Gelsecorydines A–E, Five Gelsedine–Corynanthe-Type Bisindole Alkaloids from the Fruits of Gelsemium elegans

Five monoterpenoid bisindole alkaloids with new carbon skeletons, gelsecorydines A–E (1–5), together with their biogenetic precursors were isolated from the fruits of Gelsemium elegans. Compounds 1–5 represent the first examples of heterodimeric frameworks composed of a gelsedine-type alkaloid and a modified corynanthe-type one. Notably, compound 2 featured an unprecedented caged skeleton with a 6/5/7/6/5/6 heterohexacyclic ring system, which possessed a pyridine ring that linked the two monomers. Their structures and absolute configurations were elucidated by spectroscopic analysis, X-ray diffraction, and electronic circular dichroism (ECD) calculation. A plausible biosynthetic pathway for compounds 1–5 is proposed. Compounds 1, 3, 4, and 5 exhibited a significant inhibitory effect against nitric oxide (NO) production in macrophages.