Search

Seven undescribed monoterpene indole alkaloids alstoscholarinines A‒G, along with nineteen known alkaloids, were isolated from the branches of Alstonia scholaris (L.) R. Br. The isolated alkaloids were classified into ten framework types. The structures of the undescribed alkaloids were elucidated by extensive spectroscopic analysis, ECD calculation, and single-crystal X-ray diffraction analysis. Alstoscholarinine A is an unreported and unusual monoterpene indole alkaloid incorporating three nitrogen atoms, characterized by a compact 6/5/6/6/6/5 hexacyclic system bearing a piperidine ring and a unique oxazolidine ring. Alstoscholarinine B represents the first naturally C-17 nor-isositsirikine-type alkaloid. Plausible biosynthetic pathways of alstoscholarinines A and B were proposed. All isolates were evaluated for their vasorelaxant activities against phenylephrine-induced contraction of rat mesenteric arteries. Among them, seven alkaloids showed significant vasorelaxant activities with EC 50 values less than 10 μM. Importantly, the akuammicine-type alkaloids in this study showed much better vasorelaxant activities than other framework type alkaloids, indicating that this type of alkaloid may be a valuable source for the discovery of vasodilators. A preliminary structure-activity relationship for vasorelaxant activities of the isolated akuammicine-type alkaloids is also discussed. Seven undescribed alkaloids, including an unprecedented N 3 -type monoterpene indole alkaloid featured by a unique 6/5/6/6/6/5 carbocyclic skeleton, were isolated from the branches of Alstonia scholaris. Some alkaloids showed significant vasorelaxant activities. [Display omitted] • Seven undescribed alkaloids, alstoscholarinine A-G, were isolated from A. scholaris. • Alstoscholarinine A possessed a unique 6/5/6/6/6/5 carbocyclic skeleton. • Alstoscholarinine B was the first naturally C-17 nor-isositsirikine-type alkaloid. • Alstoscholarinine G was the first C-20 epi-tubotaiwine-type alkaloid. • Seven alkaloids showed significant vasorelaxant activities. [ABSTRACT FROM AUTHOR]