A new type of GeZsemiuna alkaloid, 11-methoxygelscmsmid (2), having a 1,2secoindole system, was synthesized for the first time from a sarpagine type indole alkaloid, gsrdaerine (6). based on a suggested biogenetic sequence.
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From Gelsemium elegans Benth. (Loganiaceae),l an original plant of Chinese folk medicine "Kou-Wen", two novel alkaloids, gelsemamide (1) and ll-methoxygelsemamide (2), have been isolated by Cordell et aE.2 The characteristic of these two alkaloids are the presence of both an N-methoxyaniline and a novel aliphatic part having a cage structure. Biogenetically, 1 and 2 might be derived from the humantenine-type oxindole alkaloids, such as rankinidine (3) and humantenirine (4), by bond-cleavage between the N1 and C2 and bond-formation between the N4 and C2 positions. The humantenine-type oxindole alkaloids, the precursor of (1) and (2). would be formed from the sarpagine-type indole alkaloid, such as koumidine (6), by the C/D ring cleavage, oxidative transformation to the oxindole derivative, and introduction of a methoxy group onto the N1 function, as illustrated in scheme I. During the chemical study on the Gelsemium alkaloids,3 we were interested in the unusual structure as well as the biomimetic chemical synthesis of these two seco indole alkaloids. Here we describe the successful accomplishment of the first synthesis of 11-methoxygelsemamide (2) from the indole alkaloid, gardnerine (6).4 R=H: Gelsemmnide (1) RPH: Rankinane (S) Ft.&Me: 11-Methoxygelsemamide (2) R&Me: Humantmirine (4) Kmmidine (5) Scheme I