Indole alkaloids Hunteria type

Tacamonine, an indole alkaloid of the Iboga type, isolated from Tabermemontana eglandulosa, the root of which is used to treat snake bites in Zaire, bears structural similarity to the Hunteria alkaloids, eburnamonines, which possess vasodilator and hypotensive activities. Its synthesis in racemic and homochiral form was accomplished by incorporating a classic 6-exo-trig radical cyclization in the key step of the synthesis (Scheme 6)71. The radical precursor 6 was constructed in a 7-step synthesis by starting from racemic or chiral propane-1,3-diol. The radical cyclization of 6 produced the piperidinone in 72% yield as a diastereomeric mixture, which was then transformed into tacamonine. 

C19H26N2, Mr 282.43, mp. 147-149 °C soluble in acetone, chloroform, dilute acids. A monoterpenoid indole alkaloid of the Aspidosperma type, Q. occurs in both enantiomeric forms the (+)-form, (aJu +98° (CHCI3), in leaves and root bark of Pleiocarpa species as well as the bark of Stemmadenia species the (-)-form, [aJi, -100° (CHCI3), in Aspidosperma quebra-cho-blanco and other Aspidosperma species as well as Gonioma, Hunteria, and Rhazya species. 

Within the family Apocynaceae, only four of seven tribes contain indole alkaloids. These are the Carisseae, Plumerieae, Rauvolfieae, and Tabemaemontaneae. All major skeletal types occur in the Carisseae. Within this group, the simpler alkaloids are more widely distributed than the more derived types. Alkaloids of the most complicated skeletal type are found only in one species of Hunteria. The Tabemaemontaneae represent a rather uniform tribe in comparison to the Plumerieae. The genus Tabernaemontana is quite complex many other genera that were formerly recog-... 

Two new indole alkaloids, bisnicalaterine D (1), consisting of an ebumane and a corynanthe type of skeletons, and nicalaterine A (2) were isolated from the bark of Hunteria zeylanica. Their structures were elucidated by various spectroscopic data such as NMR and CD spectra. A series of bisnicalaterines and nicalaterine A showed potent anti-plasmodial activity against P. falciparum 3D7 [28]. 

An X-ray analysis of hunteracine from Hunteria eburnea shows it to be a salt (64) of a novel type. Its relationship to the other alkaloids of this group can be seen by assuming the loss of the C(16)-substituents of stemmadenine (65) and C(7)-hydroxylation, followed by nucleophilic closure of the basic nitrogen on to the indole a-position. 

Ceridimine (303), from Pagiantha cerifera was the first example of a qua-sidimeric-type alkaloid in which the additional tryptamine unit has a free side chain and is linked by its aromatic moiety to the monoterpenoid indole unit 203). The primary amine function was readily acetylated and methylated, and its partial synthesis via coupling of vobasinol with tryptamine confirmed the structure assignment. Demethylceridimine (304) was later obtained from P. buchtieni T. buchtieni), which also provided another quasidimer, buchtienine (254) vide supra) 153). The third member of this group, hunteriatryptamine (305), was obtained from Hunteria zeylanica from Thailand. It differs from the previous two alkaloids by the presence of a hydroxymethyl function on the vobasinyl C(16) in place of H 204). 

---