Iboga bases

Under this heading are collected the alkaloids from the plants given in Table I, with the exception of the iboga bases already considered and those dealt with under Voacanga alkaloids. 

From Ervatamia, besides coronaridine, the 2-acylindoles taber-naemontanine and dregamine (vide infra) were obtained from various Tahernaemontana species, iboga bases, voacamine, and olivacine (LVIII), mp 318°, were identified (10). 

Gonopharyngia durissima has afforded iboga bases, two dimeric alkaloids discussed under Voacanga alkaloids, and a trace of a base, alkaloid E, mp 191°-193°, pif a 7.26, UV-maxima at 210 and 305 mp, which differed from the other isolates in having no carbonyl absorption in the IR-spectrum (14). Gonopharyngia pachysiphon, in contrast to C. durissima, has yielded only steroidal bases (45). 

Wenkert, E., Biosynthesis of indole alkaloids. The aspidosperma and iboga bases, J. Amer. Chem. Soc., 84, 98 (1962). 

Thus the critical synthetic 1,6-dihydropyridine precursor for the unique isoquinuclidine system of the iboga alkaloids, was generated by reduction of a pyridinium salt with sodium borohydride in base (137-140). Lithium aluminum hydride reduction of phenylisoquinolinium and indole-3-ethylisoquinolinium salts gave enamines, which could be cyclized to the skeletons found in norcoralydine (141) and the yohimbane-type alkaloids (142,143). 

A rare and biogenetically interesting example of the ethyl chain functionalization of the iboga skeleton is represented by 18-hydroxycoronaridine (al-bifloranine) (128, C21H26N203, MP 192-194°C, [a]D -210°) isolated from T. albiflora (28). The base peak in its mass spectrum appeared at m/z 323 (M+--31), and this suggested that a hydroxy group was present at C-18. A detailed H-NMR study of 128 in comparison with 97 and heyneanine (122) was reported and shown in Table IV. 

Iboga alkaloids devoid of the 19-hydroxy group are significantly more stable toward oxidation than are the corresponding hydroxy bases. Abstraction of the hydroxy proton of 7-hydroxyindolenines by bases leads to concomitant carbon migration and formation of pseudoindoxyls. In some cases the rearrangement is better accomplished by warm HC1. The interrelationship among indoles, 7-hydroxyindolenines, and pseudoindoxyls has been exhaustively treated by Cordell (see Ref. 6). 

A quantitive, spontaneous cyclization of the 16-carbomethoxy C-20-C-21 unsaturated cleavamine to coronaridine (HO) and the general failure of dehydrosecodine to serve as a synthetic precursor of catharanthine (see Ref. HI for literature review) suggest, however, that the Iboga alkaloids may be preferably assigned a biogenetic origin based on cleavamine cyclizations rather than on a dehydrosecodine. 

The versatility of strictosidine as a central intermediate for the biosynthesis of a variety of alkaloids is based on the highly reactive dialdehyde produced by the action of strictosidine p-D-glucosidase. This reactive intermediate is converted by uncharacterized enzymes into the major corynanthe, iboga, and aspidosperma skeletons that are elaborated into die several hundred alkaloids found in Catharanthns roseus. Since the biosynthesis of strictosidine appears to occur within plant vacuoles, there has been much speculation, but little is known, about the factors that regulate the accumulation of strictosidine within the vacuole, or which trigger its mobilization for further elaboration. It is well known that glycosides of different natural product classes are located within plant vacuoles. 

The alkaloids isolated from V. rosea are listed in Table I the well-known yohimbinoid bases, including lochneridine (I) and lochnericine, will not be discussed catharanthine (II) and lochnerine (O-methyl-sarpagine) belong to the iboga and ajmaline-sarpagine chapters, respectively. And vindoline (III) and vindolinine (IV) are special cases of... 

Stemmadenine (CCXIII) was first isolated from Stemmadenia donnell-smithii (8), where it occurs with (+ )-quebrachamine (I) and some iboga-type bases. Subsequently, it has also been found in Diplorrhyncus condylocarpon ssp. mossambicensis, where it occurs together with condylocarpine (CCXV), normacusine-B (Section VIII, B), two yohimbines, norfluorocurarine, and mossambine (Section VI, B and C, Ref. 116). 

The subsequent steps from geissoschizine to form the Strychnos alkaloids, the secodines, the Aspidosperma alkaloids and the iboga alkaloids remain speculative, based on low levels of incorporation of early precursors or alkaloid time course studies. Joining C-2 and C-16 while moving C-3 to C-7, yields the strychnan skeleton... 

Figure 9 STM images (a-c) and marble models (d) of the Pt jSn(001 )-surface after low temperature annealing. a)Overview, scan width (IbOGA) ", U =0.6V, I(=l. OnA. b)( 104)-facet on the side of a pyramid near the top. Scan width (lOOA), U, =0.2V, I,= 1.0nA. c)( 102)-facet on the side of a pyramid near the base. Scan width (120A)", Uy=0.4V, I,= l.0nA. d)Marble models of the (104)-facet (left panel) and the (102)-facet (right panel). For better visibility the models correspond to a chemically ordered bulk (Pt atoms light grey, Sn atoms dark grey), whereas the real pyramids are substitutionally disordered in the bulk. The unit cells seen by STM are indicated. From Ref. [27].

Since the last review on Picralima alkaloids was written (for Volume X) activity in this field has considerably abated and in consequence there is comparatively little new work to be reported. The main features of indole alkaloid biosynthesis have now been elucidated and the reader is referred to Battersby (1) for an authoritative summary of this fascinating topic. Preakuammicine (1) appears to be involved in the direct pathway to the Strychnos, Aspidosperma, and Iboga alkaloids, and although it has not been isolated from Picralima it is appropriate to include it here, and to note that its presence in very young seedlings of Vinca rosea has been established (2). Preakuammicine is almost certainly the precursor of akuammicine (2), a transformation which can also be achieved by treatment with base (2). 

It is now quite certain that the iboga alkaloids originate from tryptophan or its equivalent and two mevalonate residues (2). The latter are linked head-to-tail since geraniol can also function as a precursor of the hydroaromatic portion (3). These results along with incorporation of the same precursors in other indole alkaloids (4) confirm the earlier hypothesis (5) which was based solely on the classic method of recognizing similar units within apparently dissimilar natural products. 

Oxidation and rearrangement products of parent bases D, Some other bases isolated with the iboga alkaloids ... 

Plants of the Voacanga genus have given rise to four groups of bases apart from the iboga type represented by voacangine, voacristine, and... 

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