Through the ICBG (International Cooperative Biodiversity Group) program and a continuing search for anticancer compounds, plant extracts were obtianed from Suriname and Madagascar and screened for cytotoxic activity in the A2780 human ovarian cancer cell line. Fractionation of a leaf and flower extract of Casearia nigrescens led to the isolation of six new clerodane diterpenes. Four were new natural products and the other two were previously unreported hydrolysis products. Their structures were determined using mass spectrometry and 1-D and 2-D NMR. All six compounds were cytotoxic in the A2780 human ovarian cancer cell line. Fractionation of a leaf extract of Vernonia pachyclada led to the isolation of four new sesquiterpene lactones. Their structures were determined using mass spectrometry, 1-D and 2-D NMR, and (in one case) single crystal X-ray diffraction. All four compounds were cytotoxic in the A2780 human ovarian cancer cell line. Fractionation of an extract of Casimirella ampla led to the isolation of three new diterpenes and two known diterpenes. Their structures were determined using mass spectrometry and 1-D and 2-D NMR. All five compounds were cytotoxic in the A2780 human ovarian cancer cell line. Fractionation of root and stem extracts of Mendoncia cowanii led to the isolation of two new naphthaquinones, and two known naphthaquinones. Their structures were determined using mass spectrometry and 1-D and 2-D NMR. All four compounds were cytotoxic in the A2780 human ovarian cancer cell line and three compounds exhibited weak inhibition of Akt kinase. The fractionation of five additional extracts resulted in the isolation of twelve known compounds. Their structures were determined using mass spectrometry, 1-D and 2-D NMR, and comparison to literature data. All twelve compounds were cytotoxic in the A2780 human ovarian cancer cell line. / Ph. D.

Rh(III) and Ir(III) dimeric complexes with tunable cyclopentadienyl (Cp) rings have proven versatile for both catalysis and as synthetic precursors. An efficient microwave method to synthesize Rh(III) and Ir(III) dimeric complexes [(η5-ring)MCl]2(μ2-Cl)2, (where (η5-ring)MCl = (η5-Me4C5R)Rh(III)Cl or (η5-Me4C5R)Ir(III)Cl) was developed. A modular design for the substituted cyclopentadienes HC5Me4R was based on Grignard reactions of 2,3,4,5-tetramethylcyclopent-2-en-1-one (R = alkyl, 12 examples; R = aryl, 3 examples) or by SNAr reactions of potassium tetramethylcyclopentadienide with perfluoroarenes (R = perfluoroaryl, 3 examples). Reaction of the Me4CpHR ligands with [M(COD)](μ2-Cl)2 (M = Rh, Ir; COD = 1,5-cyclooctadiene) produced the dimeric complexes [Cp*RMCl]2(μ2-Cl)2 in moderate to excellent yield. The resulting dimers were characterized by nuclear magnetic resonance (NMR) spectroscopy, single-crystal X-ray diffraction (XRD), high-resolution mass spectrometry (HRMS), elemental analysis, and examined as catalysts for oxidative lactonization of 1,4- and 1,5-diols. Oxidative lactonization of 1,4-butanediol to afford γ-butyrolactone proceeded selectively and efficiently using [(η5-Me4C5R)IrCl]2(μ2-Cl)2 as the catalyst. Several R substituents were tested to assess electronic substituent effects. The most active complex contained an electron donating group, R = CHMe2 and successfully catalyzed the formation of diols to lactones across a range of 1,4- and 1,5-diols, generally in high yield. Computational analysis of the rate-determining b-hydrogen elimination reactions provided an atomistic account of observed trends in reaction yield and selectivity as a function of substrate structure, while accounting neatly for the observed selective formation of lactones (vs. succinaldehyde) in the transfer dehydrogenation of 1,4-butyrolactone. / Doctor of Philosophy / Rhodium(III) and iridium(III) complexes are useful synthetic precursors, catalysts, and biologically active compounds. This dissertation explores a rapid synthesis of these metal complexes and their subsequent catalytic applications with 1,4- and 1,5-diols. The oxidative lactonization of diols with rhodium and iridium complexes is an attractive one-pot synthesis, opening a variety of lactones to be produced. Structural studies involving novel fluorinated rhodium and iridium chloro-bridged dimers are discussed in detail.

