Mountain Scholar :: Browsing by Author "Kennan, Alan J., committee member"

Browsing by Author "Kennan, Alan J., committee member"

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Open Access
A clonable selenium nanoparticle in action: high resolution localization of FtsZ using electron tomography
(Colorado State University. Libraries, 2021) Borgognoni, Kanda, author; Ackerson, Christopher J., advisor; Neilson, James, committee member; Kennan, Alan J., committee member; Tsunoda, Susan, committee member
A meaningful understanding of biochemistry requires that we understand the function of proteins, which is heavily dependent on their structure and location within an organism. As the Resolution Revolution of cryo-electron microscopy gains unprecedented ground largely due to the recent development of commercially available direct electron detectors, energy filters, and high-end computation, thousands of protein structures have been solved at atomic or near-atomic resolution, with the highest resolution structure to date being solved at 1.2 Å. A major challenge that has limited the broad use of cryo-electron tomography (cryo-ET) is locating a protein of interest in an organism, as no commercially available high-contrast markers which can be generated in vivo exist. Herein, we present a breakthrough study which aims to solve this problem by synthesizing high contrast metal nanoparticles labeling desired proteins in situ. We isolated a Glutathione Reductase-like Metalloid Reductase (GRLMR), which can reduce selenite and selenate into selenium nanoparticles (SeNPs), from Pseudomonas moraviensis stanleyae found in the roots of a Se hyperaccumulator Stanleya pinnata, or Desert Princes' Plume. A recombinant variant, denoted as a clonable Selenium NanoParticle (cSeNP), was fused to filamentous temperature sensitive protein Z (FtsZ), and the chimera was expressed in vivo using a T7 expression system in model organism E. coli for a proof-of-concept study. Because the SeNPs biogenically produced are amorphous, they exist in a quasistable state and are composed of polymeric Sen in the form of chains and rings that are constantly breaking and reforming. To stabilize the particles during cellular preservation ex aqua, a disproportionation-like reaction can be done either in vivo or as a post-fixation step to form crystalline metal selenide (MSe) NPs that can withstand the processing liquids used. Thereafter, electron tomography was used to acquire a tilt series that was reconstructed into a tomogram and segmented using IMOD, generating a model representing MSeNPs labeling FtsZ filaments. As such, we have demonstrated the potential of using cSeNP as a high resolution marker for cryo-ET. While our study relied on traditional preservation and embedment techniques, we anticipate that for cells preserved via vitrification, cloned SeNPs can be used without subsequent transformation to MSeNPs, as the amorphous particles are stable in aqueous media. Prospectively, we expect that clonable nanoparticle technology will revolutionize cryo-ET, allowing us to localize proteins in vivo at high resolution while maintaining organism viability through metal immobilization. Furthermore, this technique can be expanded to other imaging modalities, such as light microscopy and X-ray tomography, through the discovery and engineering of other clonable nanoparticles.
Open Access
A study of magnetostructural parameters related to spin crossover and single molecule magnetism
(Colorado State University. Libraries, 2013) Fiedler, Stephanie R., author; Shores, Matthew P., advisor; Kennan, Alan J., committee member; Anderson, Oren P., committee member; Crans, Debbie C., committee member; Patton, Carl E., committee member
Herein are described several methods to probe transition metal complexes that were designed by systematic structural modifications to allow for comparison of the resultant magnetic properties. In Chapter 1, a brief introduction is presented to introduce the broader goal of our research: controlling spin on the synthetic level. The introduction provides background regarding spin crossover and single molecule magnetism as well as some previous research to put our projects in context relative to endeavors by other researchers. In Chapter 2, heteroleptic complexes of the form [Fe(H2bip)2(pizR)]Br2 and [Fe(H2bip)2(pizR)](BPh4)2 are described, which have the opportunity to chelate an anion via hydrogen bonding to the H2bip ligand. The third ligand, pizR, is varied between two ligands that we predict will have similar ligand field strengths: pizH and pizMe. Because pizH has an additional hydrogen-bonding site, while pizMe does not, we selected these ligands in order to understand the effect of hydrogen bonding on the anion-binding/spin-state switching event independent from ligand field strength. From these studies, the pizH anion hydrogen bond is observed in crystallographic studies, but does not affect the anion-binding or spin-state switching properties in solution. In Chapter 3, we further investigate the geometry of the pizR ligand in Fe(II) complexes. What began as attempts to study hydrogen bonding in solution revealed unexpected structural distortions of the ligand that are correlated to the spin state of the complexes. The R-substituted nitrogen atom on the imidazoline moiety of the pizR ligand switches between a planar geometry, which is observed for high-spin species, and a pyramidalized geometry, which is observed for low-spin species. We reason that this occurs as a result of the weak-field, non-pizR ligands that influence the ligand field in the high-spin species. Chapters 4 and 5 delve deeper into understanding the relationship between structural parameters and magnetic properties in complexes with non-covalent interactions. In Chapter 4, a series of complexes with metallophilic Pt-Pt interactions show antiferromagnetic magnetic coupling of non-bonded transition metals through a Pt-Pt bond. By comparing complexes with Pt-Pt interactions to those without Pt-Pt interactions, we are able