Humantenine-type oxindole alkaloids

Biogenetically, the humantenine-type oxindole alkaloids would be generated from the sarpagine-type compounds such as 19(Z)-anhydrovobasinediol (6) through p-oxidation of the indole portion and subsequent rearrangement to the oxindoles (Fig. 13). 

The humantenine-type Gelsemium alkaloids, which so far include human-tenine, humantenirine, and rankinidine, are oxindole alkaloids with a novel skeleton similar to that of gelsemine but lacking a bond between C-6 and C-20. From a biogenetic viewpoint, humantenine-type alkaloids are less evolved than those of the gelsemine series and might well be immediate precursors of them... 

By employing the newly developed methods, humantenirine (87), a representative humantenine-type Gelsemium alkaloid, was synthesized from a sarpagine-type indole alkaloid (63). The oxindole derivative 103 prepared from gardnerine (28) was used for further transformation (Scheme 20). 

Humantenine-type alkaloids are the oxindole derivatives of the C/D ring-cleaved sarpagine-type indole alkaloids. Only three humantenine-type JVa-methoxyoxindole alkaloids, i.e., humantenine (86) (7,49-52), humanteni-rine (87) (49,51,52), and rankinidine (88) (52), had been reported up to... 

Hydroxyrankinidine (90), which was determined by single-crystal X-ray analysis (53), and 11-hydroxyhumantenine (91) are the first Gelsemium oxindole alkaloids with a phenol group, although some other oxindole alkaloids have an 11-methoxy group. iSvo A a-desmethoxy humantenine-type alkaloids 92 (55) and 93 (54) were also found in nature for the first time. 20-Hydroxy-dihydrorankinidine (95), which was determined by... 

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