In our continuing search for biologically active natural products from Madagascar as part of an International Cooperative Biodiversity Group (ICBG) program, a total of four antiproliferative extracts were studied, leading to the isolation of twelve novel compounds with antiproliferative activity against the A2780 human ovarian cancer line, and one extract with antimalarial activities was studied, which led to the isolation of five new natural products with antimalarial activities against the Dd2 and HB3 malarial parasites. The plants and their metabolites are discussed in the following order: one new xanthone and two known guttiferones from Symphonia tanalensis Jum. & H. Perrier (Clusiaceae); four new diphenyl propanes and one new cyclohepta-dibenzofuran skeleton from Bussea sakalava (Fabaceae); four new cardiac glycosides from Leptadenia madagascariensis Decne. (Apocynaceae); two new and four known alkaloids from Ambavia gerrardii (Baill.) Le Thomas (Annonaceae); five new sesquiterpene lactones from Polycline proteiformis Humbert (Asteraceae). The structures of all compounds were determined by analysis of their mass spectrometric, 1D and 2D NMR, UV and IR spectroscopic and optical rotation data. Other than structure elucidation, this dissertation also involve bioactivity evaluation of all the isolates, synthesis of two interesting alkaloids, as well as a proposal for the possible biosynthetic pathway of the new cyclohepta-dibenzofuran skeleton. / Ph. D.

Seeds of the root parasitic plants of the genus Orobanche germinate specifically in response to host-derived germination signals, which enables parasites to detect and attack preferred hosts. The best characterized class of germination stimulants is the strigolactones (SLs), although some species respond to non-SL compounds, such as dehydrocostus lactone (DCL). Recent work indicates that SLs are perceived by members of the KARRIKIN-INSENSITIVE2 (KAI2) gene family, and suggests that within parasitic Orobanchaceae the KAI2 genes have undergone duplication and specialization. The "diverged" clade of these genes, termed KAI2d, has been shown to bind SL germination stimulants in model system assays, but the precise role for KAI2d in regulating germination specificity in a parasitic plant has not been demonstrated. To address this issue, we used genetic and genomic approaches involving two closely related species, Orobanche cernua and O. cumana, which differ primarily in host range and stimulant preference. Orobanche cernua parasitizes tomato (and other Solanaceous crops) and responds to orobanchol, the major SL from tomato roots, whereas O. cumana specifically parasitizes sunflower and responds to DCL. Crosses between O. cernua and O. cumana produced hybrid populations that segregate for stimulant specificity, creating a tractable genetic system. Orobanche cernua contains four KAI2d genes (numbered OrceKAI2d1-4), while O. cumana contains six genes (OrcuKAI2d1-6). The DNA from 94 F2 hybrids was genotyped to identify the KAI2d gene composition and these were correlated with germination phenotype. The pattern of segregation indicated that the KAI2d genes are linked, but pointed to OrceKAI2d2 as a likely orobanchol receptor. Response to DCL was associated with inheritance of all O. cumana KAI2d genes together. Each KAI2d gene was expressed in the Arabidopsis thaliana kai2 mutant background and tested for ability to recover the mutant phenotype when exposed to SLs (including orobanchol, 5-deoxystrigol and GR24) or DCL. One O. cernua gene, OrceKAI2d2, responded to all SLs, but not DCL in this system. No DCL-specific KAI2 genes were identified. In summary, we have identified the likely SL receptor in O. cernua, and show evidence that the DCL receptor is either not a KAI2d protein, or uses KAI2d in combination with other signaling pathway components. / Ph. D.

