352 research outputs found
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Synthesis of the Tricyclic Core of Guanacastepene A, Decarboxylative Rearrangement of Allenylic N-Tosyl Carbamates and Phosphine-catalyzed Intramolecular γ-Umpolung Addition of α-Aminoalkylallenic Esters, and the Enantioselective Total Synthesis of (+)-Ibophyllidine
No full textChapter 1 The synthesis of the tricyclic core of the diterpene guanacastepene A is described. Based on previous studies in the laboratory of professor Ohyun Kwon, the densely functionalized six-membered ring of the natural product was constructed utilizing an intermolecular Diels-Alder cycloaddition between maleic anhydride and a highly substituted alkoxydiene. The requisite diene was synthesized via a Stille cross coupling reaction for which an efficient synthesis of the necessary vinyl stannane was developed. An alternative synthesis of the diene based on copper mediated coupling of vinyl boronates with alcohols was also devised, which allowed for the preparation of gram quantities with minimal purification. The five-membered ring of guanacastepene A was appended through coupling with a highly functionalized zinc cuprate. While previous studies in the Kwon laboratory showed that a conjugate addition/Mukaiyama aldol strategy to install the C11 methyl group and forge the seven-membered ring was efficient in forming the two desired carbon-carbon bonds, poor diastereoselectivity of the initial conjugate addition was observed. The research described here uses the Nozaki-Hiyama-Kishi reaction to close the central seven-membered ring without prior installation of the C11 methyl group.Chapter 2 The synthesis of 3-carbethoxy-2-substituted-3-pyrrolines via the phosphine-catalyzed intramolecular γ-umpolung addition of N-tosyl α-aminoalkylallenic esters is described. The current transformation provides a compliment to the known syntheses of similarly substituted pyrrolines via the phosphine-catalyzed [3 + 2] annulation between imines and allenoates. The flexibility in substitution at the 2-position, including alkyl groups, offers benefits over many of the previously reported syntheses of similarly substituted pyrrolines. The N-tosyl α-aminoalkylallenic esters were prepared via a novel decarboxylative rearrangement of allenylic carbamates. Chapter 3 The enantioselective total synthesis of the monoterpene indole alkaloid (+)-ibophyllidine is described. The employed strategy was based on a key asymmetric phosphine-catalyzed [3 + 2] annulations between a γ-ethyl allenoate and the N-tosyl imine derived from indole-3-aldehyde using Kwon's L-4-hydroxyproline-derived chiral phosphine catalyst. This key transformation constructed the stereochemically dense D-ring with exceptional levels of diastereo- and enantioselectivity. A subsequent diastereoselective hydrogenation of the pyrroline double bond led to the fully functionalized, all syn pyrrolidine ring of the natural product. An intramolecular alkylation at the C3 position of indole formed the C-ring while an intramolecular aza Baylis-Hillman reaction was utilized to forge the E-ring leading to the pentacyclic skeleton of (+)-ibophyllidine
A high-performance positive-working photosensitive polyimide: Effects of reactive end groups on the physical properties of the films
No full textTo investigate the effects of reactive end-cappers on the performance of PI precursors and the resulting polyimide (PI) films, poly(amic acid)s (PAAs) as a base polymer of the positive-working photosensitive polyimides (PSPIs) were synthesized via ring-opening polymerization of 4,4'-oxydiphthalic anhydride and 4,4'-oxydianiline with four different reactive end-cappers [maleic anhydride, citraconic anhydride, 2,3-dimethylmaleic anhydride (DMA), and 5-norbornene-2,3-dicarboxylic anhydride (NDA)]. During imidization of these end-capped PAAs to form the corresponding PI films, chain extension and crosslinking reactions of the respective end groups occurred, resulting in an improvement in the mechanical and thermal properties despite the low molecular weight of the precursors. However, the UV transmittance at similar to 365 nm, an important property of PSPIs for thick-film applications, such as stress buffer layers, was strongly influenced by the type of end-capper used. These behaviors were understood in terms of the optimized geometries and the simulated UV-vis spectra of modeled end groups determined from density functional theory calculations. (c) 2006 Wiley Periodicals, Inc
Replication Data and Code for: The Reallocation Effect of Emissions Cap-and-Trade: Evidence from China
No full textThe data and programs replicate tables and figures from "The Reallocation Effect of Emissions Cap-and-Trade: Evidence from China", by Kwon, Zhao, and Zhao. Please see the ReadMe file for additional details
