Synthesis and Biological Activity of Pyrrolizidine Alkaloids and Analogues

The work presented in this thesis is concerned with the synthesis and biological activity of pyrrolizidine alkaloids and analogues and has been divided into seven main areas: (a) the isolation and preparation of simple derivatives of pyrrolizidine alkaloids isolated from plants and root cultures available within the University; (b) the synthesis of synthanecine A, and derivatives of this compound for investigation of their anti-tumour activity; (c) approaches to the synthesis of one enantiomer of synthanecine A; (d) approaches to the synthesis of a novel synthanecine; (e) attempted synthesis of novel necic acids for esterification with synthanecine A; (f) the synthesis of novel pyrrolizidine alkaloid analogues via a 1,3-dipolar cycloaddition; (g) investigation of the biological activity of some of the compounds isolated and synthesised during the project. Those topics of greatest significance in the thesis are summarised more fully below. Isolation and Derivatisation of Pyrrolizidine Alkaloids. A number of pyrrolizidine alkaloids are available in the Chemistry Department, either from plant sources or from root cultures. Samples of these compounds were requested by Dr. Bryan Hanley, MAFF, Norwich, for metabolic studies. These compounds were also required to be radiolabelled with 14C. This was achieved by feeding [1,4-14C]putrescine (A) to the plants and root cultures. The N- oxides of the pyrrolizidine alkaloids isolated were synthesised. N-Oxides of the radiolabelled alkaloids were also synthesised. The quaternary methiodide derivatives were also prepared for biological testing. Rosmarinine (B) can be readily isolated in gram quantities from Senecio pleistocephalus. This compound can be converted into the toxic alkaloid senecionine (C) by elimination of the hydroxyl group to give a double bond in the 1,2-position. This elimination was carried out with limited success. Approaches to the Synthesis of One Enantiomer of Synthanecine A In order to prepare a closer pyrrolizidine alkaloid analogue, the synthesis of one enantiomer of synthanecine A (H, R=H) was undertaken. Several approaches were tried in this synthesis. In all approaches the key intermediate methyl (R)-3-(N-methylamino)-4-hydroxybutanoate was identified (D). Syntheses starting from (S)- malic acid (E) and from D-aspartic acid (F) were attempted. L-Aspartic acid was also used because it is cheaper and more readily available. The synthesis from L-aspartic acid proved most successful, and this route was advanced beyond the key intermediate to the diester (G). Approaches to the Synthesis of a Novel Optically Active Synthanecine Synthanecine A differs from a pyrrolizidine alkaloid in the number of carbon atoms in the molecule. Thus it was decided to attempt a synthesis of the novel synthanecine (H, R=Me). The substituted pentanoic acid derivative (I) was identified as the key intermediate. This compound could be derived from L-threonine. Synthesis of Novel Pyrrolizidine Alkaloids Analogues via a 1,3-Dipolar Cycloaddition The 1,3-dipolar cycloaddition reaction of the azomethine ylide (J) derived from N-benzyl-N-(trimethylsilylmethyl)aminomethyl methyl ether (K) with a variety of dipolarophiles was used to prepare a range of pyrrolidines of the general structure (L) and the 3-pyrroline (M). Simple derivatives of the pyrrolidines were prepared by removal of the benzyl groups and by reduction and further reaction of the ester functions

Decoupled electrolysis using a silicotungstic acid electron-coupled-proton buffer in a proton exchange membrane cell

