(7c, 7d, 9c, and 9d), which are conformationally restricted analogues of fentanyl, were synthesized and separately tested for analgesic activity and affinity for the opiate receptor of rat brain. Stereochemical assignments for 7c, 7d, 9c, and 9d were deduced from NMR spectral analyses. Conformational analysis revealed that the 2a isomers (7d and 9d) exist in solution as mixtures of cis-and trans-fused conformers with ca. 90 and 45% cis form, ...