Gelsekoumidines A and B: Two Pairs of Atropisomeric Bisindole Alkaloids from the Roots of Gelsemium elegans

Two pairs of atropisomeric bisindole alkaloids, gelsekoumidines A (1) and B (2), with a new carbon skeleton, were isolated from the roots of Gelsemium elegans. Compounds 1 and 2 represent the first examples of seco-koumine–gelsedine type alkaloids, which feature an unprecedented 20,21-seco-koumine scaffold fused with a gelsedine framework via a double bond. Their structures including absolute stereochemistry were elucidated by spectroscopic data, single-crystal X-ray diffraction, and electronic circular dichroism (ECD) calculation. A plausible biogenetic pathway for the new compounds is also proposed. Compound 2 exhibited a moderate inhibitory effect against nitric oxide (NO) production.