549 research outputs found
New synthetic methodologies: Diamination and aza-diels-alder
No full textThe quest for novel synthetic methodologies that allow for the synthesis of highly functionalized chiral "building blocks" is a challenging and medicinally important topic in organic chemistry. The Li group has made significant contributions in this area by developing efficient and cost effective asymmetric and stereoselective methodologies for the synthesis of highly functionalized nitrogen-containing chiral building blocks. The synthesis of these highly functionalized molecules requires the development of asymmetric and stereoselective methodologies, which continues to remain a daunting task for the modern synthetic organic chemist. In this thesis, two novel synthetic methodologies for the synthesis of vicinal diamines and aza-Diels-Alder products will be discussed.
The synthesis of vicinal diamines from á,â-unsaturated ketones and esters will be the first methodology presented. The 1,2-diamino functionality is present in numerous naturally occurring and medicinally important compounds. Furthermore, these vicinal diamine products closely resemble the structure of naturally occurring amino acids and thus these compounds are invaluable for peptidomimetic studies, in that they can mimic both á- and â- amino acids, as well as the development of novel organocatalysts.
The development of an aza-Diels-Alder reaction utilizing a novel iodo-diene will also be discussed. These reactions involve carbon-carbon bond formation and facilitate the synthesis of nitrogen-containing heterocycles. These valuable products are commonly found in modern medicines as well as a plethora of natural products
The Study of New Chirality and Asymmetric Catalytic Control
No full textStarting with Lous Pasteur's pioneering work that tartaric acid enantiomers can rotate polarized light in opposite directions, there has been a significant expansion of research into chirality's impact in the pharmaceutical industry, enantioselective synthesis and catalysis, and materials science. This increasing understanding stimulates the chemistry community to keep unlocking the possibilities of new chirality, and two novel chirality – multilayer 3D chirality and orientation chirality – have been discovered and developed recently. The present dissertation focuses on the modification of multilayer chirality, aiming to simplify its chiral separation and purification, as well as the discovery of orientational chirality, featuring a remote block to control the orientation of C(sp)-C(sp3) or C(sp2)- C(sp3) axis-anchored chiral center. Among the development of these two chirality, the chiral amide installed in the targeted molecules plays a key role in achieving asymmetric control within the molecular architecture.
Additionally, this candidate examines the use of Aggregation-Induced Emission (AIE) in identifying chiral aggregates in various compounds and introduces Aggregation-Induced Polarization (AIP), which serves as a novel technique developed in response to the limitations of AIE, particularly for non-fluorescent small molecules. Based on the hypothesis that chiral aggregates can lead to the enhancement or adjustment of optical rotation, this technique has shown promise in monitoring chiral aggregates in chiral folding polymers and BINOL/BINAP derivatives.
Lastly, the dissertation showcases the application of chiral aggregates in enhancing the stereoselectivity in asymmetric catalytic reactions, especially in the Sharpless Asymmetric Dihydroxylation (AD) reaction, significantly improving the enantioselective ratio of the diol product from 78:22 (R:S) to 97:3 (R:S) through the manipulation of chiral quinine aggregation.Embargo status: Restricted until 06/2030. To request the author grant access, click on the PDF link to the left
Design and Synthesis of Organic Building Blocks, Monomers and Polymers
No full textChirality is not only fundamental to research in science, but also intrinsically associated with people's everyday lives. Chirality can be found in every scientific discipline, from astrophysics and geophysics to chemistry and biology, from history and linguistics to computer and information sciences. Therefore, both the research of chirality's genesis and the creation of new chiral applications have switched their focus to the production of advanced chiral building blocks. The current work, which has been addressing several topics including design background, substrate scope increasing, synthetic process, and molecular application, focuses on the idea of multi-layer 3D chirality. Through dual-coupling processes, single C-C bond anchored multi-layer 3D polymer has been created and described. The three layers that make up this new chirality are arranged almost parallel to one another and prevent rotational isomerization. The advancement of optical rotation, UV-visible, and AIE investigation of multilayer 3D polymer provided a great surprise. This research is anticipated to have a significant influence on chiral materials and asymmetric chemistry in the future.