to determine that the Pt-Pt bond is a unique superexchange pathway for the transition metal coupling. Off-set complexes, exhibiting two Pt S interactions instead of one Pt-Pt interaction, do not show evidence of magnetic coupling between transition metals. Furthermore, by comparing magnetic properties of complexes where the apical ligand varies, we determine that the presence or absence of intermolecular interactions is largely independent from the strength of coupling through the Pt-Pt bond. In Chapter 5, an asymmetric trinuclear manganese complex with unique magnetic exchange properties and two high-spin square planar complexes of iron and cobalt, are investigated. The trinuclear manganese complex consists of a central octahedral Mn(II) ion that is coupled antiferromagnetically to another octahedral Mn(II) ion and ferromagnetically to a terminal tetrahedral Mn(II) ion. The different coupling is rationalized as a result of the change in geometry, which affects the orbital overlap that is predicted for each pair of ions. The high-spin square-planar Fe(II) and Co(II) complexes illustrate an unusual pairing of spin-state with square-planar geometry. Moreover, the Fe(II) complex exhibits signs of easy-axis molecular anisotropy and slow-relaxation of magnetization, albeit in the presence of a magnetic field. Lastly, in Chapter 6, we investigate a trinuclear Fe(III) complex bridged by a triethynylmesitylene ligand. The magnetic properties of the complex are compared to a previous Fe(III) complex bridged by a triethynylbenzene ligand. Steric interactions between the aromatic core of the ethynylmesitylene ligand and the auxiliary dimethylphosphinoethane ligands on Fe(III) are predicted to engender a ligand conformation to promote strong orbital overlap. Magnetic susceptibility data for the two complexes both exhibit ferromagnetic coupling between metal centers as expected. Further studies are necessary to confirm the observed behavior, but the new triethynylmesitylene complex appears to have slightly stronger coupling than the previous triethynylbenzene complex.
Open Access
Approaches towards the synthesis of saxitoxin alkaloids
(Colorado State University. Libraries, 2013) Pearson, Aaron Daniel, author; Williams, Robert M., advisor; Wood, John L., committee member; Kennan, Alan J., committee member; Fisher, Ellen R., committee member; Woody, Robert W., committee member
Zetekitoxin AB is a toxin isolated from the Panamanian golden frog (Atelopus zeteki). The structure and activity of zetekitoxin AB was a mystery for 30 years until 2004 when it was elucidated by Yamashita and coworkers. It was found to be a potent analog of Saxitoxin, a marine neurotoxin. Saxitoxin is a sodium channel blocker and has been used extensively as a research probe. Zetekitoxin AB shows an affinity profile similar to saxitoxin, but is considerably more potent. Due to the endangerment of the Panamanian golden frog there is no source of zetekitoxin AB, preventing further studies. Presented herein is a concise synthesis of 4,5-epi-11-hydroxy-saxitoxinol, which utilizes D-ribose to direct an asymmetric Mannich reaction. This approach allows many modes of reactivity, which can be used to potentially access various analogs of saxitoxin with novel bioactivity.
Embargo
Characterization and application of a novel composite nanomaterial comprised of porous protein crystals and synthetic DNA
(Colorado State University. Libraries, 2022) Stuart, Julius D., author; Snow, Christopher D., advisor; Kennan, Alan J., committee member; Shores, Matthew P., committee member; De Long, Susan K., committee member
Composite nanomaterials are systems comprised of multiple components boasting enhanced properties over those exhibited by the individual constituents when isolated. Such systems are highly tunable, allowing one to vary component types (e.g., polymer, metal, ceramic) for influencing performance in various contexts. Moreover, composite nanomaterials can be further modified using biofunctionalization for use in biological settings. Composite nanomaterials have been tested in applications including, but not limited to, textile, defense, food, energy and biomedical engineering. A sub-domain within composite nanomaterials involves porous protein crystals soaking, or, separately, encapsulating various guest molecules. Porous protein crystals are ordered, insoluble assemblies forming a network of nanopores capable of allowing inward diffusion of guest molecules. Moreover, recombinant protein variants can be engineered for probing guest molecule binding to host crystal nanopores further highlighting the tunability of this novel composite nanomaterial. The goal of this work is to evaluate a novel composite nanomaterial comprised of host porous protein crystals and guest double stranded DNA. We show that guest DNA loads into host crystals predominantly along the axial nanopores. Equilibrium adsorption isotherm results suggest guest DNA unbinds from host crystals relatively slowly. Computational modeling and Fluorescence Recovery After Photobleaching (FRAP) studies suggest intra-nanopore guest diffusion is attenuated relative to bulk diffusion. We also show that porous protein crystals loading with synthetic DNA barcodes can be used for tracking mosquitoes. Fluorescently labeled crystals can be ingested by mosquito larvae and adults, followed by detection using fluorescence confocal microscopy. Crystal-bound DNA can be liberated from host crystals by incubation with solution containing deoxynucleotide triphosphates (dNTPs). Previously ingested barcode-loaded crystals can be recovered using standard molecular biological techniques. Lastly, we show a DNA barcode sequence construction strategy that is modular, economical and scalable. Computational sequence design and scoring allowing identification of top candidates for experimental validation. Analysis of next-generation sequencing datasets informs barcode construction specificity while simultaneously reinforcing the multiplexing capabilities boasted by modular DNA barcodes.