The storage of renewably-generated energy as hydrogen via the electrolysis of water is a fundamental cornerstone of a sustainable hydrogen economy. Conventional electrolysers usually require stable power inputs in order to operate effectively and safely and so may be unsuited to harnessing renewable power, which is often intermittent and diffuse. Electrolysis mediated by Electron-Coupled-Proton Buffers has the potential to overcome some of the challenges surrounding electrolysis using low and/or sporadic power inputs (especially those related to gas crossover) as the use of Electron-Coupled-Proton Buffers allows the oxygen and hydrogen evolution reactions to be completely decoupled from one another. Herein, we show that silicotungstic acid can be used as an Electron-Coupled-Proton Buffer in a proton exchange membrane cell, decoupling the hydrogen and oxygen evolution reactions at steady state current densities as high as 500 mA cm−2. O2 and H2 can be produced continuously by this system by cycling a fixed volume of the Electron-Coupled-Proton Buffer solution. Even at current densities as low as 25 mA cm−2, the level of hydrogen in the oxygen stream is <0.4%, whereas a conventional proton exchange membrane electrolyser operating at this current density produces oxygen containing nearly 2% hydrogen (unacceptable for most applications). Furthermore, using silicotungstic acid as an Electron-Coupled-Proton Buffer also confers greater tolerance to non-deionised water inputs and reduces fluoride release from the perfluorosulfonated membrane (a marker for membrane degradation) relative to a conventional proton exchange membrane electrolyser. Together, these results highlight the promise and potential advantages of Electron-Coupled-Proton Buffers (and silicotungstic acid in particular) for the electrolytic production of hydrogen and oxygen over a wide range of current densities, such as might be produced by renewable power inputs

A practical, organic-mediated, hybrid electrolyser that decouples hydrogen production at high current densities

Hydrogen is seen as a sustainable fuel of the future, yet the vast majority of global hydrogen production comes from the reformation of fossil fuels. Electrolytic water splitting using proton exchange membrane electrolysers (PEMEs) provides a pathway to sustainable hydrogen production through coupling to renewable energy sources, but can suffer from gas crossover at low current densities and high operating pressures, causing explosive gas mixtures and decreasing cell lifetimes. Here we demonstrate the application of a highly stable, organic electron-coupled proton buffer (ECPB) which allows the decoupling of hydrogen and oxygen production during water splitting. By merging concepts from redox flow battery and PEM electrolysis research, we have built a hybrid electrolyser device capable of decoupling the gas evolution reactions during water splitting. The device improves on both gas purity and operational safety, while still working at industrially relevant, high current density. Anthraquinone-2,7-disulfonic acid was used as an organic redox mediator in this two-step process, producing H2 at current densities of up to 3.71 A cm−2 at 2.00 V, extending the concept of the ECPB

Aqueous solutions of super reduced polyoxotungstates as electron storage systems

Due to the increasing energy density demands of battery technology, it is vital to develop electrolytes with high electron storage capacity. Polyoxometalate (POM) clusters can act as electron sponges, storing and releasing multiple electrons and have potential as electron storage electrolytes for flow batteries. Despite this rational design of clusters for high storage ability can not yet be achieved as little is known about the features influencing storage ability. Here we report that the large POM clusters, {P5W30} and {P8W48}, can store up to 23 e− and 28 e− per cluster in acidic aqueous solution, respectively. Our investigations reveal key structural and speciation factors influencing the improved behaviour of these POMs over those previously reported (P2W18). We show, using NMR and MS, that for these polyoxotungstates hydrolysis equilibria for the different tungstate salts is key to explaining unexpected storage trends while the performance limit for {P5W30} and {P8W48}, can be attributed to unavoidable hydrogen generation, evidenced by GC. NMR spectroscopy, in combination with the MS analysis, provided experimental evidence for a cation/proton exchange process during the reduction/reoxidation process of {P5W30} which likely occurs due to this hydrogen generation. Our study offers a deeper understanding of the factors affecting the electron storage ability of POMs and provides insights allowing for further development of these materials for energy storage

Convergence of multiple synthetic paradigms in a universally programmable chemical synthesis machine

Although the automatic synthesis of molecules has been established, each reaction class uses bespoke hardware. This means that the connection of multi-step syntheses in a single machine to run many different protocols and reactions is not possible, as manual intervention is required. Here we show how the Chemputer synthesis robot can be programmed to perform many different reactions, including solid-phase peptide synthesis, iterative cross-coupling and accessing reactive, unstable diazirines in a single, unified system with high yields and purity. Developing universal and modular hardware that can be automated using one software system makes a wide variety of batch chemistry accessible. This is shown by our system, which performed around 8,500 operations while reusing only 22 distinct steps in 10 unique modules, with the code able to access 17 different reactions. We also demonstrate a complex convergent robotic synthesis of a peptide reacted with a diazirine-a process requiring 12 synthetic steps

Blood pressure monitoring in high-risk pregnancy to improve the detection and monitoring of hypertension (the BUMP 1 and 2 trials): protocol for two linked randomised controlled trials.