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STUDIES CONCERNING NUCLEOPHILIC PHOSPHINE CATALYSIS AND DESIGNS OF NEW CHIRAL AMINOPHOSPHINES TOWARD ASYMMETRIC PHOSPHINE-CATALYZED REACTIONS
No full textPhosphinocatalysis has been used among us as a short term for nucleophilic phosphine catalysis. The information in chapter 1 will focus on how phosphinocatalysis was discovered, who contributed to the early-day developments of this field, and what have been achieved in the field. Chapter 2 will cover phosphine-allene chemistry in which the equillibrium between phosphonium dienolate and vinylogous ylide was reaffirmed. Two new phosphine-mediated transformations were discovered in this chemistry: vinylogous aldol/P-to-C aryl migration by reaction of phosphonium dienolate with an aromatic aldehyde and vinylogous Wittig olefination by reaction of vinylogous ylide with an aromatic aldehyde. Chapter 3 will discuss the development of a one-pot procedure for phosphine-initiated general base-catalyzed quinoline synthesis and of its variation to quinolone synthesis. A number of 3-substituted and 3,4-disubstituted quinolines, as well as 3-substituted 4-quinolones have been generated from this methodology. Chapter 4 involves the designs of new chiral aminophosphines toward the asymmetric version of phosphine-catalyzed double Michael reaction. The aminophosphines were particularly designed based on the presumption that the anchimeric assistance of the amino group onto the phosphonium phosphorous was essentially significant to the reaction's success. The chiral element was designed to be on the amino group, which would endow the asymmetric environment to the reactive center via anchimeric assistance during the reaction. A small collection of chiral aminophosphines were eventually prepared based on this design. Chapter 5 was an extension on the design of chiral aminophosphines. However, the new design of chiral aminophosphines was not based on any specific asymmetric chemical transformations. This design was centered on the steric-directing mode of asymmetric induction and then would be tested toward various phosphinocatalysis reactions
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Catalytic and Stoichiometric Wittig Reactions for the Synthesis of Arenes and 1,2-Dihydroquinolines
No full textIn the first chapter, a phosphine-mediated multi-component reaction between o-phthalaldehydes, nucleophiles and monosubstituted allenes furnished the functionalized non-C2-symmetric naphthalenes in synthetically useful yields. Also, when the o-phthalaldehydes were reacted with 1,3-disubstituted allenes in the presence of diethylphenylphosphine, naphthalene derivatives were obtained in yields up to quantitative. The mechanism of the latter transformation is straightforward, involving aldol addition followed by Wittig olefination and dehydration. The mechanism of the former reaction has been established as a tandem γ-umpolung/aldol/Wittig/dehydration process through careful analysis of data based on the preparation of putative reaction intermediates and mass spectrometry. The current method can also be applied iteratively to prepare anthracenes and tetracenes when carboxylic acids are employed as pronucleophiles.In the second chapter, a novel bridged [2.2.1] bicyclic phosphine oxide, devised to circumvent the burdens of waste generation and purification that are typical of Wittig olefination, has been prepared from commercially available cyclopent-3-ene-1-carboxylic acid through three steps. This new catalyst was designed based on the theoretical transition state geometry of the silane-mediated reduction of phosphine oxides. The superior performance of this novel phosphine oxide has been verified experimentally through kinetic analysis of its silane-mediated reduction, as well as catalytic Staudinger reduction, in comparison to the current bests. It has also been applied successfully in halide-/base-free catalytic γ-umpolung addition–Wittig olefinations of allenoates and 2-amidobenzaldehydes to produce 1,2-dihydroquinolines with extraordinary efficiency
Effect of Wavelength and Intensity of Light on a-InGaZnO TFTs under Negative Bias Illumination Stress
No full textWe investigated degradation mechanism of a-IGZO TFTs under NBIS with different wavelengths. and intensities IL of light. Negative gate bias was applied for 4000 s while drain and source were grounded, and illuminations with lambda = 450, 530, or 700 nm were applied. Illumination with photon energy exceeding similar to 2.3 eV (530 nm) induced noticeable change in threshold voltage shift Delta V-th, which can be interpreted in terms of ionization of oxygen vacancies V-O. In addition, I-L of blue illumination (450 nm) was varied from 6 to 200 lux and saturation in Delta V-th was observed after exceeding a certain I-L. We suggest that the saturation occurs because V-O-ionization rate is saturated by outward relaxation of metal atoms in the a-IGZO film. (C) The Author(s) 2016. Published by ECS.1174Ysciescopu
