Although carbon dioxide as a greenhouse gas is a serious environmental concern, it remains a valuable C1 source if viable methods are available for its conversion into useful products. Herein, we present recent progress in the synthesis of aliphatic, aromatic, cyclic, and bicyclic beta-ketocarboxylic acids and the promising results from subsequent asymmetric hydrogenation to give beta-hydroxycarboxylic acids. For the synthesis of the beta-ketocarboxylic acids, we investigated the effects of temperature, reaction time, and amount of 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU), which is a promoter for carbon-carbon bond formation with CO2. The highest-yielding conditions for this DBU-promoted carboxylation reaction were used to carboxylate a number of aliphatic and aromatic substrates. In order to determine whether the hydrogenation reaction will effectively compete with the in situ decarboxylation of the beta-ketocarboxylic acids, 1H NMR spectroscopy was used to monitor the rate of decarboxylation. The solvent, electronic, and steric effect on the rate of decarboxylation was investigated by testing a variety of beta-ketocarboxylic acids. Using œRuCl2{(S)-BINAP} catalyst precursor, we determined the effect that solvent, H2 pressure, base, and substrate substitution had on the enantioselectivity of the asymmetric hydrogenation. CH2Cl2 and MeOH were determined to be the best solvents because of the high hydrogenation selectivity, high enantioselectivity, and decreased reaction times. These standard conditions were used to hydrogenate the variety of aliphatic and aromatic beta-ketocarboxylic acids previously synthesized. Additional experiments, including deuterium labelling, were performed in an attempt to elucidate the hydrogenation mechanism and the actively hydrogenated tautomer. These results lead us to believe that different reaction pathways occur in protic versus aprotic solvents. The results discussed herein represent the first in depth investigation of transition metal catalyzed hydrogenation of beta-ketocarboxylic acids. These results are very encouraging because enantioselectivities greater than 99 % were achieved for multiple beta-keto acids. This synthesis is industrially advantageous due to the limited number of reactants required, their low-cost, and the potential for recycling unused materials. / Thesis (Master, Chemistry) -- Queen's University, 2008-08-26 10:17:34.703

Les travaux rapportés dans ce mémoire concernent la synthèse et la réactivité des disilanes, en vue de leur application vers la synthèse de la 10-silatestostérone. Dans un premier temps, l’intérêt potentiel de la cible choisie, la 10-silatestostérone, a été étayé au moyen d’exemples de molécules d’intérêt biologique présentant un atome de silicium, issus de la littérature. L’objectif de la silasubstitution serait d’empêcher la biotransformation de la molécule en composé aromatique potentiellement cancérigène. Enfin, la substitution en position 10 de l’atome de carbone quaternaire permet d’éviter l’introduction de substituants supplémentaires, susceptibles d’affecter l’activité hormonale de la molécule. Dans un second temps, nous avons optimisés les outils nécessaires à la synthèse de la 10-silatestostérone. Une nouvelle stratégie pour accéder à des disilanes non symétriques a été développée par réaction de déphénylation chlorative. Puis l’étude de la réaction de coupure hétérolytique de la liaison Si-Si de phényldisilanes non symétriques par piégeages des silylures ainsi générés et par une analyse par RMN ¹H a permis de mettre en évidence une sélectivité en faveur du phénylsilylure. Enfin cette réaction a été appliquée pour accéder à des composés carbonylés β-silylés par réaction d’addition-1,4 de silylcuprolithiens sur des composés carbonylés α,β-éthyléniques tels que le cyclohexènecarboxylate de méthyle, un modèle simplifié de l’ester bicyclique que nous envisageons d’utiliser pour apporter la partie C&D de la 10-silatestostérone. Enfin le troisième chapitre porte sur la synthèse des précurseurs de la 10-silatestostérone. Dans une première partie un disilane hautement fonctionnalisé, le 1-isopropényl-1-(3-oxopropyl)disilane a pu être préparé en mettant à profit la réaction de déphénylation chlorative. Puis la réaction ène intramoléculaire de cet aldéhyde a conduit au 1-méthyl-1-triméthylsilyl-2-méthylidène-1-silacyclohexan-4-ol précurseur du cycle A de la 10-silatestostérone ainsi qu’au 1,2-diméthyl-2-triméthylsilyl-1-silacyclohexane-1,4-diol issu d’un réarrangement hautement diastéréosélectif. Dans une seconde partie, est présentée la synthèse énantiosélective de la partie C&D de la 10-silatestostérone par une réaction clé de carboxylation de la dicétone d’Hajos-Parrish. L’étape suivante consistera à réaliser la coupure sélective du disilane porteur du cycle A, et à additionner le silylure ainsi généré sur l’ester α,β-insaturé bicyclique. / This thesis is devoted to the synthesis and the study of the reactivity of disilanes in order to involve them in the 10-silatesosterone synthesis. The first part focuses on the usefulness of 10-silatestosterone by developing some relevant examples, from the literature, of biologically active molecules containing a silicon atom. Thus in the testosterone field, the substitution of a carbon atom by a silicon one is supposed to avoid aromatization of the molecule during its biotransformation. Moreover the 10 position of the silasubstitution prevents from the introduction of new groups, which is known to affect the biological activity of the molecule. In the second part the chemical tools for the 10-silatestosterone synthesis are described. A new strategy has been developed to obtain non symmetrical disilanes by a chlorinative dephenylation sequence. The heterolytic cleavage of the Si-Si bond of non symmetrical phenyldisilanes has been studied by trapping the generated silylanion and by ¹H NMR experiments. Selectivity for the phenylsilylanion formation has been established. Finally this reaction was fully exploited to access to β-silylated carbonyl compounds after 1,4-addition of silylcuprates on α,β-unsaturated carbonyl compounds as methylcyclohexenecarboxylate, model of the C&D rings of 10-silatesosterone. The third part presents the 10-silatestosterone precursors synthesis. At First high functionalized 1-isopropenyl-1-(3-oxopropyl)disilane has been obtained by using the previously performed chlorinative dephenylation reaction. Then intramolecular ene reaction of this aldehyde leads to a mixture of 1-methyl-1-trimethylsilyl-2-methylidene-1-silacyclohexan-4-ol, which corresponds to the 10-silatestosterone A ring and 1,2-dimethyl-2-trimethylsilyl-1-silacyclohexane-1,4-diol coming from diastereoselective rearrangement. Eventually, enantioselective synthesis of 10-silatestosterone C&D rings was achieved through a sequence involving a Hajos-Parrish ketone carboxylation key step. Further work would consist on a selective cleavage of A ring disilane followed by the 1,4-addition of the generated silylanion on α,β-unsaturated bicyclic ester.