Open Access
Enantioselective β-functionalization of enals via N-heterocyclic carbene catalysis
(Colorado State University. Libraries, 2015) White, Nicholas Andrew, author; Rovis, Tomislav, advisor; Kennan, Alan J., committee member; Chen, Eugene Y.-X., committee member; Williams, Robert M., committee member; Kipper, Matt J., committee member
A series of δ-nitroesters were synthesized through the N-heterocyclic carbene catalyzed coupling of enals and nitroalkenes. The asymmetric coupling of these substrates via the homoenolate pathway afford δ-nitroesters in good yield, diastereoselectivity, and enantioselectivity. This methodology allows for the rapid synthesis of δ-lactams. Using this approach, we synthesized the pharmaceutically relevant piperidines paroxetine and femoxetine. A novel single-electron oxidation pathway for the N-heterocyclic carbene generated Breslow intermediate has been developed. Nitroarenes have been shown to transfer an oxygen from the nitro group to the β-position of an enal in an asymmetric fashion to generate β-hydroxy esters. This reaction affords desired β-hydroxy ester products in good yield and enantioselectivity and tolerates a wide range of enal substrates. A dimerization of aromatic enals to form 3,4-disubstituted cyclopentanones has been investigated. Using a single-electron oxidant, aromatic enals couple to form cyclopenanone products in good yield, good enantioselectivity, and excellent diastereoselectivity. A cross coupling has also been developed to afford non-symmetrical cyclopentanone products.
Open Access
Fundamental research into gold nanocluster properties
(Colorado State University. Libraries, 2021) Window, Phillip S., author; Ackerson, Christopher J., advisor; Neilson, James R., committee member; Kennan, Alan J., committee member; Peersen, Olve, committee member
Gold materials are popular for research into many applications with their interesting properties, such as magnetism, bio-inactivity, and other size-dependent properties. As the size of the gold material decreases from a bulk material to the nanoscale, new properties are introduced moving through different size regimes. As the particle size reaches the 2-3 nm range and move into the quantum-confined particle range, the most interesting particle changes occur and gold nanomaterials have extremely interesting research potential. These materials exist between the bulk and molecular systems and have similar properties to both; however, they are different enough from both of these to have their own unique application possibilities. Some properties of gold nanoclusters can be attributed more to the core or more to the ligand layer of the nanocluster. Certain properties, like electronics and magnetism, are due to the superatomic electron count and electronic structure from the core and depend on the number of gold atoms in the nanocluster. Extensive research has been done on investigating and altering these properties in small nanoclusters, however, larger nanoclusters have hardly been studied as they can be more difficult to work with. Within this work is investigated the magnetism and thus electronic structure of Au102(SPh)44 and Au133(tBBT)52 in different oxidation states. Paramagnetism up to two unpaired electrons is observed with both these nanoclusters through solution phase magnetic studies. Through this, electronic structure information has been obtained to elucidate the behavior of unique superatomic 1G and 1H orbitals. Looking at the outside of a nanocluster structure, interactions of nanoclusters with other nanoclusters, molecules, surfaces, and solvents are all due to the ligand layer of the nanocluster. Investigations of the ligand layer have been performed extensively through many techniques. However, further studies are always helpful since controlling the ligand layer is essential for functionalization for potential applications. Within this work is investigated the interactions of Au25(SR)18 with other Au25 nanoclusters in both solution and solid phase, as well as ligand exchange reactions of Au133(tBBT)52. Studies on Au25(SR)18 within solution include investigations of a supramolecular assembly, or supercluster, formed solely of the nanocluster itself with control over its growth and size. Studies on Au25(SR)18 within the solid-phase include controlled crystallization techniques that result in different solid-phase structures with previously unseen properties. Ligand exchange studies have also been expanded from small nanocluster materials only in previously published studies to the large nanocluster, Au133(tBBT)52. Within this dissertation, some of the first empirical studies into the oxidation state- dependent properties of large gold nanoclusters, Au102(SPh)44 and Au133(tBBT)52, were performed. This betters the field's understanding of how many unpaired electron spins these large gold nanocluster can sustain at room temperature and further elucidates the behavior of superatomic electronic structure and behavior based on electron count. Furthermore, this dissertation presents the first investigations into the formation of supramolecular assemblies of gold nanocluster as recyclable materials, and more interactions of gold nanoclusters based on ligand layer interactions through polymorphism studies and ligand exchange studies. These investigations all help understand how to control the ligand layer for future applications of gold nanoclusters and nanoparticles, from molecular to bulk materials.
Open Access
Interrogating reactions of gold nanoclusters: insights into catalysis and the Brust-Schiffrin synthesis
(Colorado State University. Libraries, 2017) Dreier, Timothy Andrew, author; Ackerson, Christopher J., advisor; Kennan, Alan J., committee member; Henry, Charles, committee member; Peebles, Christie, committee member
Over the past several decades, interest in the synthesis and behavior of atomically precise gold nanoclusters has gained substantial momentum. Herein, both catalytic behavior and synthetic mechanisms are explored using techniques more typically applied to organic chemistry. In the case of catalysis, Au25(SR)18 has emerged as a well-studied model system. In an effort to investigate their potential as intact, homogeneous, unsupported catalysts, we have discovered that Au25(SR)18 clusters are not stable in oxidizing conditions reported for catalytic styrene oxidation. Further investigation suggests that the active catalytic species is an Au(I) species resulting from oxidative decomposition of the starting gold cluster. Equally important to chemical behavior is an understanding of the reaction dynamics during the synthesis of atomically precise clusters. Because the Brust-Schiffrin method is the standard procedure by which gold nanoclusters are synthesized, the role of oxygen in it has been investigated for both organic and aqueous systems. In either case, it is clear obtaining the desired product depends on a radically mediated etching step. These results give new insight into how the Brust-Schiffrin method might be modified to further synthesis of uniquely interesting nanocluster systems.