INTRODUCTION: Self-monitoring of blood pressure (BP) in pregnancy could improve the detection and management of pregnancy hypertension, while also empowering and engaging women in their own care. Two linked trials aim to evaluate whether BP self-monitoring in pregnancy improves the detection of raised BP during higher risk pregnancies (BUMP 1) and whether self-monitoring reduces systolic BP during hypertensive pregnancy (BUMP 2). METHODS AND ANALYSES: Both are multicentre, non-masked, parallel group, randomised controlled trials. Participants will be randomised to self-monitoring with telemonitoring or usual care. BUMP 1 will recruit a minimum of 2262 pregnant women at higher risk of pregnancy hypertension and BUMP 2 will recruit a minimum of 512 pregnant women with either gestational or chronic hypertension. The BUMP 1 primary outcome is the time to the first recording of raised BP by a healthcare professional. The BUMP 2 primary outcome is mean systolic BP between baseline and delivery recorded by healthcare professionals. Other outcomes will include maternal and perinatal outcomes, quality of life and adverse events. An economic evaluation of BP self-monitoring in addition to usual care compared with usual care alone will be assessed across both study populations within trial and with modelling to estimate long-term cost-effectiveness. A linked process evaluation will combine quantitative and qualitative data to examine how BP self-monitoring in pregnancy is implemented and accepted in both daily life and routine clinical practice. ETHICS AND DISSEMINATION: The trials have been approved by a Research Ethics Committee (17/WM/0241) and relevant research authorities. They will be published in peer-reviewed journals and presented at national and international conferences. If shown to be effective, BP self-monitoring would be applicable to a large population of pregnant women. TRIAL REGISTRATION NUMBER: NCT03334149.This work is funded from a National Institute for Health Research (NIHR) Programme grant for applied research (RP-PG- 1209-10051) and an NIHR Professorship awarded to RJM (NIHR-RP- R2- 12-015). RJM and KLT receive funding from the National Institute for Health Research (NIHR) Collaboration for Leadership in Applied Health Research and Care Oxford at Oxford Health NHS Foundation Trust. JS is a National Institute for Health Research (NIHR) Senior Investigator and supported by the National Institute for Health Research (NIHR) Collaboration for Leadership in Applied Health Research and Care South London (NIHR CLAHRC South London) at King’s College Hospital NHS Foundation Trust. Service support costs will be administered through the NIHR Clinical Research Network

Effective storage of electrons in water by the formation of highly reduced polyoxometalate clusters

Aqueous solutions of polyoxometalates (POMs) have been shown to have potential as high-capacity energy storage materials due to their potential for multi-electron redox processes, yet the mechanism of reduction and practical limits are currently unknown. Herein, we explore the mechanism of multi-electron redox processes that allow the highly reduced POM clusters of the form {MO3}y to absorb y electrons in aqueous solution, focusing mechanistically on the Wells–Dawson structure X6[P2W18O62], which comprises 18 metal centers and can uptake up to 18 electrons reversibly (y = 18) per cluster in aqueous solution when the countercations are lithium. This unconventional redox activity is rationalized by density functional theory, molecular dynamics simulations, UV–vis, electron paramagnetic resonance spectroscopy, and small-angle X-ray scattering spectra. These data point to a new phenomenon showing that cluster protonation and aggregation allow the formation of highly electron-rich meta-stable systems in aqueous solution, which produce H2 when the solution is diluted. Finally, we show that this understanding is transferrable to other salts of [P5W30O110]15– and [P8W48O184]40– anions, which can be charged to 23 and 27 electrons per cluster, respectively

