Quantum chemical studies of meperidine and prodine.

Extensive quantum chemical calculations have been made of the electronic distribution and conformational behavior of meperidine and desmethyl-, (+)-alpha, and (+)-beta-prodine using PCILO, a semiempirical molecular orbital method. For this series of opiates, a phenyl equatorial conformation was preferred over a phenyl axial one, with the equatorial conformer most favored in the most potent compounds. Using the low-energy equatorial conformer obtained for each compound, together with calculated net atomic charges, their observed potency variation could successfully be explained. From the results, these compounds appear to act at the morphine receptor with an identical piperidine rather than phenyl ring site.