Bonabiline A, a monoterpenoid 3alpha-acyloxytropane from the roots of Bonamia spectabilis showing M3 receptor antagonist activity.

Two 3alpha-acyloxytropanes with unique monoterpenoic acyl moieties, bonabiline A and its anhydro derivative bonabiline B, have been isolated from the roots of Bonamia spectabilis. Their structures were elucidated by detailed spectroscopic analysis. Due to the structural similarity of bonabiline A to atropine/hyoscyamine, the affinity of both bonabilines to the muscarinic M (3) receptor was studied in the isolated guinea-pig ileum. Bonabiline A (pA (2) 6.65 +/- 0.03) proved to be a more potent antagonist than bonabiline B (pA (2) 5.50 +/- 0.03).