3D printed flow plates for the electrolysis of water: an economic and adaptable approach to device manufacture

The electrolysis of water is considered a promising route to the production of hydrogen from renewable energy sources. Electrolysers based on proton exchange membranes (PEMs) have a number of advantages including high current density, high product gas purity and the ability to operate at high pressure. Despite these advantages the high cost of such devices is an impediment to their widespread deployment. A principal factor in this cost are the materials and machining of flow plates for distribution of the liquid reagents and gaseous products in the electrochemical cell. We demonstrate the production and operation of a PEM electrolyser constructed from silver coated 3D printed components fabricated from polypropylene. This approach allows construction of light weight, low cost electrolysers and the rapid prototyping of flow field design. Furthermore we provide data on the operation of this electrolyser wherein we show that performance is excellent for a first generation device in terms of overall efficiency, internal resistances and current–voltage response. This development opens the door to the fabrication of light weight and cheap electrolysers as well as related electrochemical devices such as flow batteries and fuel cells

Decoupled Electrolysis using a Silicotungstic Acid Electron-Coupled-Proton Buffer in a Proton Exchange Membrane Cell

The storage of renewably-generated energy as hydrogen via the electrolysis of water is a fundamental cornerstone of a sustainable hydrogen economy. Conventional electrolysers usually require stable power inputs in order to operate effectively and safely and so may be unsuited to harnessing renewable power, which is often intermittent and diffuse. Electrolysis mediated by Electron-Coupled-Proton Buffers has the potential to overcome some of the challenges surrounding electrolysis using low and/or sporadic power inputs (especially those related to gas crossover) as the use of Electron-Coupled-Proton Buffers allows the oxygen and hydrogen evolution reactions to be completely decoupled from one another. Herein, we show that silicotungstic acid can be used as an Electron-Coupled-Proton Buffer in a proton exchange membrane cell, decoupling the hydrogen and oxygen evolution reactions at steady state current densities as high as 500 mA cm−2. O2 and H2 can be produced continuously by this system by cycling a fixed volume of the Electron-Coupled-Proton Buffer solution. Even at current densities as low as 25 mA cm−2, the level of hydrogen in the oxygen stream is <0.4%, whereas a conventional proton exchange membrane electrolyser operating at this current density produces oxygen containing nearly 2% hydrogen (unacceptable for most applications). Furthermore, using silicotungstic acid as an Electron-Coupled-Proton Buffer also confers greater tolerance to non-deionised water inputs and reduces fluoride release from the perfluorosulfonated membrane (a marker for membrane degradation) relative to a conventional proton exchange membrane electrolyser. Together, these results highlight the promise and potential advantages of Electron-Coupled-Proton Buffers (and silicotungstic acid in particular) for the electrolytic production of hydrogen and oxygen over a wide range of current densities, such as might be produced by renewable power inputs

CFD based design optimization of a cabinet nitrogen generator

The design of mechanical enclosures is evolving to be more compact and quieter and this compromises the cooling of the internal components. Computational Fluid Dynamics (CFD) based optimization could significantly improve the cooling efficiency of the critical parts of the components to ensure their performance and reliability. This work presents the CFD surrogate based optimization of the forced cooling of two reciprocating compressors located in an enclosure from a gas generator. Due to the challenging project time constraints, the accuracy of the results was compromised to make optimization feasible. The parameters to be optimized were related to the position of the compressors and the cooling fans. The boundary conditions associated to the cooling of the critical parts were derived by experimental data. Artificial Neural Networks (ANNs) were used to construct a surrogate model of the computational model to reduce the time and resources required. The combination of the ANN model with a multi start-gradient based algorithm optimized the position of compressors and cooling fans to minimize the average temperature on the critical parts. A set of new enclosure designs were found with outstanding CFD based performance compared with the design elaborated by engineering intuition