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Phosphine Organocatalysis and Dealkenylative C(sp3)–C(sp2) Bond Functionalization
No full textThis dissertation features several projects involving catalyst design and reaction development in the areas of organocatalysis and dealkenylative C(sp3)–C(sp2) bond functionalizations. The underlying theme throughout all of this work is the introduction of robust and modular transformations aimed at the synthesis of pharmaceutically-relevant scaffolds. Chemical reactions with these attributes are of utmost importance with regards to the synthesis and modification of biologically-active small-molecules, so these works seek to contribute to this goal.Chapter One focuses on the design and synthesis of a new family of carvone-derived phosphine organocatalysts and their implementation in enantioselective allene–imine [3 + 2] annulations to give functionalized nitrogen heterocycles. A background of nucleophilic phosphine organocatalysis is presented to give context in this area. During these studies, a new fragmentation reaction was developed which was exploited in later work. The introduction of this catalyst family also enabled the enantioselective synthesis of a biologically-active small-molecule, efsevin.Chapter Two discusses the development of the aforementioned C(sp3)–C(sp2) bond fragmentation reaction, hydrodealkenylation, in which an alkene C(sp3)–C(sp2) bond is converted to a C(sp3)–H bond. A history of ferrous-mediated fragmentations of organoperoxides is provided to show the extensive, yet unrealized potential of this type of transformation. This robust transformation was applicable in the syntheses of many desirable chiral scaffolds from abundant feedstock reagents. It was also shown that this transformation can be applied in the formal syntheses of five complex natural product scaffolds.Chapter Three describes further development of the alkene C(sp3)–C(sp2) bond fragmentation in a new variation, dealkenylative thiylation, in which an alkene C(sp3)–C(sp2) bond is converted to a C(sp3)–S bond. Carbon–heteroatom bonds are commonly found in both natural products and pharmaceutical leads. Therefore, the development of reactions that forge these bonds are important. Contrary to known methods of synthesis, this dealkenylative radical approach enables the synthesis of complex chiral scaffolds containing aryl-sulfide motifs under relatively mild reaction conditions. These products were then subjected to various transformations to highlight the synthetic utility of the products.Chapter Four highlights the development of a one-pot procedure, referred to as oxodealkenylation, in which an alkene C(sp3)–C(sp2) bond is converted to a C=O bond. Carbonyls are among the most versatile functional group handles, therefore conversion of the alkenes commonly found in abundant chiral pool feedstock into carbonyls would be highly desirable. This process was demonstrated on a variety of chiral pool-derived materials, providing novel building blocks in a facile fashion. Synthetic transformations of the products and mechanistic investigations are also provided in this work.Chapter Five focuses on the synthesis of chiral pool-derived sulfinate salts and their applications in synthesis. Sulfinate salts are versatile and robust radical precursors. However, the synthesis of chiral sulfinates is seldom reported. Here, we utilized our previously developed dealkenylative thiylation to enable streamlined access to these types of sulfinates. Examples are also provided that demonstrate how these products can be used in the diversification of pharmaceutically-relevant scaffolds to give C(sp3)- and stereochemically-rich heterocyclic derivatives
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Phosphine-Mediated Multi-Component γ-Umpolung/Aldol/Wittig Cascade Reaction for the Synthesis of Functionalized Naphthalenes
No full textThis study describes an efficient and convenient triphenylphosphine-mediated γ-umpolung/aldol/Wittig cascade reaction. This is the first time that phosphine has been used to mediate a multi-component coupling reaction involving alleonates, nucleophiles, and substituted o-phthalaldehyde to synthesize naphthalenes. The reaction condition has been optimized and employed in the synthesis of eight functionalized naphthalenes. This reaction proceeds under mild conditions and produces good to excellent yields
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Synthesis of Functionalized α,α-Dibromo Esters through Claisen Rearrangements of Dibromoketene Acetals and the Investigation of the Phosphine-Catalyzed [4 + 2] Annulation of Imines and Allenoates
