Gelserancines A–E, monoterpenoid indole alkaloids with unusual skeletons from Gelsemium elegans

Five monoterpenoid indole alkaloids (MIAs) with unusual skeletons, gelserancines A–E (1–5), along with their postulated biosynthetic intermediates (6–10) were isolated from the roots of Gelsemium elegans. Compound 1 features a new skeleton with an unusual trimethyl-dihydrofuranone unit. Compounds 2 and 3 are two novel gelsedine-iridoid adducts constructed through unusual C19–C11′ and N4–C3′ linkages forming an additional pyridine ring. Compounds 4 and 5 are a pair of photochemical E/Z tautomeric MIAs with a highly conjugated skeleton. Their structures and absolute configurations were elucidated by spectroscopic analysis, X-ray diffraction, and electronic circular dichroism (ECD) comparison. The plausible biosynthetic pathways of these alkaloids and their anti-inflammatory effects on zebrafish acute inflammatory models are also discussed.