THE MINOR ALKALOIDS OF GELSEMIUM SEMPERVIRENS1

Aza-cope rearrangement-mannich cyclizations for the formation of complex tricyclic amines: stereocontrolled total synthesis of (+/-)-gelsemine

A detailed examination of the use of aza-Cope rearrangement-Mannich cyclization sequences for assembling the azatricyclo[4.4.0.0(2,8)]decane core of gelsemine is described. Iminium ions and N-acyloxyiminium ions derived from endo-oriented 1-methoxy- or 1-hydroxybicyclo[2.2.2]oct-5-enylamines do not undergo the first step of this sequence, cationic aza-Cope rearrangement, to form cis-hydroisoquinolinium ions. However, the analogous base-promoted oxy-aza-Cope rearrangement does take place to form cis-hydroisoquinolones containing functionality that allows iminium ions or N-acyloxyiminium ions to be generated regioselectively in a subsequent step. Mannich cyclization of cis-hydroisoquinolones prepared in this way efficiently assembles the azatricyclo[4.4.0.0(2,8)]decane unit of gelsemine. Using a sequential base-promoted oxy-aza-Cope rearrangement/Mannich cyclization sequence, gram quantities of azatricyclo[4.4.0.0(2,8)]decanone 18, a central intermediate in our total of (+/-)-gelsemine, were prepared from 3-methylanisole in 12 steps and 16% overall yield.

THREE MINOR ALKALOIDS OF GELSEMIUMSEMPERVIRENS AIT

After the removal of gelsemine and sempervirine from the extract of the root of Gelsemiumsempervirens Ait. the residual alkaloids left in the mother liquors were benzoylated. It was thus possible to separate the tertiary bases from the secondary bases which were obtained as a mixture of the neutral benżoyl derivatives. Purification by chromatography yielded the benzoylated mixture in crystalline form. This was hydrolyzed and the recovered secondary bases were separated by fractional crystallization of the perchlorates into alkaloid A and alkaloid B. Alkaloid A was shown by direct comparison to be identical with the gelsemicine (C20H26O4N2) of Chou. Alkaloid B, now designated gelsedine, was shown to be distinct from either gelsemicine or gelsemine. It appears to be identical with the alkaloid C19H24O3N2 claimed by Janot and his co-workers to be gelsemicine. From the tertiary bases there was isolated an apparently new base, gelseverine, as its crystalline perchlorate. Whereas gelsemine is a 3,3-disubstituted oxindole, gelsedine appears to be a 1,3-disubstituted oxindole and gelseverine a 1,3,3-tri-substituted oxindole. Gelsemicine shows many of the characteristics of an indole alkaloid.