As part of an International Cooperative Biodiversity Group (ICBG) program and a continuing search for antiproliferative natural products from the Madagascar rainforest, and a collaborative research project established between Virginia Tech and the Institute for Hepatitis and Virus Research (IHVR) focusing on searching for bioactive natural products from tropical forests in South Africa, 20 extracts were selected for investigation based on their antiproliferative activities against A2780 human ovarian cancer cell line or antimalarial activities against the Dd2 strain of Plasmodium falciparum. Bioassay-guided fractionation of seven of the extracts yielded twenty new compounds and twenty-four known compounds, and their structures were elucidated by using a combination of 1D (1H and 13C) and 2D NMR spectroscopy including COSY, HASQC, HMQC, HMBC, and NOESY sequences, mass spectrometry, UV, IR, ECD, optical rotation, and chemical conversions. In addition, ten known compounds were isolated from another five of the extracts, while studies on the remaining extracts were suspended due to loss of activity, unworkable small amounts of material, or low structural interest. The plants and their metabolites are discussed in the following order: five new antimalarial 5,6-dihydro-𝛼-pyrones and six bicyclic tetrahydro-𝛼-pyrone derivatives from Cryptocarya rigidifolia (Lauraceae); two new and five known antiproliferative lignans from Cleistanthus boivinianus (Phyllanthaceae); two new and two known antiproliferative sesquiterpenes lactones from Piptocoma antillana (Asteraceae); one new antiproliferative 1,4-naphthoquinone, one known antiproliferative isoflovonoid, and five known antiproliferative stilbenoids from Stuhlmannia moavi (Leguminosae); four known antiproliferative bisbenzylisoquinoline alkaloids from Anisocycla grandidieri (Menispermaceae); one new and two known antiproliferative butanolides, and two new antiproliferative secobutanolides from Ocotea macrocarpa (Lauraceae); one new antiproliferative and five known antiproliferative diterpenoids from Malleastrum rakotozafyi (Meliceae); and 10 known compounds from Monoporus sp. (Myrsinaceae), Premna corymbosa (Verbenaceae), Premna perplexanes (Verbenaceae), Epallage longipes (Asteraceae), and Cinnamosma fragrans (Canellaceae). / Ph. D.

Yes / N-acylhomoserine lactone (AHL)-based quorum sensing (QS) is important for the regulation of proteobacterial virulence determinants. Thus, the inhibition of AHL synthases offers non-antibiotics-based therapeutic potentials against QS-mediated bacterial infections. In this work, functional AHL synthases of Pseudomonas aeruginosa LasI and RhlI were heterologously expressed in an AHL-negative Escherichia coli followed by assessments on their AHLs production using AHL biosensors and high resolution liquid chromatography–mass spectrometry (LCMS). These AHL-producing E. coli served as tools for screening AHL synthase inhibitors. Based on a campaign of screening synthetic molecules and natural products using our approach, three strongest inhibitors namely are salicylic acid, tannic acid and trans-cinnamaldehyde have been identified. LCMS analysis further confirmed tannic acid and trans-cinnemaldehyde efficiently inhibited AHL production by RhlI. We further demonstrated the application of trans-cinnemaldehyde inhibiting Rhl QS system regulated pyocyanin production in P. aeruginosa up to 42.06%. Molecular docking analysis suggested that trans-cinnemaldehyde binds to the LasI and EsaI with known structures mainly interacting with their substrate binding sites. Our data suggested a new class of QS-inhibiting agents from natural products targeting AHL synthase and provided a potential approach for facilitating the discovery of anti-QS signal synthesis as basis of novel anti-infective approach. / University of Malaya High Impact Research (HIR) Grant (UM-MOHE HIR Grant UM.C/625/1/HIR/MOHE/CHAN/14/1, no. H-50001-A000027) given to K.G.C. and National Natural Science Foundation of China (no. 81260481) given to H.W.

Yes / Bacteria sense their own population size, tune the expression of responding genes, and behave accordingly to environmental stimuli by secreting signaling molecules. This phenomenon is termed as quorum sensing (QS). By exogenously manipulating the signal transduction bacterial population behaviors could be controlled, which may be done through quorum quenching (QQ). QS related regulatory networks have been proven their involvement in regulating many virulence determinants in pathogenic bacteria in the course of infections. Interfering with QS signaling system could be a novel strategy against bacterial infections and therefore requires more understanding of their fundamental mechanisms. Here we review the development of studies specifically on the inhibition of production of N-acyl-homoserine lactone (AHL), a common proteobacterial QS signal. The opportunistic pathogen, Pseudomonas aeruginosa, equips the alkylquinolone (AQ)-mediated QS which also plays crucial roles in its pathogenicity. The studies in QQ targeting on AQ are also discussed. / University of Malaya High Impact Research Grants (UMC/625/1/HIR/MOHE/CHAN/01, A-000001-50001,and UMC/625/1/HIR/MOHE/CHAN/14/1, H-50001-A000027)

