M.Sc. / Cephalanthus natalensis of the Rubiaceae family of plants is used in traditional medical systems. Different parts of this shrubby plant are used to treat different ailments. In this study, the use of C. natalensis for medicinal purposes was investigated. Plant materials (leaves, bark and fruits) were collected from different localities in Swaziland. Extraction of phytochemicals in C. natalensis was done by maceration in four solvents of increasing polarities: hexane, dichloromethane, ethyl acetate and methanol. Amino acids and alkaloids were also extracted in boiling water and mild acid, respectively. Phytochemical screening of compounds was done by means of TLC, HPLC and GC-MS. Previously reported TLC screening of C. natalensis indicated the presence of alkaloids. This study also confirmed the presence of alkaloids, although these were not isolated. No other chemical compounds have been reported from this plant in literature. Phenolic compounds and terpenes were extracted and detected by the chromatographic methods employed. GC-MS analysis was also used to identify and quantify amino acids in the extracts. A phytochemical assessment of compounds was done by investigating the leaves, bark and fruits of C. natalensis. A further phytochemical variation study was conducted by comparing these phytochemicals and their relative concentrations in C. natalensis plants collected from five different localities. While the same compounds were present in all the plants sampled, the concentrations of the different compounds seemed to vary irrespective of the geographic location of the plants sampled. In the different plant parts collected, some common compounds were present in the leaves, bark and fruits and other compounds varied between the different parts studied. The traditional usage of C. natalensis as fever and malaria remedy was investigated in vitro. Antibacterial and antimalarial tests of crude extracts were done. The antibacterial test on the extracts against five bacteria: Bacillus cereus, Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli and Pseudomonas aeruginosa showed an average activity in all the plant parts tested. The average MIC values ranged from 0.917 mg/mL to >100 mg/mL. In the antimalarial test the extracts inhibited 50% of Plasmodium falciparum growth at concentrations ranging from 27 μg/mL to >100 μg/mL. Hexane extracts of the bark had notable antimalarial activity with the highest inhibition at the lowest concentration of 27 μg/mL. The fruits were not as active as expected since they are particularly used as malaria remedy in traditional medicine. Two compounds were also isolated from methanol extracts of C. natalensis for the first time. A volatile isoprenoid ketone, (5E,9E)-6,10,14-trimethylpentadeca-5,9,13-trien-2-one, was isolated and characterised. The structure of the second compound could not be completely characterised due to complex data obtained from the NMR experiments.

La découverte de nouvelles entités naturelles est un processus particulièrement long et coûteux ; il faut donc réussir le plus tôt possible à détecter les composés connus dans les matrices étudiées, afin de ne pas les ré-isoler inutilement. C’est tout l’enjeu de l’annotation (c’est-à-dire l’identification partielle mais rapide de composés) et de la déréplication (autrement dit, l’application de l’annotation en vue d’écarter les molécules déjà connues du processus d’isolement).Grâce à sa sensibilité inégalée et à l’avènement de l’ère du « big data », la chromatographie liquide à haute performance couplée à la spectrométrie de masse tandem (CLHP-SM/SM) s’est imposée comme l’un des outils de référence dans cette démarche. En effet, son utilisation implique bien souvent le recours à des méthodes de traitement informatiques, comme le « molecular networking », de par la quantité considérable de données générées.La thèse se présente donc en deux parties : la première, portée sur l'extraction de nouveaux composés à partir d'une plante de la famille des Apocynaceae déjà très étudiée, Picralima nitida. La seconde propose une application des réseaux moléculaires à de la chimie biomimétique en mélanges, autour des alcaloïdes de type « 3-alkylpipéridines ». / The discovery of new natural products is a very long and expensive process ; it is therefore important to detect previously known compounds in the studied matrices, to avoid an useless second isolation. This is the aim of annotation (partial but quick identification of compounds) and of dereplication (application of annotation to avoid reisolation).Thanks to its unequaled sensibility and to the "big data" era, the high performance liquid chromatography hyphenated to tandem mass spectrometry has become a reference technique in this field. Indeed, its use is generally implemented by informatic tools, as "molecular networking", because of the large amount of generated data.This thesis is divided into two parts : the first one deals with extraction of new compounds from a very well-known Apocynaceae plant, Picralima nitida. The second one is about the application of "molecular networking" to biomimetic chemistry in mixtures, especially about "3-alkylpiperidines" alkaloids.

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