Geleganidines A–C, Unusual Monoterpenoid Indole Alkaloids from Gelsemium elegans

The first rotameric monoterpenoid indole alkaloids (MIAs), 1a and 1b, and two unusual dimeric MIAs, 2 and 3, with new dimerization patterns, together with their putative biosynthetic intermediates 4–7, were isolated from the roots of Gelsemium elegans. Compounds 2 and 3 represent the first natural aromatic azo- and the first urea-linked dimeric MIAs, respectively. Their structures and absolute configurations were elucidated by means of NMR spectroscopy, single-crystal X-ray diffraction, and electronic circular dichroism data analyses. The interconverting mechanism of rotamers 1a and 1b was studied by density functional theory computation. Compounds 2 and 3 showed moderate cytotoxic activity against MCF-7 and PC-12 cells, respectively. In addition, a plausible biosynthesis pathway for the new alkaloids was proposed on the basis of the coexistence of their biosynthetic precursors.