Yes / Burkholderia spp. rely on N-acyl homoserine lactone as quorum-sensing signal molecules which coordinate their phenotype at the population level. In this work, we present the whole genome of Burkholderia sp. strain A9, which enables the discovery of its N-acyl homoserine lactone synthase gene. / UM High Impact Research Grants (UM-MOHE HIR grant UM C/625/1/HIR/MOHE/CHAN/01, H-50001-A000001 and UMMOHE HIR Grant UM C/625/1/HIR/MOHE/CHAN/14/1, H-50001- A000027)

La mise au point de méthodologies de synthèse permettant d’obtenir des substances énantiopures présentant une diversité structurale importante est une préoccupation majeure de la chimie contemporaine. L’efficacité de ces approches doit être désormais évaluée au regard de critères tel que la pureté optique et l’analyse de l’impact écologique des processus mis en jeu. Au cours de ce travail nous avons développé de nouvelles méthodologies permettant d’accéder à des γ-lactones polyfonctionnelles chirales. La 5-hydroxyfuran-2(5H)-one, petite molécule bio-sourcée, a servi d’élément clé pour la construction du cycle γ-lactone. Dans une première approche, des γ-lactones chirales possédant une grande diversité structurale ont été obtenues par une séquence réactionnelle « one pot ». Une première étape de catalyse organique a permis d’activer le transfert énantiosélectif d’acides boroniques sur la 5-hydroxyfuran-2(5H)-one ; l’adduit chiral obtenu a ensuite été engagé dans une réaction de Passerini pour construire le cycle lactone. Dans une deuxième partie, nous avons mis au point des réactions d’alkylation de γ-lactones, catalytiques et énantiosélectives, pour la formation de γ-lactones possédant un centre quaternaire. L’utilisation de complexes du palladium et de l’iridium a permis d’effectuer des réactions d’Alkylation Allylique Asymétrique avec un très bon contrôle de l’induction asymétrique. Le squelette carboné des lactones obtenues par AAA a permis d’effectuer une fonctionnalisation inédite d’hétérocycles aromatiques par des réactions sigmatropiques [3,3]. Enfin des réactions d’alkylations énantiosélectives induites par un catalyseur organique ont été évaluées. / The development of new synthetic pathway leading to enantiopure compounds with significant structural diversity is an ongoing challenge in many fields of chemistry. The efficiency of these processes has to be evaluated, not only in term of quantitative criteria, such as yields and/or optical purities of obtained compounds, but also by analyzing the environmental impact of the different processes involved. In this work, we sought to develop new methodologies for the synthesis of polyfunctional chiral γ-lactones. The 5-hydroxyfuran-2(5H)-one, a bio-based molecule, was used as a platform to the construction of the γ-lactone ring. In the first part of our work, a variety of chiral γ-lactone displaying a great structural diversity were obtained by a one-pot sequential reaction. First, a step of enantioselective organocatalysis allowed the activation for the transfer of boronic acids on the hydroxyfuran-2(5H)-one; then the chiral adduct formed was engaged in a Passerini reaction leading to the construction of the lactone ring. In a second part, our efforts focused then on the development of catalytic asymmetric alkylation reactions leading to the construction of γ-lactones bearing an all-carbon quaternary stereocenter. Asymmetric allylic reactions were carried out with a very good control of the selectivity of the reaction by the use of palladium and iridium complexes catalysts. Theses lactones bearing an [1,5]-diene scaffold were then engaged in a sigmatropic [3,3] reaction opening a path for a new approach to the functionalization of aromatic heterocycles. Finally, the use of organic enantioselective catalysis was envisioned for the creation of all-quaternary stereocenters.

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