Stereochemical studies on medicinal agents. 20. Absolute configuration and analgetic potency alpha-promedol enantiomers. The role of the C-4 chiral center in conferring stereoselectivity in axial- and equatorial-phenyl prodine congeners.

Opical antipodes of the axial phenyl analgetic, alpha-promedol hydrochloride (3), were prepared and the absolute stereochemistry wasd determined by relating one of the enantiomers to its gamme diastereomer having the 2S, 4S,5R configuration. The analegetic potency of (+)-(2R,4S,5S)-3 is 20 times that of morphine, while its enantiomer, (-)-(2S,4R,5R)-3, is inactive at 50 mg/kg. These results are in accord with prior reports which indicate that substitution of a 3-or 5-alkyl group on the pro-4S enantiotopic edge of the piperidine ring leads to enantiomers which have greater potency than those substituted in an identical position on the pro-4R edge. This coupled with the fact that the torsion angle between the axial phenyl group and piperidine ring in (+)-3 is of the same sign as its equatorial congeners, suggests that the C(3)-C(4)-C(5) moiety and its substituents at C(4) are located in a similar chiral environment on the receptor. In contrast, the C(2)-N-C(6) portion of the axial and equatorial molecules does not bind in the same receptor environment, and it is suggested that different modes of interaction in the prodine series arise from different orientations of this moiety.