Open Access
Mechanistic investigations and ligand development for rhodium catalyzed [2+2+2] and zinc catalyzed [4+2] cycloadditions
(Colorado State University. Libraries, 2013) Dalton, Derek M., author; Rovis, Tomislav, advisor; Wood, John L., committee member; Kennan, Alan J., committee member; Rappé, Anthony K., committee member; Snow, Christopher D., committee member
Described herein are mechanistic studies and ligand development for Rh(I) catalyzed [2+2+2] cycloaddition reactions of alkene tethered isocyanates and exogenous alkynes. A mechanistic hypothesis has been proposed and supported through experiment. Novel perfluoroaryl Taddol phosphoramidite ligands were developed based on the mechanistic hypothesis. Improvements in product and enantioselectivity were found using the perfluoroaryl Taddol phosphoramidite ligand, CKphos. This catalyst system was studied by NMR, X-ray and DFT calculations. Rh(I)-C6F5 and Co(-1)-C6F5 interactions were found in the course of studying the CKphos catalysts. The Rh-CKphos catalyst system was used in the synthesis of the tricyclic core structure of the cylindricine and lepadiformine alkaloids. Finally a Zn(II)-catalyzed [4+2] cycloaddition of 1- azabutadienes and nitro olefins was discovered and developed as an efficient and selective means to synthesize tetrahydropyridines.
Open Access
Mitomycin alkaloids: synthetic studies
(Colorado State University. Libraries, 2009) Gubler, Daniel Alan, author; Williams, Robert M., advisor; Rovis, Tomislav, committee member; Kennan, Alan J., committee member; Elliott, C. Michael, committee member; Luger, Karolin, committee member
Documented herein are efforts towards the first asymmetric total synthesis of the mitomycin family of natural products. Methods have been developed that efficiently construct eight-membered ring precursors of the natural products. Additionally, a tetracyclic mitosane compound containing all the core features of the mitomycins except the C9a methyl aminal has been constructed. The above-mentioned synthetic efforts help set the stage for future completion of the asymmetric total synthesis of this family of compounds. The studies mentioned herein, while not totally successful, shed new light on the reactivity of the mitomycins as well as the remarkable electronic effect of the electron-rich arene ring.
Open Access
New approaches to fluoromodifications of electron acceptor molecules for organic photovoltaics
(Colorado State University. Libraries, 2021) Brook, Colin P., author; Strauss, Steven H., advisor; Prieto, Amy L., committee member; Kennan, Alan J., committee member; Gelfand, Martin P., committee member
The overall goal of this work is to advance fundamental science and applications of organic electron acceptors based on fluorinated fullerenes and polycyclic aromatic hydrocarbons. The synthetic part of the dissertation focused on the development of new synthetic approaches towards the fluoromodification of large conjugated organic molecules and on the improvements of existing methods for the preparation of high-performing fullerene-based n-type semiconductors. Chapter 1 describes development and application of a new configuration of the gradient-temperature gas-solid reactor for the efficient and high-yielding trifluoromethylation of fullerenes. Significant improvements were achieved in the yields and selectivity of bis-trifluoromethylated C60 and C70 fullerenes: 2-fold and 10-fold yield increase compared to prior state of the art, respectively. An approach to maintain a constant reactive gas pressure in the reactor has been introduced by creating a liquid-gas reservoir of CF3I by submerging the reservoir in one of several low-temperature slush baths available that resulted in improvements in both yields and selectivity for trifluoromethylfullerenes, while also solving a problem of previously unproductive use of the gaseous reagent. Chapter 2 presents the author's work in partnership with the National Renewable Energy Lab (NREL) aimed at investigation of the prominent stabilizing effect of perfluoroalkylated fullerenes on the rate of photobleaching of common high-performance donor-polymers used in OPV devices, compared to the pure polymer films and blends with prototypical non-fluorinated fullerene, PC60BM. It is demonstrated that rationalizing complex photobleaching behaviour ultimately required consideration of the electron affinity of the fullerene as well as the relative miscibility of the polymer–fullerene blend. The ability of the bis-trifluoromethylfullerene and Fauxhawk fullerene to stabilize certain donor materials against photodegradation, to blend well with fluorinated (and even certain non-fluorinated) polymers, and to quench excited states efficiently was thoroughly studied and correlated with structure-property relationships amongst several polymer donor and fullerene acceptor combinations. Chapter 3 describes a new approach to the direct fluoromodification of polycyclic aromatic hydrocarbons based on replacing commonly used perfluoroalkyl groups (CnF2n+1, or RF) with perfluorobenzyl groups (C6F5CF2, or BnF). For the first time, solution-phase direct perfluorobenzylation of an electron-rich perylene (PERY) and electron-poor perylene diimide (PDI), has been achieved. Five new bay- and peri-substituted compounds of perylene, PERY-(BnF)n, where n = 1, 2 and 3; and three new bay-substituted compounds of perylene diimide, PDI-(BnF)n, where n = 1, 2; were synthesized and fully characterized, revealing that electron withdrawing BnF group is comparable to RF in increasing acceptor strength of new compounds. Chapter 4 deals with a new type of annulative pi-extension (APEX) reaction discovered in this work that occurs via reductive dehydrofluorination/aromatization reactions involving perfluorobenzylated PDI compounds, that afforded fluorinated benzoperylene and coronene-based derivatives with prodigious electron acceptor properties. Another type of annulation leading to the transition-metal free formation of new compounds with all-carbon seven-membered rings across the bay regions of PDIs, consequently forming as rare examples of a newly-recognized fundamental type of conformational isomers, named akamptisomers, is also reported here for the first time. Studies of the likely reaction pathways in both types of reactions and effects of varying reaction parameters on the preferred product formation are presented. Single crystal crystallographic studies of many of the new compounds prepared in this work provide rich and unique data for in-depth analysis of the solid-state packing motifs and influences of the type and position of fluorinated functional groups on the intermolecular interactions, and ultimately, charge transport in these new organic n-type semiconductor materials.