Leaf traits have long been used to classify and characterise species in natural ecosystems. In addition, leaf traits provide important information about plants’ strategies for the use of resources and can be used to improve our understanding of ecosystem level processes such as nutrient cycling and carbon allocation. To explore the linkages between leaf traits and productivity, we worked in two resource-limited ecosystems (a grassland and a forest), and used leaf traits to understand how species respond to changes in available resources and their relationship to ecosystem processes. We worked in a species rich limestone-grassland located in central England, which has been subjected to long-term climatic manipulation (winter warming, summer drought and extra summer rainfall). We characterised species composition in terms of their identity, abundance and leaf structural properties (nitrogen content and leaf mass per area (LMA)) in the main treatments and the control. We found that change in species abundance was the most important factor to understand the differences in productivity (above ground biomass and total foliar nitrogen). We then measured CO2 exchange at ecosystem level, using a chamber technique, and assessed the treatments’ effect on the gross primary productivity (GPP) and ecosystem respiration (Reco). GPP and Reco were controlled by soil moisture and above ground biomass but also influenced by the conditions experienced during the growing season prior to the measuring period. Our second location was a post-disturbance chronosequence in a seasonally dry tropical forest in Costa Rica and we used leaf level gas exchange measurements to explore the role of nitrogen (N) and phosphorus (P) on the temporal-spatial variation of photosynthesis of dominant species. We found that photosynthetic efficiency was strongly linked to leaf N and P content, but that there was an important seasonal pattern on this relationship likely associated to P remobilization. Additionally we found seasonal changes in resources (water, nutrients) had a larger impact on the photosynthetic parameters than changes along the chronosequence. The two ecosystems studied for this thesis are contrasting in their physiognomy, species composition and climate, but are also characterised by species whose structural traits (high LMA and high C:N ratio) are likely to have a significant impact on the nutrient cycling processes. We learned that leaf traits provide important information about species strategies and their usage of resources and they can also aid to address questions at ecosystem level in time and space, either through simple aggregation or as emergent properties. Additionally, the traits explored are important input information to up-scale processes from leaf to the ecosystem level, a step needed to address the effect changes in resources will have on the seasonally dry tropical forest and grasslands, which represent a significant fraction of the total global carbon storage.

