Browsing by Author "Samwel, Stephen"

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    Antimicrobial, antiviral, larvicidal and other secondary metabolites from Tanzanian annonaceae species
    (University of Dar es Salaam, 2010) Samwel, Stephen
    This Thesis reports the results from investigations of five Tanzanian Annonaceae species in which the leaves of Toussaintia orientalis Verdc. yielded the new indolidinoids toussaintine A - C, the N-cinnamoyl hydrobenzofuranoid toussaintine D, the N-cinnamoyl acetoxymethylcyclohexenoid toussaintine E, the β-glucoglycerylaminooctenyl adipate named orientalin, and the glycoflavonoid 3'-dehydroxyquercitrin. The leaves of another Toussaintia species, T. patriciae Q. Luke & Deroin which was only recently described, gave ursolic acid, its derivatives 19α,23-dihydroxyursolic acid and 29-hydroxyursolic acid, together with ethyl gallate. The stem and root barks of Artabotrys modestus Verdc. yielded the new diphenylpentanoid artabotrinone, aminotriketide diterpenoid artabotramide and known metabolites. The new N-methoxyoxoaporphinoid artamonteirine and the naphthalene derivative artamonterone were obtained from the stem bark of A. monteiroae Oliv. The leaves, stem and root barks of the new Annonaceae species Mwasumbia alba Couvreur & D.M. Johnson yielded the cyclitol quebrachitol, together with polycarpol. The structures of the isolated compounds were established based on analysis of their spectroscopic data, some structures also being further confirmed by X-ray diffraction analysis. Some of the isolated compounds exhibited in vitro antimicrobial potency against Staphylococcus aureus, Escherichia coli, Candida albicans and Cryptococcus neoformans, in ovo antiviral efficacy against Infectious Bursal Disease Virus and Newcastle Disease Virus, and larvicidal activity against Anopheles gambiae s.s Giles mosquito larvae. These results demonstrated some of the compounds to be potential leads for drug development.
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    Oxygenated heptanoids and c-benzylflayanones from cleistochlamys kirkii and uvaria faulknerae
    (University of Dar es Salaam, 2003) Samwel, Stephen
    The phytochemical investigations reported in this Thesis were aimed at the isolation of biologically active and other constituents from two plant species of the family Annonaceae, viz. Cleistochlamys kirkii Benth (Oliv.) and Uvaria faulknerae Verde. The two plant species belong to the genera of some Tanzanian plants that have been a focus for phytochemical investigations being carried out in the Chemistry Department at the University of Dar es Salaam. Whereas the monotypic genus Cleistochlamys was just recently included in these studies Uvaria species have been extensively investigated. Crude extracts from C. kirkii are used for the treatment of tuberculosis, rheumatism and haemorrhoid wounds in Mozambique. In these investigations C. kirkii yielded three known oxygenated heptanoids (Z)-acetylmelodorinol, melodorinol and 1,6-desoxy-(3-senepoxide, together with two hitherto new heptanoids that were named cleistenolide and cleistodienol. Acetylmelodorinol, cleistenolide and cleistodienol exhibited mild antifungal activity against Candida albicans and antibacterial activity against Staphylococcus aureus and Bacillus anthracis at a concentration of 10 mg/ml. They also showed cytotoxic activity in the brine shrimp test (LD50 values of 16.32, 90.55 and 117.89 |xg/ml, respectively). The root bark of Uvaria faulknerae yielded 5-hydroxy-7-methoxyflavanone (pinostrobin), chamanetin, dichamanetin and a mixture of uvarinol and isouvarinol, whereby chamanetin, dichamanetin, and the mixture of uvarinol and isouvarinol exhibited antibacterial activity in vitro against S. aureus and B. anthracis with a minimum inhibitory concentration of 78, 19.7 and 39.5 fig/ml, respectively. They also exhibited a strong cytotoxicity in the brine shrimp lethality test (LD50 values of 9.28, 2.50 and 2.50 fxg/ml, respectively). Pinostrobin exhibited neither antibacterial nor antifungal activity in these investigations but it showed mild cytotoxicity in the brine shrimp lethality test ( LD50 value of 132.25 ng/ml). These results demonstrate the diversity of structural frameworks among annonaceae natural products, as it has been previously observed.