Gelstriamine A, a Triamino Monoterpene Indole Alkaloid with a Caged 6/5/7/6/6/5 Scaffold and Analgesic Alkaloids from Gelsemium elegans Stems

A novel triamino monoterpene indole alkaloid with an unprecedented skeleton, gelstriamine A (1), four new monoterpene indole alkaloids (2–5), and 12 known analogues (6–17) were isolated from Gelsemium elegans. The structures of 1–5 were established using extensive spectroscopic techniques, NMR calculations with iJ/dJ-DP4 and 2D C–H COSY ANNs analysis, ECD calculations, chemical methods, and single crystal X-ray diffraction analysis. Gelstriamine A (1) possesses an unprecedented 6/5/7/6/6/5 heterohexacyclic scaffold bearing a unique hexahydrooxazolo­[4,5-b]­pyridin-2­(3H)-one motif, and a plausible biosynthetic pathway was proposed. All the isolated alkaloids 1–17 showed discernible analgesic activities in an acetic acid-induced writhing test in mice, and N-desmethoxyhumantenine N4-oxide (3) exhibited more potent analgesic activities than those of morphine at doses of 0.04 and 0.2 mg/kg.