KeywordsEmbargo status: Restricted until 01/2030. To request the author grant access, click on the PDF link
to the left
Applications of chiral N-phosphinyl auxiliaries
No full textChiral auxiliary chemistry is one of the rapidly developing areas in asymmetric synthesis. The advantages in applications of chiral auxiliaries include high reliability, efficiency, and selectivity in asymmetric transformations. Several universal chiral auxiliaries, such as Evan’s auxiliary and tert-butanesulfinamides, have been developed since the concept of chiral auxiliaries was formed. Our research group has focused on new chiral auxiliaries based on N-phosphonyl amides, and these new auxiliaries have provided efficient asymmetric induction for many different reactions. Recently, we have developed a series of chiral N-phosphinyl auxiliaries. One of the most surprising facts is that we found that products with our auxiliaries are solids, and can be purified simply by washing with common organic solvents. This concept has been termed Group-Assisted Purification (GAP)
Asymmetric synthesis of Bis-indoles and Indole Derivatives aided by Chiral Phosphoric acid ion pairing
No full textDue to the significance of the indole core within natural products, synthetic efforts for better methodology has been an active area of research within the synthesis field. While there is plenty of literature on the topic, there is always a need for asymmetric variants.
The candidate first demonstrates a facile, amenable, and mild method for the synthesis of bis-indoles and indole derivatives through a DDQ mediated cross dehydrogenative coupling. A substrate scope proves the reaction applicable to different types of functional groups. A series of chiral phosphoric acids were then screened in an effort to induce enantioselectivity. The candidate shows the results and findings from the screening of more than ten different CPAs.
Lastly the candidate also attempts the synthesis of chiral bis-indoles through the chiral phosphoric acid catalyzed dehydration of a tertiary alcohol. Results and findings are reported from a series of reaction conditions
GAP chemistry & its applications
No full textAsymmetric synthesis and catalysis are important tools in chemistry, pharmaceutical and material sciences. Increasingly large numbers of drugs and building blocks of materials show chirality in their structures. Group-assisted-purification (GAP) technology plays an important role in both general organic synthesis and asymmetric chemistry. It involves new achiral and chiral reagents and introduces alternative method of purification and recrystallization. A large amount of organic solvents and silica gels can be avoided by using GAP technology resulting in environmentally friendly work-up.
Till now, our group identified different applications of GAP chemistry in asymmetric synthesis and catalysis including the new form of chirality, known as Multi-layer 3D chirality. Sandwich typed organic molecule with multi-layer 3D chirality shows fluorescent and luminescent properties, which could open a new research area in chemical, medicinal, and material science in the future. It could also be a new catalyst besides BINOL/BINAP and others in asymmetric synthesis and catalysis.
This thesis has invoved building block molecules to synthesize multi-layer 3D chiral organic compounds and one pot synthesis of N-phosphonyl amide. Besides that, it also demonstrated the synthesis of α-aminomethyl enaminones and α- bromo nitroalkane using GAP chemistry
Multi-layer 3D chirality: Design, synthesis and applications
No full textChirality is not only closely bound up with people’s daily life, but also an essential issue in scientific research. From astronomy to earth science, from chemistry to biology, from history to information and communication science, chirality is omnipresent. Therefore, the construction of advanced chiral building blocks has become a significant concern in the research in both the origin of chirality and the development of chiral applications.
The present study focuses the concept of Multi-layer 3D chirality, has been addressing four issues including design background, structure extension, synthetic methodology, and molecular application. As specifically, single C-N/C-C bond anchored multi-layer 3D enantiomer has been synthesized and characterized through dual-coupling reactions. X-ray diffraction revealed the key features of this novel chirality: three layers arrange nearly parallel to each other and restrict each other from rotational isomerizaiton. Subsequently, structurally condensed triple-column/multi-layer 3D polymers have been achieved as the development of optical and AIE study of multi-layer 3D molecules. This work is believed to have profound impact on asymmetric chemistry and chiral materials in future
Synthesis of new proton-ionizable macrocyclic ligands
No full textCalix[4]arene is a scaffold for the construction of new metal ion receptors. Introduction of a crown ether loop at the lower rim of calix[4]arenes not only increases the cation binding ability of the parent calix[4]arene, but allows control of the selectivity through the modulation of the crown ether ring size. However, research on 1,2-bridged calix[4]crowns is rare. Known members are limited. In general, they showed low binding ability and selectivity toward metal cations.