No full textAllylic alcohols can be transformed into γ,δ-unsaturated α,α-dibromo esters through a two-step process: formation of a bromal-derived mixed acetal, followed by tandem dehydrobromination/Claisen rearrangement. The scope and chemoselectivity of this tandem process is broad and it tolerates many functional groups and classes of allylic alcohol starting material. The diastereoselectivity of the Claisen rearrangement was investigated with moderate to excellent diastereomeric selectivity for the formation of the γ,δ-unsaturated α,α-dibromo esters. The product α,α-dibromo esters are also shown to be valuable chemical building blocks. They were used in the synthesis of the ynolate reaction intermediate, as well as other carbon–carbon bond-forming reactions. Highly functionalized lactones were also shown to be simply prepared from the γ,δ-unsaturated α,α-dibromo ester starting materials formed via the Cliasen rearrangement.A phosphine-catalyzed [4 + 2] annulation of imines and allenoates is also investigated herein. A synthesis of the core structure of the ergot alkaloid lysergic acid was attempted using an annulation of the alkyl imine derived from tert-butyl 4-bromo-3-(2-oxoethyl)-1H-indole-1-carboxylate and ethyl 2-methylbuta-2,3-dienoate to form the key tetrahydropyridine ring. The imine investigated was shown to have rapid tatomerization to the more stable enamine moiety, which was not conducive for the annulation process. Other routes for the synthesis of the ergot alkaloid core structure were also investigated. Chiral bicyclic phosphines were applied to the annulation between ethyl 2-methylbuta-2,3-dienoate and various imines, which produces 6-substituted guvacine analogues in good yield with excellent enantiomeric excess. The bridged bicyclic chiral phosphines can be accessed quickly from trans-4-hydroxyproline. A new chiral phosphine (1S,4S,5R)-5-(4-anisyl)-2-tosyl-2-aza-5-phosphabicyclo[2.2.1]heptane was identified as an efficacious catalyst for the [4 + 2] annulation between ethyl 2-methylbuta-2,3-dienoate and aryl imines. A variety of aryl and heteroaryl imines were tested under this annulation process, affording 6-substituted guvacine esters. Utilizing this method, both (R) and (S)-aplexone were synthesized and tested to reveal that (R)-aplexone is the eutomer responsible for the reduction of cellular levels of cholesterol in the zebrafish mode
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Synthetic Studies toward Indole Alkaloids and Catalytic Asymmetric Staudinger–Aza-Wittig Reaction via Desymmetrization
No full textIn Chapter 1, we developed a catalytic asymmetric total synthesis of (–)-actinophyllic acid, with the key step being a chiral phosphine-catalyzed [3 + 2] annulation between an imine and an allenoate to form a pyrroline intermediate in 99% yield and 94% ee. The synthesis also features a CuI-catalyzed coupling between a ketoester and a 2-iodoindole to shape the tetrahydroazocine ring; intramolecular alkylative lactonization; SmI2-mediated intramolecular pinacol coupling between ketone and lactone subunits to assemble the complex skeleton of (–)-actinophyllic acid; and an unprecedented regioselective dehydroxylation.In Chapter 2, we applied our phosphine-catalyzed [4 + 2] annulation between an imine and an allenoate toward the synthetic studies on akuammiline indole alkaloids. We successfully constructed indole fused [3.3.1]azabicycles from [4 + 2] annulation precursor via Barbier reaction in high yields. We also developed a Lewis acid promoted dehydration to form the key indole–quinonemethide intermediate, which could generate a challenging quaternary carbon by trapping with a carbon nucleophile. By adopting this strategy, we proposed a pathway to finish formal total syntheses of aspidodasycarpine and lonicerine.In Chapter 3, we have successfully developed the catalytic asymmetric Staudinger–aza-Wittig reaction. Historically, both Staudinger and Wittig reactions need stoichiometric amounts of phosphine, which impede the development of catalytic asymmetric versions of these reactions. In 2006, Marsden and co-workers reported the first asymmetric Staudinger–aza-Wittig reaction with stoichiometric chiral phosphine, which was undesirable considering economic efficiency and environmental concerns. Based on our previous study, we found that the bridged [2.2.1]bicyclic phosphine oxide had higher efficiency than the known phosphine oxides during the reduction cycle in the presence of silane. In this project, after screening various Brønsted acids, we could realize the catalytic asymmetric Staudinger–aza-Wittig reaction at room temperature
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