A study of Mesembryanthemaceae alkaloids.

Since prehistoric times medicinal plants have been an invaluable source of medicinal preparations to mankind due to their respective chemical constituents. The family of the Mesembryanthemaceae is almost entirely endemic to southern Africa, and the subfamily Mesembryanthemoideae is a key source of previous reports of plants with traditional uses, both medicinal and culinary. In this thesis, phytochemical investigations were performed on a representative sample of the Mesembryanthemaceae genera and species, with emphasis on the subfamily Mesembryanthemoideae. The aims of this study were to record all ethnobotanical information on the medicinal and culinary uses of the Mesembryanthemaceae, and to isolate and identify alkaloids from selected taxa (especially those of medicinal interest) with specific emphasis on mesembrane alkaloids A review on the structural classification, biosynthesis and biological properties of the mesembrane alkaloids is presented in this dissertation. Experimentally, extracts of the 67 plants encompassing 15 genera and approximately 26 species which were investigated were prepared using one of two alkaloid extraction procedures, following which the extracts were screened for the presence of alkaloids by thin-layer chromatography. The extracts in which alkaloids were detected were analysed further using gas chromatography/mass spectrometry. Where possible, pure alkaloids were isolated from the crude alkaloid extracts and the pure compounds characterised using 1H and 13C nuclear magnetic resonance spectroscopy. Six of the mesembrane alkaloids as well as the tyrosine-derived non-mesembrane alkaloid hordenine were found to be distributed amongst the various genera and species of the Mesembryanthemoideae. The mesembrane alkaloids detected include mesembrine, mesembrenone, mesembrenol, mesembranol, 4’-Omethylsceletenone and 4,5-dihydro-4’-O-methylsceletenone. Within the genus Sceletium mesembrine, mesembrenone, mesembrenol, mesembranol and hordenine were identified. Mesembrine, mesembrenone, mesembranol and hordenine were also detected in Aridaria. The sceletenone derivatives 4’-Omethylsceletenone and 4,5-dihydro-4’-O-methylsceletenone as well as hordenine were identified as the major alkaloid constituents of the genus Aptenia. No alkaloids were detected in the species of Brownanthus and Prenia investigated in this study. Amongst the genera Mesembryanthemum, Phyllobolus and Psilocaulon, a random distribution of the alkaloid hordenine was observed. During the course of this study, the isolation and characterisation of 4’-O-methylsceletenone and 4,5-dihydro-4’-O-methylsceletenone from both Aptenia cordifolia and Aptenia lancifolia, as well as the non-mesembrane alkaloid hordenine from a number of species, was achieved. The two mesembrane alkaloids have been prepared synthetically, but this is the first report of the presence of the compounds in a natural source.Prof. F.R. van Heerde