Indole alkaloids coronaridine group

The Nagata group also developed a new approach to the construction of bridged aziridines via nitrene intermediates that led to the total synthesis of the indole alkaloids, ibogamine, velbanamine, and coronaridine (1968-1971). 

II. With the formula of the starting material written as in la, it is seen that the cyclization involves attack of a methylene group adjacent to nitrogen on the indole nucleus, with migration of the double bond. Two minor reaction products were characterized as the epimeric iboga alkaloids coronaridine (111) and dihydro-catharantine (IV). The production of III and IV implies reaction of I as written in Ib. An iminium intermediate from Ib with a 5,6-double bond in equilibrium with the enamine with a 4,5-double bond accounts for isomerization at C4 during the reaction. 

In an extensive study of indole alkaloids from Tabernaemontana dichotoma, Perera and co-workers reported the isolation and characterization of eight bisindole alkaloids of the vobasine-coronaridine type (90,93). Of these, the previously known alkaloids tabernamine (173), voacamine (175), and 3 R/S-hydroxyvoacamine (178) were identified from their spectral characteristics and co-TLC with authentic samples (91). UV, IR, and XH NMR data of three of the bisindole alkaloids indicated a close structural relationship to tabernamine (173), and the XH NMR of one of these alkaloids was identical to that of tabernamine, except that the signal at 6 2.62 ppm due to the N-Me group was missing. The MS and IR spectroscopic data... 

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