Über die Struktur eines neuartigen Indolalkaloids,des Talbotins. 141. Mitteilung über Alkaloide [1]

From the leaves of the African Apocynacea Pleiocarpa talbotii Wernham a novel indole alkaloid, talbotine, C₂₁H₂₄N₂O₄, has been isolated. Talbotine ( 1 ) contains a secondary N_(b)-atom and a cyclic hemiacetal group. Catalytic hydrogenation leads to 19, 20-dihydrotalbotine ( 6 ), hydrogenation in the presence of formaldehyde gives N_(b)-methyl-19, 20-dihydrotalbotine ( 8 ). In the presence of sodium methoxide and methanol, 1 is converted into the lactone 12 and the methyl ester 13 . In these reactions carbon 17 is lost as formic acid. These data, together with the analyses of the NMR. spectra of talbotine and its derivatives as well as the interpretation of the various types of the mass spectral fragmentation, lead to formula 1 for the alkaloid. Dehydrogenation of talbotine methyl ether ( 3 ) with palladium and maleic acid gives the ß-carboline derivative 26 . The N_(b)-methiodide of the latter is converted into N_(b)-methyl-talbotine methyl ether on reduction with sodium borohydride. From these data as well as from the analyses of NMR. and IR. spectra the complete relative stereochemistry of talbotine could be derived. Application of the Horeau method to the nitrogen atom b of the methyl ether 3 on the one hand and to the hydroxyl group on C17 in N_(b)-methyl-19, 20-dihydrotalbotine ( 8 ) on the other hand gives consistent results and establishes S configuration of centre 15.