In this dissertation, five series of di-ionizable calix[4]arene-1,2-crown ethers in the cone conformation were prepared as potential metal ion extractants. The conformations and regioselectivities of the new ligands were verified by 1H and 13C NMR spectroscopy. With the variation of proton-ionizable groups, which include oxyacetic acid and N-(X)sulfonyl oxyacetamide units with X = methyl, phenyl, 4-nitrophenyl, and trifluoromethyl, ¡°tunable¡± acidity of the ligand is obtained. Competitive solvent extraction of alkaline earth metal cations reveals that di-ionizable calix[4]arene-1,2-crown compounds are effective extractants with high selectivity for Ba2+. To our knowledge, this is the first example of a calix[4]arene-1,2-crown ether ligand is efficient and selective in metal cation separations.
To explore the influence of additional cation-¦Ð interactions between the aromatic pendant on lariat ether and the bound metal ion, one series of proton-ionizable dibenzo-16-crown-5 ethers with a sterically very bulky, 9-phenanthryl group was synthesized. The ligands are found to be effective extractants with high selectivity for Na+ in competitive solvent extraction of alkali metal cations. The Na+/K+ selectivity is three times larger than the corresponding values of those geminal aliphatic, proton-ionizable dibenzo-16-crown-5 compounds reported in literature
Oxalate and perchlorate: Two trace components in the environment
No full textOxalic acid is the most abundant atmospheric dicarboxylic acid. Its salts are very hygroscopic and can act as cloud condensation nuclei. In this work, vapor pressure of oxalic acid was measured in the laboratory at atmospherically relevant temperatures and these data were used to explain the observed gas-aerosol partitioning of the summertime oxalic acid data collected from three major US cities. We inferred that below 50% relative humidity, the oxalic acid aerosol in the atmosphere behaves like pure oxalic acid. The median oxalate concentration is nearly an order of magnitude greater than gas phase oxalic acid concentration. Data analysis shows that the main mechanism of formation is aqueous phase photochemical production. There was no correlation between oxalic acid and traffic markers such as NOx and CO, suggesting that traffic emissions are not a significant source for oxalic acid or its precursors.
Perchlorate has received much attention following its discovery in waterways in the US. Perchlorate interferes with the uptake of iodide by the thyroid gland affecting thyroid hormone production. A sensitive and selective method for determination of perchlorate in various matrices using ion chromatography-mass spectrometry (IC-MS) was developed. Perchlorate forms an ion-pair with a dicationic reagent; the ion-pair is detected by MS. The detection limit for perchlorate in real samples was 100 ng/L. A rapid and simple pretreatment technique for the determination of urinary perchlorate was developed. The sensitivity for measuring perchlorate is high using this method since there was no dilution of sample and the matrix effects are reduced. Perchlorate and iodide levels in seaweed samples were measured and their bioconcentration factors were calculated to assess the efficiency of seaweed as a source of iodide. We examined the possibility of the formation of perchlorate in the atmosphere and studied its mechanism. We showed that perchlorate is readily formed when chloride aerosol was exposed to electrical discharge. Many precipitation samples contain detectable levels of perchlorate. This strongly suggests that some perchlorate is formed in the atmosphere and a natural perchlorate background of atmospheric origin should exist
Mechanistic studies and synthetic application of monohydrolysis of symmetric diesers
No full textThe hydrolysis of esters is among the most common and useful reactions in organic chemistry. The monohydrolysis of symmetric diesters can produce half esters, which form intermediates for total syntheses of various important compounds. However, the efficiency for the monohydrolysis of symmetric diesters is low by classical saponification. In order to obtain high yields of products of monohydrolysis from symmetric diesters, enzymes are used in total syntheses. However, because mechanisms of enzyme reactions are not well understood, they often require extensive screening with no clue for the mononhydrolysis of symmetric diesters. Due to these reasons, a new method has been proposed and developed, which can produce a very high yield up to 99% for the mononhydrolysis of symmetric diesters.
In this research, we studied co-solvent effects for the reaction rate in the monohydrolysis reaction. Moreover, we expanded the research result of the co-solvent system from organic-aqueous systems to polar-less polar systems, which are used for the drug conjugation, pegylation and coupling reaction. The product of the mononhydrolysis of symmetric dimethyl bicyclo[2.2.1] hept-2,5-diene-2,3-dicarboxylate was used as the starting material for the total synthesis of CMP-New5AC mimics and an important intermediate for the syntheses of natural products with the methylene-bicylo[2.2.1] heptane framework. Asymmetric monohydrolysis of dimethyl bicyclo[2.2.1]
Texas Tech University, Hezhen Wang, December 2010
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hept-2,5-diene-2,3-dicarboxylate was studied without enzymes. In addition, chiral phase transfer catalysts, chiral bases, chiral ionic liquids and chiral catalysts were examined for the asymmetric reaction
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