Open Access
Part 1: Formation and nucleophilic interception of α,β-unsaturated platinum carbenes. Part 2: Efforts toward controlling magnetic properties of cobalt and iron coordination complexes
(Colorado State University. Libraries, 2017) Ozumerzifon, Tarik J., author; Shores, Matthew P., advisor; Kennan, Alan J., committee member; Neilson, James R., committee member; Achter, Jeffrey D., committee member
Presented in this dissertation are a series of studies describing the use of transition metals in several different applications. Part 1 concerns the development of novel platinum(II)-catalyzed reaction manifolds toward C-C bond formation, as well as the formal synthesis of a natural product. Meanwhile, Part 2 describes three separate efforts toward modulation of either single-molecule magnet properties in cobalt(II) or spin state control of iron(II) coordination complexes. The first chapter is a general introduction to single-molecule magnetism (SMM) and spin crossover, as these topics specifically relate to Co(II) and Fe(II) complexes, respectively. The physical origins of both phenomena are discussed, as well as some general terminology that are used throughout Chapters 3-5. Chapter 2 describes the use of Pt(II) salts in alkyne activation reactivity. The vinylogous addition of carbon nucleophiles into α,β-unsaturated platinum carbenes is discussed, and the optimization and scope of enol incorporation is provided. This is followed by a description of how Pt(II) catalysis enables the rapid formal synthesis of frondosin B, a sesquiterpene natural product. In Chapter 3, the synthesis and characterization of several salts of a trigonal prismatic cobalt(II) complex are detailed. The capping ligand used in these podands is cis-,cis-,1,3,5-triaminocyclohexane (tach), a rigid backbone which dictates coordination geometry and the iminopyridine contains pendant tert-butylamide moieties which are meant to enable guest association. The single-molecule magnet behavior (measured via slow magnetic relaxation) of these compounds is also explored, where the cation-binding tetraphenylborate salt shows slow magnetic relaxation at both zero and applied dc fields. A brief discussion of theoretical considerations of the effect of trigonal distortion on axial anisotropy is also presented, which suggests systems in an intermediate twisting geometry may give rise to guest-dependent magnetic properties in SMMs. Chapter 4 presents initial efforts toward the development of an Fe(II) system which can undergo a spin-state switch upon addition of a reagent. The chemoselective process is intended to be the result of an irreversible ligand modification. The first target toward this goal is manifested in desilylation of a 5-siloxy substituted podand. Spectroscopic and spectrometric as well as electrochemical and magnetic data indicate qualitatively that ligand desilylation is occurring as a function of fluoride addition, affecting a decrease in high-spin:low-spin ratio. Last, Chapter 5 details the systematic study of electronic character of 5-pyridyl substitution in the Fe(II) tren iminopyridine tripodal system. The Fe(II) species magnetic susceptibility and Ni(II) analog d-d transition energy data are compared to the Hammett parameter of the respective substituent, which define the complexes' electron-donating or -withdrawing properties. Overall, electron withdrawing substituents at this position lead to stabilization of the HS state. A comparison of these iminopyridine complexes to Fe(II) podands which undergo spin crossover is provided in an effort to explain the observed low-spin behavior of these complexes.
