Transformation of Indole Alkaloids. II. On the C/D Ring Opening and Closing Reactions of Indole Alkaloids and the Syntheses of Vobasine Type Alkaloids

The chemistry of mavacurane alkaloids: a rich source of bis-indole alkaloids

“…It could indeed also be envisioned that such oxidative cyclization might proceed through ionic pathways via formation of electrophilic intermediates at the enamine of the indole ring or the C16formyl ester. The C16-formyl ester of geissoschizine (51) is obviously also involved in other oxidative cyclizations of geissoschizine (with C5 for the sarpaganes 40 and N4 for the excelsinidines 41 ). Thus, it is interesting to take into consideration the potential influence of the conformation of geissoschizine (51) in these cyclization processes (Scheme 12).…”

“…Soon after (1976) the first assembly of the mavacurane skeleton, Sakai was interested in the biomimetic transformation of geissoschizine (51) to pleiocarpamine (3). 50 Its strategy was based on the formation of the E ring, via an intramolecular alkylation of the indolic nitrogen with a chloroester moiety.…”

“…In 2018, slightly-modified Ma's conditions were applied by our groups to (+)-geissoschizine (51) itself which was deprotonated with KHMDS (instead of LiHMDS) followed by addition of I2 and it was noticed that only (-)-17-nor-excelsinidine (147) was formed (Scheme 38). 64,65 Neither the N1-C16, nor the C7-C16 bond formation were obtained as planned.…”

“…64,65 In order to limit the loss of substrate, (±)-166 was epimerized to (±)-145 in three steps: 1) oxidation into a 7-chloroindolenine; 2) isomerisation of the latter into C3=N4 iminium (±)-167; 3) diastereoselective reduction of this iminium promoted by Noyori's catalyst. (±)-16-Desformyl-geissoschizine (145) was formylated into (±)-geissoschizine (51) and then converted into diazo compound (±)-168 using Regitz diazo transfer reaction.…”