Abstract
In the last few years, numerous new indole and oxindole alkaloids were isolated from the Chinese medicinal plant, Gelsemium elegans Benth., by us and other researchers. Our interest in the relationship of various skeletons of Gelsemium alkaloids has led us to consider their biogenetic pathway and to investigate our proposal by chemical transformations. Here, we describe the following three newly developed chemistry on the Gelsemium alkaloids. I: 3,3-Disubstituted oxindole (16) could be converted to the Na-methoxy-oxindole (20) via tungstate-catalyzed oxidation of the indoline derivative (17). This method was applied to the synthesis of the Gelsemium alkaloids, (5), (27), (28) and (56). II: Based on a biogenetic speculation, a humantenine-type Na-methoxyoxindole alkaloid, humantenirine (27), was prepared from a sarpagine-type indole alkaloid, gardnerine (21), via stereoselective rearrangement to the oxindole, olefin inversion, and introduction of a methoxy group onto the Na-function. III: 20-Hydroxydihydrorankinidine (28), a biogenetically hypothetical intermediate to the new skeletal type alkaloid, gelselegine (8) (Fig. 5), was synthesized from ajmaline (33) in 22 steps. Gardnerine (21) was transformed into the gelselegine skeleton (53), which was then converted to the natural gelsedine-type alkaloid, gelsemicine (56) in a highly stereoselective manner. Starting from 21 the first and biomimetic construction of another gelselegine-type alkaloid (60) having a hydroxy group at C-19 was achieved via a biogenetically hypothetical aziridine intermediate (59).
