Monoterpenoid Indole Alkaloids Bearing an N-4-Iridoid from Gelsemium elegans

Five new alkaloids, gelseganines A-D (1-4) and humantenine N-4-oxide (5), were isolated from the stems and leaves of Gelsemium elegans. Compounds 1-4 represent a rare class of monoterpenoid indole alkaloids that bear an N-4-iridoid unit. The structures of 1-5 were determined by spectroscopic analysis, single-crystal X-ray diffraction, and chemical correlation, and their absolute configurations were elucidated by CD analysis. A plausible biogenetic pathway for alkaloids 1-5 was also postulated.