Open Access
Part one: The formal total synthesis of dehydrogliotoxin and the first synthesis of an epidiselenodiketopiperazine and Part two: Towards the total synthesis of the tetrapetalones
(Colorado State University. Libraries, 2013) McMahon, Travis Chandler, author; Wood, John L., advisor; Kennan, Alan J., committee member; Ferreira, Eric M., committee member; Bailey, Travis S., committee member; Crick, Dean C., committee member
Tuberculosis (TB), caused by Mycobacterium tuberculosis (MTB), affects approximately one third of the global population and is associated with nearly two million deaths annually. Although there are known cures for TB, current treatment plans suffer due to length, usually taking 6-9 months to complete. Additionally, developing countries lack the infrastructure and resources necessary to both efficiently diagnose and treat patients. Of particular concern are an increasing number of strains of TB that are becoming resistant to the current drug regimens, which has been a result of patients beginning, but not completing their treatment. In light of these facts it is clear there is a continuing need to develop simplified and shorter treatments for TB, and with the increasing prevalence of resistant strains, chemically unique targets should be investigated. As part of a collaborative effort with the Hung group at the Broad Institute, we identified two related epidithiodiketopiperazine (ETP) natural products, gliotoxin and dehydrogliotoxin, as potential candidates for exploration as anti-TB agents. We initially targeted a synthesis of dehydrogliotoxin, as it had also never been tested against MTB, whereas gliotoxin was known to be active. Additionally, as dehydrogliotoxin was the simpler of the two compounds, we believed it could be synthesized more rapidly and also be more amenable to derivatization to form structural analogs. The synthetic studies towards dehydrogliotoxin culminated in a formal total synthesis that featured a key two step amidation-intramolecular ring-closure. With access to dehydrogliotoxin we were able to test it against MTB and found its activity to be comparable to gliotoxin. We next turned our attention to the synthesis of structural analogs in hopes of identifying a compound that could potentially be used as an anti-TB therapeutic. In that regard, we targeted a compound wherein the disulfide region of the natural product was replaced with a diselenide. As an epidiselenodiketopiperazine (ESeP) had never been synthesized before, we initially explored the installation of this functional group in a model system. These synthetic efforts resulted in the synthesis of an ESeP, both from a simple diketopiperazine and directly from an ETP. Additionally, in these model systems, the ESeP exhibited comparable activity towards MTB as the ETP. Tetrapetalone A was isolated in 2003 by Hirota and coworkers from a culture filtrate of Streptomyces sp. USF-4727. The related compounds tetrapetalones B, C, and D were isolated from the same Streptomyces strain in 2004. We became interested in this family of natural products due to their interesting structural features and the synthetic challenge they present. Salient features of the tetrapetalones include a tetracyclic core containing a tetramic acid, a seven-membered ring possessing a trisubstituted double bond, a p-quinol, and a five-membered ring with a pendant β-rhodinose. Several strategies towards the synthesis of the tetrapetalones have been explored. In our initial approach we hoped to form the seven-membered ring of the natural product through nucleophilic attack of the aromatic ring onto a pendant palladium π-allyl species. While exploring this process, we found that the desired seven-membered product was not formed, instead we isolated a product containing a five-membered ring, the result of attack at the wrong position of the palladium π-allyl species. Attempts to bias the substrate towards formation of the desired seven-membered ring through a transannular palladium π-allyl approach proved unfruitful. Our current route features a Friedel-Crafts acylation to form the seven-membered ring containing the trisubstituted double bond. The precursor for this approach was built up rapidly from simple starting materials, and the desired Friedel-Crafts reaction proceeds smoothly. Furthermore, we have implemented a C-H oxidation protocol to install a synthetic handle, which can ultimately be converted to an alkyne that we envision can be transformed into the five-membered ring bearing the sugar moiety in order to finish the natural product. Concurrent to the approaches described above, we have also targeted the related natural product, ansaetherone, which was isolated from the same Streptomyces strain as the tetrapetalones and is proposed to be a biosynthetic precursor to the family. The ultimate goal in this approach was to develop a synthesis of ansaetherone and explore methods to convert it to one of the members of the tetrapetalones in a biomimetic fashion. Our proposed synthesis included a key tandem enyne-cross metathesis to form the eleven-membered ring present in the natural product. Although this synthesis is still in its infancy, we have accessed a compound that is a few synthetic steps away from the precursor to explore the key step. We are currently exploring an improved synthesis of this intermediate and ways to elaborate it to the natural product.
Open Access
Progress toward the synthesis of providencin
(Colorado State University. Libraries, 2011) Stevens, Sarah Jean, author; Wood, John L., advisor; Williams, Robert M., committee member; Kennan, Alan J., committee member; Kurosu, Michio, committee member; Rappé, Anthony K., committee member
Providencin, a highly oxygenated diterpene, was isolated from the sea plume Pseudopterogorgia kallos in 2003 by Rodriguez and co-workers. Providencin was revealed to be a cembrane-based diterpene containing an unprecedented [12.2.0]hexadecane ring-system. Providencin was found to possess anti-cancer activity against human breast (MCF7), lung (NCI-H460) and CNS (SF-268) cancer cell lines. The unique structure and biological activity make providencin an attractive target for total synthesis and our work toward providencin began shortly after its isolation. The initial focus of each approach has centered on the unique trans-fused cyclobutanol moiety. A formal [2 + 2] cycloaddition is our chosen approach to the synthesis of the cyclobutane moiety. Further elaboration of our cyclobutane compounds has led to the synthesis of several highly functionalized intermediates. Our efforts toward the synthesis of providencin are discussed herein.
Open Access
Progress towards the total synthesis of the welwitindolinone alkaloids and the discovery of a novel tandem O-H insertion Conia-ene cyclization
(Colorado State University. Libraries, 2011) Freeman, David Blandy, author; Wood, John L., advisor; Kennan, Alan J., committee member; Ferreira, Eric M., committee member; Shores, Matthew P., committee member; McNeil, Michael R., committee member
The broth of blue-green algae Hapalosiphon wewitschii and Westiella intricate was shown to possess interesting biological activity, including insecticidal and P-glycoprotein inhibiting capabilities. Upon further investigation, the welwitindolinone alkaloids were isolated from the lipophilic extracts and shown to be responsible for the observed biological activity. Herein are described efforts towards the total synthesis of the welwitindolinone alkaloids and novel chemistry developed in the process. In efforts towards N-methylwelwitindolinone C isothiocyanate, we employed a sequential O-H insertion Claisen rearrangement to provide compounds capable of undergoing a [3+2] dipolar cycloaddition to access the bicyclo[4.3.1] core. Attempts to install the requisite quaternary center of N-methylwelwitindolinone C isothiocyanate during the [3+2] dipolar cycloaddition event were unsuccessful. Ultimately, a chloronium-ion semi-Pinacol rearrangement was utilized to install the key quaternary center. Due to complications encountered during the synthesis of N-methylwelwitindolinone C isothiocyanate we shifted our focus to N-methylwelwitindolinone D isonitrile. Investigation into the construction of the bridged ether embedded within N-methylwelwitindolinone D isonitrile led to the discovery of a novel tandem O-H insertion Conia-ene cyclization.
