New phenylpropanoids and monoterpene alkaloids with vasorelaxant activities from the branches of Alstonia scholaris.

Two new phenylpropanoids (1-2), one new nor-monoterpenoid alkaloid (3), one new monoterpene alkaloid (4), together with nine known compounds (5-13) were obtained from the branches of Alstonia scholaris. The structures of the undescribed compounds were determined by extensive spectroscopic analysis. Alkaloid 3 represented the first example of C-4 methylated nor-monoterpenoid alkaloids. A possible biosynthetic pathway for this new type of monoterpene alkaloids was proposed. All the isolates were evaluated for vasorelaxant activity against phenylephrine-induced contraction of rat mesenteric arteries. Compounds 1, 4, 9, 12, and 13 showed significant vasorelaxant activity with relaxation rates above 90% at 200 μM and exhibited moderate vasorelaxant activity with IC50 values ranging from 41.87 to 93.30 μM by further studies. It was the first report on the potential vasorelaxant activity of monoterpene alkaloids. Monoterpene alkaloids 3 and 4 may be served as the potential lead compounds for the discovery of vasodilators, due to their simple and optimizable structures.