reaffirming the sensitivity of this portion of 4- anilidopiperidine analgetics to conformational restraint.