Open Access
Rhodium-catalyzed cycloadditions between alkenyl isocyanates and alkynes: study of scope, mechanism and applications toward total synthesis
(Colorado State University. Libraries, 2010) Friedman, Rebecca Ann Keller, author; Rovis, Tomislav, 1968-, advisor; Kennan, Alan J., committee member; Prieto, Amy L. (Amy Lucia), committee member; Shi, Yian, committee member; McNeil, Michael R., committee member
Rhodium-catalyzed cycloadditions between alkenyl isocyanates and unsymmetrical, internal alkynes has been studied. A wide variety of alkynes have proven successful components in the [2+2+2] cycloaddition. Excellent yields and enantioselectivities have been achieved in the resulting indolizidinone products. Furthermore, a single regioisomer is obtained for the vast majority of alkynes subjected to reaction conditions. A logical explanation for the highly regioselective insertion for internal, unsymmetrical alkynes was provided. Small variations in the electronics and/or steric bulk of the alkyne substitution were sufficient to predictably control the insertion of the alkyne into the initial rhodacycle. Mechanistic insight into the rhodium-catalyzed [4+2+2] cycloaddition between dienyl isocyanates and alkynes has been achieved. A series of competition and slow addition experiments, alongside analysis of enantioselectivity and product formation, provided evidence for a proposed mechanism of the [4+2+2] cycloaddition. It was determined that the diene preferentially coordinates to the rhodium, in the presence of a terminal alkyne, to provide eight-membered bicyclic azocene products. Steps towards the total synthesis of natural product Secu'amamine A have been made. The bicyclic core of the molecule has been successfully synthesized utilizing rhodium-catalyzed [2+2+2] methodology developed within the Rovis group. Additionally, a successful, diastereoselective 1,4-reduction of the resulting vinylogous amide product and subsequent deprotection of an enyne side-chain provided an intermediate that is hypothetically two steps (an alpha-oxidation and 2+2+1 cycloaddition) away from Secu'amamine A.
Open Access
Rhodium-catalyzed cycloadditions to construct nitrogen heterocycles and progress towards the synthesis of ionomycin
(Colorado State University. Libraries, 2014) Oberg, Kevin Martin, author; Rovis, Tomislav, advisor; Kennan, Alan J., committee member; Ferreira, Eric M., committee member; Neilson, James R., committee member; Kanatous, Shane B., committee member
The ability to construct molecules in a rapid, atom-economical fashion is a major goal of organic chemistry. This work describes four topics; pyridone synthesis, mechanistic understanding in [2+2+2] cycloadditions, pyrimidinone synthesis, and progress towards ionomycin. The first chapter describes the synthesis of 4,6-substituted 2-pyridones and 3,5-substituted 4-pyridones from the rhodium-catalyzed [2+2+2] cycloaddition of two alkynes and an isocyanate. Our group demonstrated that an enantioselective rhodium-catalyzed [2+2+2] cycloaddition of alkenyl isocyanates and alkynes generates indolizidinone and quinolizidinone products. Although trends for product and regioselectivity were established, the underlying mechanism was unclear. The second chapter describes X-ray analysis of rhodium·phosphoramidite complexes in conjunction with other mechanistic work to elucidate a theory that explains product and regioselectivity in this reaction. This system is amazing in that it illuminates the factors contributing to oxidative cycloadditions in a spectacular fashion by delivering two different products. The third chapter describes the enantioselective synthesis of pyrimidinones from a rhodium-catalyzed [4+2] cycloaddition of α, β-unsaturated imines and isocyanates. The final chapter describes our group's progress toward the synthesis of ionomycin using rhodium-catalyzed desymmetrization of anhydrides with zinc nucleophiles.
Open Access
Studies on the biosynthesis of prenylated indole secondary metabolites from Aspergillus versicolor and Aspergillus sp.; and A novel approach to tumor specific drug delivery: use of a naphthyridine drug linker with a DNA hairpin
(Colorado State University. Libraries, 2011) Finefield, Jennifer M., author; Williams, Robert Michael, advisor; Rovis, Tomislav, 1968-, committee member; Kennan, Alan J., committee member; Elliott, C. Michael, committee member; Thamm, Douglas H., committee member
Herein are documented our efforts in two projects, beginning with studies toward elucidating the biosynthesis of prenylated indole alkaloids from two different Aspergillus species. Marine-derived Aspergillus sp. and terrestrial-derived Aspergillus versicolor were found to produce antipodal metabolites, in which we have developed several putative biosynthetic pathways to determine the enantio-diverging point of these fungal cultures. Through the synthesis of several potential intermediates, both with and without isotopic labeling, as well as through bioinformatics analysis of both the (-)- and (+)-notoamide biosynthetic gene clusters, significant progress has been made toward identifying a single biosynthetic precursor that serves as an intermediate to the postulated enantio-diverging event, the intramolecular hetero Diels-Alder cycloaddition. In the second project discussed, through collaboration with Dr. James Berenson at the University of California, Los Angeles, we have developed a novel tumor specific drug delivery system. Two naphthyridine-drug derivatives were synthesized and conjugated to a modified DNA oligonucleotide specifically targeted for multiple myeloma cells. The oligonucleotide-drug conjugate was successfully delivered and activated specifically within RMI8226 multiple myeloma cells.
