Abstract
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 Heerden