Comparison of (-)-eseroline with (+)-eseroline and dihydroseco analogues in antinociceptive assays: confirmation of rubreserine structure by X-ray analysis

The enantiomers of eseroline bind to opiate receptors of rat brain membranes with equal affinities and show opiate agonist properties as inhibitors of Adenylate Cyclase in vitro. However, only (-)-eseroline shows potent narcotic agonist activity in vivo, similar to that of morphine. Neither (-)-noreseroline, (+)-eseroline, nor the open dihydroseco analogue (-)-8 shows analgetic effects in vivo. The structure of rubreserine being a resonance hybrid of an o-quinone with its zwitterionic mesomer is confirmed by solid-state X-ray diffraction analysis.