Open Access
Studies on the biosynthesis of the stephacidins and notoamides. Total synthesis of notoamide S and notoamide T. and Progress toward the synthesis of chrysogenamide A
(Colorado State University. Libraries, 2011) McAfoos, Timothy Jospeh, author; Williams, Robert M., advisor; Kennan, Alan J., committee member; Wood, John L., committee member; Finke, Richard G., committee member; Crick, Dean C., committee member
Herein I discuss my efforts toward the elucidation of the biosynthesis of the stephacidins and notoamide family of natural products. Notoamide S has been suggested to be the final common precursor between two different fungal strains, Aspergillus sp. and Aspergillus versicolor, before diverging to form enantiomerically opposite natural products (+) and (-)-stephacidin A and (+) and (-)-notoamide B. The synthesis of notoamide S comes from coupling N-Fmoc proline with a 6-hydroxy-7-prenyl-2-reverse prenyl tryptophan derivative synthesized through a late stage Claisen rearrangement. The oxidation of notoamide S affords an achiral azadiene that leads to an intramolecular Diels-Alder providing a new product, notoamide T, containing the bicyclo[2.2.2]diazaoctane ring system with the 6-hydroxy-7-prenyl indole ring of notoamide S. The synthesis of notoamide T is accomplished through a radical addition to the pyran ring of stephacidin A followed by an elimination ring opening event to provide the 6-hydroxy-7-prenyl indole. Chrysogenamide A is the newest member of the marcfortine family of natural products. Herein I discuss the synthesis of 7-prenyl-2-reverse prenyl indole through a thio-Claisen reaction and subsequent Lewis acid mediated sulfide removal. Coupling of a pipecolic acid derivative with the 7-prenyl-2-reverse prenyl tryptophan leads to the dipeptide containing all of the carbons needed in chrysogenamide A. I propose that chrysogenamide A can be synthesized through an unprecedented intramolecular Diels-Alder reaction of a monoketopiperazine by a condensation/tautomerization event leading to the appropriate azadiene for the intramolecular Diels-Alder reaction. A final oxidation of the intramolecular Diels-Alder product would lead to chrysogenamide A and what could be a newly proposed biosynthesis of a monoketopiperazine.
Open Access
Studies toward the total synthesis of microsclerodermin G
(Colorado State University. Libraries, 2010) Burnett, Cameron Moeller, author; Williams, Robert Michael, advisor; Kennan, Alan J., committee member; Rovis, Tomislav, 1968-, committee member; Bernstein, E. R. (Elliot R.), committee member; Belisle, John Theodore, committee member
We report our studies toward the synthesis of microsclerodermin G, a cyclic hexapeptide with antifungal and antitumor activity. The dehydrotryptophan amino acid was synthesized according to literature precedents. (3R)-γ-amino-β-hydroxybutyric acid (GABOB) was synthesized according to previous methodology from our research group. An aspartate-based precursor to the pyrrolidinone moiety of microsclerodermin G was prepared in four steps from known materials. 3-amino-6-methyl-12-phenyl-2,4,5-trihydroxydodeca-7,9,11-trienoic acid (AMPTD) was prepared in seven steps from known materials; the synthesis utilized Evans' chiral oxazolidinone glycolate aldol reaction and the sulfinimine-based Mannich reaction developed by Ellman. Syntheses of two dipeptides are reported, as are other attempts at coupling of the various amino acids.
Open Access
Studies towards the biomimetic synthesis of the stephacidin family of natural products and the concise and versatile synthesis of d,l-brevianamide B, C-12A-epi-malbrancheamide and structurally related analogs
(Colorado State University. Libraries, 2006) Valente, Meriah W. N., author; Williams, Robert M., advisor; Kurosu, Michio, committee member; Kennan, Alan J., committee member
The total synthesis of stephacidin A, avrainvillamide and stephacidin B was envisioned to proceed through a biomimetic intramolecular Diels-Alder cylcoaddition. The Diels-Alder precursor was thought to come from the coupling of (L)-prolinamide with a α-ketoacid indole, which would subsequently form the requisite azadiene. While the desired α-ketoacid indole was not stable enough to be synthetically formed, a pseudo- α-ketoacid was successfully designed. The coupling of this vinyl ether carboxylic acid indole moiety with (L)-prolinamide provides an interesting intramolecular Diels-Alder (IMDA) precursor, which might prove to be a useful synthetic prototype for the IMDA. A similar coupling between a pseudo-α-ketoacid and (L)-prolinamide was applied to a different system of compounds, which led to a biomimetically-inspired intramolecular Diels-Alder reaction that diastereoselectively formed the characteristic bicyclo[2.2.2]diazaoctane core of the brevianamides. A concise synthesis of d,l-brevianamide B was successfully designed through a Fischer indole reaction of the key tricyclic IMDA cycloadduct and phenyl hydrazine. This IMDA / Fischer indole route to form the indolic bicyclo[2.2.2]diazaoctane core has proven to be a versatile method to access C-12a-epi-malbrancheamide and structurally related analogs for biological activity studies.