Variations in the relative amounts of ¹²C and ¹³C in microbial biomass can be used to infer the pathway(s) autotrophs use to fix and assimilate dissolved inorganic carbon. Discrimination against ¹³C by the enzymes catalyzing autotrophic carbon fixation is a major factor dictating the stable carbon isotopic composition (δ¹³C = {[¹³C/¹²Csample/¹³C/¹²Cstandard] - 1} X 1000) of biomass. Six different forms of ribulose 1,5-bisphosphate carboxylase/oxygenase or RubisCO (IA, IB, IC, ID, II, and III), the carboxylase of the Calvin-Benson-Bassham cycle (CBB), are utilized by algae and autotrophic bacteria that rely on the CBB cycle for carbon fixation. To date, isotope discrimination has been measured for form IA, IB, and II RubisCOs. Isotopic discrimination, expressed as ε values (={[¹²k/¹³k] - 1} X 1000; ¹²k and ¹³k = rates of ¹²C and ¹³C fixation) range from 18 to 29‰, explaining the variation in biomass δ¹³C values of autotrophs that utilize these enzymes. Isotope discrimination by form IC RubisCO has not been measured, despite the presence of this enzyme in many proteobacteria of ecological interest, including marine manganese-oxidizing bacteria, some nitrifying and nitrogen-fixing bacteria, and extremely metabolically versatile organisms such as Rhodobacter sphaeroides. The purpose of this work is to determine the e value for the form IC RubisCO enzyme from R. sphaeroides. Under standard conditions (pH 7.5 and 5 mM DIC), form IC RubisCO had an ε value of 29‰. Sampling the full phylogenetic breadth of RubisCO enzymes for isotopic discrimination makes it possible to constrain the range of δ¹³C values of organisms fixing carbon via the Calvin-Benson-Bassham cycle. These results are helpful for determining the degree to which CBB cycle carbon fixation contributes to primary and secondary productivity in microbially-dominated food webs.

Considering the major role of insulin signaling on fatty acid synthesis via stimulation of lipogenic enzymes, differential effects of insulin signaling on individual carbon fluxes for fatty acid synthesis have been investigated by comparing the individual lipogenic fluxes in WT and IRS-1 knockout (IRS-1 KO) brown adipocytes. Results from experiments on WT and IRS-1 KO cells incubated with [5-¹³C] glutamine were consistent with the existence of reductive carboxylation pathway. Analysis of isotopomer distribution of nine metabolites related to the lipogenic routes from glucose and glutamine in IRS-1 KO cells using [U-¹³C] glutamine as compared to that in WT cells indicated that flux through reductive carboxylation pathway was diminished while flux through conventional TCA cycle was stimulated due to absence of insulin signaling in IRS-1 KO cells. This observation was confirmed by quantitative estimation of individual lipogenic fluxes in IRS-1 KO cells and their comparison with fluxes in WT cells. Thus, these results suggest that glutamine’s substantial contribution to fatty acid synthesis can be directly manipulated by controlling the flux through reductive carboxylation of alpha-ketoglutarate to citrate using hormone (insulin). / Singapore-MIT Alliance (SMA)

Five different forms of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO; IA, IB, IC, ID, II), the carboxylase of the Calvin-Benson-Bassham cycle (CBB), are utilized by plants, algae and autotrophic bacteria for carbon fixation. Discrimination against 13C by RubisCO is a major factor dictating the stable carbon isotopic composition (δ13C = {[13C/12C sample/13C/12C standard] - 1} X 1000) of biomass. To date, isotope discrimination, expressed as ε values (={[12k/13k] - 1} X 1000; 12k and 13k = rates of 12C and 13C fixation) has been measured for form IA, IB, and II RubisCOs from only a few species, with ε values ranging from 18 to 29 /. The aim of this study was to better characterize form ID and IC RubisCO enzymes, which differ substantially in primary structure from the IB enzymes present in many cyanobacteria and organisms with green plastids, by measuring isotopic discrimination and kinetic parameters (KCO2 and Vmax). Several major oceanic primary producers, including diatoms, coccolithophores, and some dinoflagellates have form ID RubisCO, while form IC RubisCO is present in many proteobacteria of ecological interest, including marine manganese-oxidizing bacteria, some nitrifying and nitrogen-fixing bacteria, and extremely metabolically versatile organisms such as Rhodobacter sphaeroides. The ε - values of the form ID RubisCO from the coccolithophore, Emiliania huxleyi and the diatom, Skeletonema costatum (respectively 11.1 / and 18.5 /) were measured along with form IC RubisCO from Rhodobacter sphaeroides and Ralstonia eutropha (respectively 22.9 / and 19.0 /). Isotopic discrimination by these form ID/IC RubisCOs is low when compared to form IA/IB RubisCOs (22-29 /). Since the measured form ID RubisCOs are less selective against 13C, oceanic carbon cycle models based on 13C values may need to be reevaluated to accommodate lower ε values of RubisCOs found in major marine algae. Additionally, with further isotopic studies, the extent to which form IC RubisCO from soil microorganisms contributes to the terrestrial carbon sink may also be determined.