Studies on sulfur-capped osmium and ruthenium clusters with focus on dynamics, chirality and isomerism.

The thesis begins with an introduction to salen complexes describing possible types of co-ordination to the metal ions and ways in which chirality is introduced in these complexes. A review on applications of salen complexes in asymmetric synthesis is then presented which aims to illustrate how careful catalytic system design led to significant discoveries in asymmetric synthesis. Preparation of 2-amino-R-benzaldehydes, unstable intermediates, using multistep organic synthesis is presented and the stability of these compounds generated by three different routes is assessed. Further on, synthesis of ambch ligands, which combine the donor atom environment of porphyrins with ready incorporation of chirality and arene functionalisation familiar to salen ligands, was achieved. It is followed by synthesis and characterisation of their nickel, copper, manganese, cobalt, and vanadium complexes. The structures of these complexes are discussed based on NMR, IR and X-ray crystallography data. The strategies proposed in the literature for preparation of asymmetrical ligands were tested in synthesis of ligands 4.5, 4.7and 4.10. Original strategies were developed in the search for better synthetic procedures to compounds 4.5-4.10. The results demonstrated that the route to asymmetrical ligands is not general and has to take into account the difference in reactivity of the two aldehydes and the nature of the diamine involved in the condensation reaction. Synthesis of nickel salchsal* and ambchsal complexes was achieved under mild conditions in high yields. The influence of changing the co-ordination sphere from N2O2 to N2OO*, N3O and N4 in the ligands and nickel complexes on the structure of these compounds is discussed based on NMR and X-ray crystallography data. The asymmetrical complexes allow a better tuning of the electronic and structural properties than symmetrical ones. The study demonstrates for the first time that the difference between the two aromatic moieties of the asymmetrical complexes can be tuned by appropriate substitution of the aromatic rings

Cytotoxic compounds from the genus Centaurea.

This thesis, which is divided into four chapters, represents an account on the isolation, identification and the assessment of bioactivity of cytotoxic compounds from the genus Centaurea (Family: Asteraceae alto Compositae), a large genus of about 500 species. The first three chapters deal with an introduction of natural products and Centaurea species, followed by the isolation and characterisation of compounds from twelve Centaurea species. The last chapter describes the bioactivities of extracts and isolated compounds from these species. A total of 45 compounds were isolated from twelve Centaurea species, and only C. americana, C. cyanus, C. dealbata and C. macrocephala had previously been studied. Four of these are novel compounds. Four lignans arctiin, matairesinoside, matairesinol and lappaol A were isolated fromthe methanol extract of C. macrocephala seeds. Arctiin and matairesinoside were also isolated from the methanol extract of C. americana, C. bornmuel/eri, C. dealbata, C. huber-morathii, C. mucronifera, C. pamphylica, C. schischkinii and C. urvillei. The methanol extract of C. americana also afforded 20-hydroxyecdysone, 24-hydroxyecdysone, lappaol A, arctigenin and a novel compound, 3"-O-caffeoyl(9"'->3")-arctiin. The methanol extract of C. cyanus produced lariciresinol 4-0-B-D-glucoside, berchemol, moschamine and cis-moschamine. Arctigenin, astragalin, afzelin, matairesinol and a novel indole alkaloids, schischkiniin, were isolated from the methanol extract of C. schischkinii. Extract from C. bornmuelleri afforded arctigenin, astragalin, afzelin and matairesinol. The methanol extract of C. mon/ana yielded berchemol, berchemol 4'-O-B-D-glucoside, p-coumaroylquinic acid, cis-pcoumaroylquinic acid, pinoresinol, pinoresinol mono methyl ether, pinoresinol dimethyl ether, pinoresinol 4-0-B-D-glucoside, pinoresinol 4,4'di-0-B-D-glucoside, pinoresinol 4-0-apiose-(1->2)-B-D-glucoside, centcyamine, cis-centcyamine, N-(4-hydroxycinnamoyl)-5-hydroxytryptamine, cis-N-(4-hydroxycinnamoyl)-5-hydroxytryptamine, moschamine, cis-moschamine, tryptamine and two novel compounds, flavanone-apiose-glucuronic acid and montamine. C. gigantea afforded arctiopicrin, 8-0-(4-hydroxy-3-methylbutanoyl)-salonitenolide, chlorogenic acid, cirsiliol, isoquercetrin, orientin, isoorientin and 4"-hydroxybenzoyl-isoorientin. General toxicity, cytotoxicity and antioxidant activity of the extracts and isolated compounds were evaluated, respectively, by the brine shrimp lethality assay, MTT assay on human colon cancer cell line (CaCo-2) and DPPH assay. Among all the species, the methanol extract of C. bornmuelleri, C. gigantea, C. huber-morathii and C. montana were the most toxic extracts in brine shrimp lethality and MTT assay. Arctigenin (IC50=7.0 mM), matairesinol, montamine (IC50=43.9 mM) and lappol A, schischkiniin, arctiopicrin (IC50=8.5 mM) and 8-0-(4-hydroxy-3-methylbutanoyl)-salonitenolide (IC50=26.4 mM) showed higher cytotoxicity against MTT assay. Matairesinoside (IC50=2.2 x 10-3 mg/mL), matairesinol (IC50=2.0 x 10-3 mg/mL) and schischkiniin (lC50=3.8 x 10-3 mg/mL) exhibited significant free radical scavenging activities towards DPPH assay