Sceletium alkaloids isolation

Fig. 1. Sceletium alkaloids isolated from Amaryllidaceae species...

Sceletium Alkaloids.—(-)-Mesembrane has been isolated from S. namaquense L. Bolus X-ray diffraction analysis of its hydrochloride monohydrate revealed its relative and absolute configuration (10). (-)-Mesembrine has the absolute configuration (11), in accord with (10), and it is now firmly established that alkaloids of this family belong to a single antipodal series.13... 

In order to prove the biosynthesis of the Sceletium alkaloids, an extensive study has been carried out using Sceletium subvelutinum L. Bolus that were grown from seed, and the six aUcaloids (447-452) (Fig. 26) produced by S. subvelutinum (101) isolated. These six alkaloids were separated chromato-... 

Mesembrine (-)-Mesembrine (12) is an octahydroindole alkaloid isolated from certain plants of the Sceletium genus, namely S. namaquense, S. strictum and S. tortuosum The mesembrine sttucture has been established as 12. Mesembrine was synthesized from noncarbohydrate, and its synthesis from D-mannitol has been presented herein. 

Sceletium Alkaloids.—Full details of an earlier briefly reported synthesis of ( )-mesembrine (3) have been published and a new synthesis has been described. The latter is outlined in Scheme 1 A new Sceletium alkaloid has been isolated from S. namaquense. It has been formulated as (4) on spectral evidence, the assignment of structure being confirmed by direct spectral comparison between the natural base and racemic (4), synthesized earlier. Channaine, another alkaloid of this family, from S. tortuosum, has structure (5) on the basis of an X-ray diffraction analysis of its hexahydrate. It may be an artefact arising from the condensation of a pair of iV-desmethylmesembrenone molecules (6) during isolation. ... 

Isolation procedures for Sceletium alkaloids have generally relied on column chromatography over alumina and/or silica gel for the separation and purification of the major alkaloids, with rqieated preparative-layer chromatography often necessary for separation of the minor bases. In one instance high-pressure liquid chromatography was used for purification of an alkaloid. This latter technique is likely to find increasing application in the future for isolation of the minor alkaloids of this family. 

The structural features embodied in the Sceletium alkaloids coupled with evidence from biosynthetic studies suggest that cyclohexadienone intermediates may be involved in the biosynthesis of these alkaloids. In an attempt to isolate such intermediates, a procedure was devised for the isolation of phenolic alkaloids from 5. namaquense which avoided the use of acidic conditions which might be expected to induce rearrangement of acid-sensitive cyclohexadienone systems. This procedure involved washing a chloroform... 

The pyrrolidine ring is part of numerous natural products, and we have restricted the field to those alkaloids that contain an isolated pyrrolidine ring. We have thus excluded the so-called Sceletium (mesembrine) and Dendrobium (den-drobine) alkaloids, which have also been reviewed (3-5). The pyrrolidine ring is frequently encountered in proline residues of cyclopeptide alkaloids these have been reviewed already (7) and will not be discussed here. 

A detailed account concerning the isolation and characterization of the pyrido-mesembran alkaloids A4(7) and iV-formyltortuosamine (8) and the mesembran bases (9 R = H, R = OMe), (10), and (9 R = R = H) from Sceletium namaquense and S. strictum has appeared. Their structures were established by detailed H and... 

In an initial study of the alkaloids of Sceletium joubertii, Arndt and Kruger (31) isolated three new alkaloids. The essential features of the structures of these noncrystalline bases were evident from their spectral properties. The molecular formula of joubertiamine as C16H21NO2 was established by high-resolution mass spectrometry, and fragment ions at m/e 58 and m/e 78 indicated that the alkaloid contains an A -dimethylamino-ethyl side chain. A carbonyl absorption in the IR at 1680 cm" and the UV spectrum were both in accord with the presence of an a,jS-unsaturated carbonyl group. A bathochromic shift of the absorption maxima in the UV... 

A minor, noncrystalline alkaloid in Sceletium namaquense has been recently isolated ai characterized as iV-acetyl-lV-methyl-Ar,7a-secomesem-brine (48) 34). The IR spectrum indicated the presence of both amide and ketone functions, with carbonyl absorptions at 1658 and 1750 cm respectively. The H-NMR spectrum of the alkaloid, in addition to possessing absorptions characteristic of the veratryl group, showed signals attributable to an IV-methyl-lV-acetyl group from Ae occurrence of pairs of singlets at... 

A noncrystalline, optically active base, [d]24i —10,500°, subsequently characterized as the dihydropyridone 53, has be isolated from the polar fractions resulting after alumina chromatography of the nonphenolic alkaloids of Sceletium namaquense (36). 

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