Gelsemine-type alkaloids

In the leaves of G. elegans Benth., an Ab-oxide derivative of gelsemine was found (6). The stereochemistry of the quaternary nitrogen in 235 was determined to be the R configuration by H-NMR spectral comparison of two diastereomeric N-oxides prepared by wi-CPBA oxidation of gelsemine. 

Both stereoisomers of gelsevirine derivatives (237 and 238) having a secondary hydroxy group at the C-19 position were isolated (6,98). The structure including the 19(7 ) configuration of one of these two alkaloids was established by single-crystal X-ray analysis (98). The stereochemistry of the first isolated compound initially proposed as the 19(7 ) configuration became that of 5. 

A new alkaloid 239 having an acetoxy group at the C-19 position in the gelsevirine skeleton was isolated from G. sempervirens and G. rankinii (98). The H- and C-NMR spectra of 239 were similar to those of 19(7 )-hydroxy-dihydrogelsevirine (237), except for the presence of an acetyl group in 239, suggesting that 239 had the 19(7 ) configuration. That was proved by hydrolysis of 239 to produce 237. 

The ketolactam 257 was then converted into its enol silyl ether 258, thus setting the stage for the operation of the key intramolecular Mannich reaction. In practice, the conversion of 258 into bromoketone 259, whose structure was confirmed by X-ray determination, proceeded cleanly in hot trifluoroacetic acid. This cyclization can be viewed as a normally disfavored 

Reagent i. pyridine ii. hv, MeOH iii. liAlH4 iv. AgOAc, I2, AcOH v. RuCls, NaI04 then CH2N2 vi. K2CO3, MeOH vii. TBDMSOTf, 2,6-lutidine viii. PhSeSiMes, Znl2 ix. w-CPBA  

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Gelsemine-type Alkaloids from Gelsemium Species... 

Strictosidine was experimentally incorporated into a series of different alkaloids, including yohimbine-, strychnine- and gelsemine-type alkaloids (151). 

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... 

The ability of plants to utilize both the major biogenetic routes (to CCLXIX and CCLXX) is amply demonstrated by the various Stryehnos species, some of which elaborate the strychnine family of alkaloids, some of which (the South American Stryehnos) elaborate yohimbine types (curare alkaloids). Even more striking is the biosynthesis in the one plant Gdsemium sempervirens (not a Stryehnos) of both gelsemine... 

For reasons of brevity, some skeletal variations are not clearly defined in Figs. 2 and 3 and some have been omitted. In particular, the picraline (Volume VIII, p. 147), the echitamine (Volume VIII, p. 174) 78), and the aspidodasycarpine 53) skeletons have been merged into type If. The new alstophylline type 39, 40) is somewhat hidden in the Id reprepresentation. Gelsemine (Volume VIII, p. 95) has been included in the oxindole type Ic. Type Ila Iboga) includes several closely related rearranged alkaloids (Volume VIII, Chapter 9) which are not shown in Fig. 3. An unusually modified Aspidosperma structure 83) is related to type III6 and is listed as such. 

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