¹³C‐NMR. Spectroscopy of Naturally Occurring Substances. XLV. Iboga Alkaloids

Abstract

Summary. The carbon shifts of the Iboga-type alkaloids catharanthine, voacanginc, coronaridine, ibogaine, dihydrocatharanthine and epiibogamine were recorded and correlated with the conformation of the natural bases. A 13C-NMR. analysis of heyneaninc determined its C(19) configuration and a similar study of cpihcyneanine proved its structure.In connection with problems of structure determination and synthesis of indole alkaloids of the Iboga type it became of importance to apply 13C-NMR. spectroscopy thereto. As a consequence the following study of a select group of such natural products was undertaken and isoquinuclidine (l), its N-methyl derivative (2), some monosubstituted bicyclo[2.2.2joctanes (3) and two bicyclo[2.2.2]octenes (4) used as models. The carbon shifts of the first two models are depicted on formulas 1 4 ) and 2 and those of bicycles 3 5 ) and 46) listed in Table 1. The total of the y-effects on C(l) and C(3) is 7.7 ppm greater for the hydroxy group of 3c than the methyl group of 3b, relative to 3a. The comparable magnitude of the y-effect of the two groups as a result of fixed gauche, non-bonded interactions [3] [4] and thc shielding of ycarbons by hydroxy groups truns anti-periplanar to them within rigid six-membered rings [j] suggest that the excess shielding by the hydroxy group of 3c is an acyclic equivalent of the lattcr phcnomenon. This result indicates that caution must be cxerciscd in the assessment of shielding contributions to y carbon atoms from oxygen in all staggered, acyclic conformations.The introduction of a double bond into the strained bicyclo[2.2.2]octane skeleton leads to stronger deshielding of the allylic carbons and a decrease of the cndocyclic homoallyl effect relative to the shift differences of cyclohexenes and cyclohexanes [2] [6] (cf. also the 6b + 5a change).

Chemical constituents from Tabernaemontana catharinensis root bark: a brief NMR review of indole alkaloids and in vitro cytotoxicity

“…In this work, the ethyl acetate basic fraction of the root bark of T. catharinensis yielded twelve indole alkaloids: coronaridine (1), coronaridine hydroxyindolenine (2), voacangine (3), voacangine hydroxyindolenine (4), ibogamine (5), heyneanine (6), voacristine (7), voacristine hydroxyindolenine (8), 3-oxo-coronaridine (9), 3-hydroxycoronaridine hydroxyindolenine (10), vobasine (11) and voachalotine (12). This is the first report on the presence of the alkaloids 7, 8, 9 and 12 in T. catharinensis while 10 has never been described.…”

“…Of the 12 alkaloids isolated 10 are of the ibogan class and the other 2 of the corynanthean class. They were identified by comparison with published data (UV, IR, 1 H NMR, 13 C NMR and MS) 3,5,[9][10][11][12][13][14] .…”

Aromatic chemical shifts in ar‐hydroxy‐ and ‐methoxy‐substituted indole alkaloids; reference data and substituent‐induced chemical shifts for ten different chromophoric groups

“…By far the largest group of alkaloids studied is that having an indole chromophore, but this group includes alkaloids with very diverse structures. It was at first thought that these various structural types would need to be considered separately but, since calculation showed that the SCS values for the different types were similar, the compounds have been dealt with as one group, consisting of the following types 12,23,29,79,98,112 two indole c h r o r n o p h~r e s ;~~~~~~~~.~~ various dimers, one half of which has an indole moiand miscellaneous alkaloids with an indole chromophore not belonging to the preceding…”

A new indole alkaloid isolated from Tabernaemontana hystrix steud (Apocynaceae)

“…The known indole alkaloids, ibogamine (2), 12,13 olivacine (3), 14,15 NMR experiments, which were also used to complete and unambiguous 1 H and…”