Sarpagine-type alkaloids

Sarpagine-type indole alkaloids have a structural feature bonding the C-5 and C-16 positions in the Corynanthe-typ monoterpenoid indole alkaloids. This type of compound is distributed not only in Gelsemium plants, but also in many genera in the family of Apocynaceae, Rubiaceae, and Loganiaceae. As shown in Table I, sfac sarpagine-type alkaloids 1-3, 5, 8, and 9 have been isolated from the Gelsemium plants. 

Koumidine was first obtained from the Chinese G. elegans Benth. and shown to be a C-16 epimer of normacusine B (5). The presence of an intense NOE between H-19 and H-15, together with the different C-NMR chemical shifts of C-15 and C-21 from those of gardnerine (28), 

In the original paper, koumicine was converted into koumicine N-oxide by hydrogen peroxide oxidation (9) therefore, koumicine iV-oxide (3) should also have the 19(Z) configuration (Fig. 2). 

The spectroscopic data for a new alkaloid 8 (6) showed a similarity to those reported for anhydrovobasinediol (taberpsychine) (10) 11,12), but a significant NOE observed between H-19 and H-15 in the H-NMR spectrum of 8 suggested that the configuration of the ethylidene side chain was in the Z form. The C-NMR signal due to C-15 of 8 was observed downfield (7.0 ppm) and, in contrast, that of C-21 was observed upheld (6.9 ppm) compared to the corresponding signal of 10. The structure of 8 inferred by spectroscopic analysis to be 19(Z)-anhydrovobasinediol was confirmed by chemical synthesis from ajmaline (Section II.B). 

The new alkaloid 9 is the first example of a Gelsemium alkaloid having an Na-methoxyindole moiety. Full assignment of the H- and C-NMR spectra of Na-methoxy-19(Z)-anhydrovobasinediol (9) was carried out by NMR techniques. The structure was determined by single-crystal X-ray analysis (75) (Fig. 3). This alkaloid may be a biogenetic intermediate to the Na-methoxyoxindole alkaloids. 

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G. Sarpagine type alkaloids (including gardneria bases).135... 

Sempervirine and Sarpagine-type Alkaloids from Gelsemium Species... 

The procedure thus developed was applied to the sarpagine type alkaloid, 19( ))-koumidine (9) giving A a-methoxy-19( ))-koumidine (120) in 49% yield from the corresponding dihydroindole (119) (Fig. 20) (83). 

As described in Section II.B.2, Magnus et al. have developed an efficient synthetic route from (5)-(-)-tryptophan to chiral sarpagine-type alkaloids. 

Biogenetically, the humantenine-type oxindole alkaloids would be generated from the sarpagine-type alkaloids such as 19(Z)-anhydrovobasinediol... 

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