• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 383
  • 258
  • 48
  • 37
  • 22
  • 21
  • 6
  • 5
  • 4
  • 4
  • 4
  • 3
  • 1
  • 1
  • 1
  • Tagged with
  • 975
  • 975
  • 375
  • 374
  • 224
  • 190
  • 173
  • 123
  • 70
  • 66
  • 64
  • 62
  • 56
  • 50
  • 50
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
101

Total syntheses of the regenerative natural products vinaxanthone, xanthofulvin, and eupalinilide E.

Chin, Matthew Ryan 26 August 2015 (has links)
The fungal metabolites vinaxanthone and xanthofulvin possess the remarkable ability to restore motor function in animal models of complete spinal cord transection making them the most promising small molecules for the development of spinal cord injury (SCI) therapeutics. A concise nine-step total synthesis of vinaxanthone was accomplished utilizing a biomimetic dimerization of the putative precursor 5,6-dehydropolivione and the first reported synthesis of xanthofulvin was achieved in 15-steps highlighted by an unprecedented enaminone O-to-C carboxyl transfer to forge key carbon-carbon bonds. Both natural products were also identified as positive allosteric modulators of the G-protein coupled receptor (GPCR), GPR91, thus elucidating their modes of action accounting for their regenerative capabilities. Furthermore, a unique ynone coupling reaction was developed in order to access various vinaxanthone analogs for structure activity relationship (SAR) studies. This resulted in the preparation of a small molecule library of 25 vinaxanthone analogs that demonstrated pronounced neuronal regeneration within laser axotomy assays performed in vivo on C. elegans. The plant derived natural product eupalinilide E has been found to promote the ex vivo expansion of hematopoietic stem and progenitor cells (HSPCs) which have the potential to improve the success of medical procedures such as bone marrow transplants. In light of its promising applications, unknown mechanism of action, and scarcity in nature the total synthesis of eupalinilide E was undertaken. Efforts culminated in the first enantioselective total synthesis of the natural product in 20-steps, which showcases a Favorskii rearrangement, borylative enyne cyclization, aldehyde-ene ring closure, and a dual allylic oxidation. / text
102

Total Syntheses of Fastigiatine and the Hibarimicin Aglycons

Liau, Brian Bor-Jen 07 June 2014 (has links)
Part one of this two-part thesis describes my efforts toward the total syntheses of the complex polycyclic alkaloids himeradine A and fastigiatine, which are members of the Lycopodium family of natural products. A cascade reaction sequence featuring a biosynthesis-inspired transannular Mannich reaction was planned to construct the strained and densely functionalized pentacyclic cores of the molecules from acyclic starting materials. After difficulties were encountered in a first-generation synthesis plan toward himeradine A, a second-generation synthesis plan was eventually successful in accomplishing the first total synthesis of fastigiatine via a formal [3+3]-cycloaddition reaction and a retro-aldol tandem transannular Mannich reaction sequence. In part two of this thesis, syntheses of the hibarimicin aglycons, including HMP-Y1, atrop-HMP-Y1, hibarimicinone, atrop-hibarimicinone, and HMP-P1, are reported. These natural products are amongst the largest and most complex type-II polyketides isolated. A novel benzylic fluoride Michael-Claisen reaction sequence was developed to construct the complete carbon skeleton of HMP-Y1 and atrop-HMP-Y1 via a symmetrical bidirectional double annulation reaction. Through efforts to convert HMP-Y1 derivatives to hibarimicinone and HMP-P1, a biomimetic mono-oxidation to desymmetrize protected HMP-Y1 was realized. A bidirectional unsymmetrical double annulation and biomimetic etherification were developed to construct the polycyclic and highly-oxidized skeleton of hibarimicinone, atrop-hibarimicinone, and HMP-P1. Lastly, a pH-dependent rotational barrier about the C2-C2' bond of hibarimicinone was discovered, which provides valuable information for achieving the syntheses of the glycosylated congeners of hibarimicinone. / Chemistry and Chemical Biology
103

A carbolithiation approach toward the synthesis of 8-methyl-pyridoxatin

Axelrod, Abram Joseph 17 June 2011 (has links)
A stereoselective approach toward the synthesis of 8-methyl-pyridoxatin using an intramolecular carbolithiation strategy is discussed. Model studies have proven this approach is not feasible for the synthesis of 8-methyl-pyridoxatin. / text
104

Studies of potential intermediates for the total synthesis of the antitumor compound (+)-pancratistatin

Edge, Mark 05 1900 (has links)
No description available.
105

Structure and synthesis of Gunnera perpensa secondary metabolites.

Peter, Xolani Kevin. January 2007 (has links)
The project focused on the isolation, characterization and synthesis of secondary metabolites of Gunnera perpensa L. (Gunneraceae), a South African medicinal plant used by many South African women to induce or augment labour and as an antenatal medication to tone the uterus. From the methanol extracts of the rhizomes we have isolated the compounds Z-venusol, methyl lespedezate, 4-6>-/?-D-glucopyranosyl-3,3',4'-tri-0- methylellagic acid and punicallagin. Structural elucidation of the compounds was performed using NMR spectroscopy. The presence of ellagic acid derivatives and hydrolysable tannins have not previously been reported from the family Gunneraceae. The study also focuses on the development of an HPLC analytical method to fingerprint the crude extracts of G perpensa. This method was used to determine the chemical composition of the rhizomes of the G. perpensa collected in different parts of South Africa. It is clear from the HPLC study that the rhizomes contain large concentrations of the hydrolysable tannin punicalagin and the second most abundant metabolite was Z-venusol. However, it was observed from plants collected in different regions that the ratio between punicalagin and Z-venusol differs substantially in the different extracts. An ellagic acid derivative isolated from G. perpensa contains a biaryl structure derived from gallic acid. The synthesis described in this thesis focused on reaction methods to access unsymmetrical biaryls and two synthetic routes were investigated - one that relies on the Ullmann reaction and the second that uses the Heck coupling reaction. Success of this coupling reaction towards the formation of ellagic acid derivatives was accomplished by the Heck coupling reaction method. One of the most important considerations towards the synthesis was the manipulation of hydroxyl groups of gallic acid by selective protection reactions that provide entry to the aforementioned preparation of unsymmetrical ellagic acid derivatives. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2007.
106

Application of selective methods in the search for new bioactive natural products from fungi

Chamyuang, Sunita January 2010 (has links)
The work undertaken explored the potential for discovery of new bioactive metabolites from soil fungi. The research developed selective mycological isolation techniques and maximised metabolite production from active selected fungi by application of the OSMAC approach and concept of hormesis. Novel active compounds were discovered and many other known compounds characterised.
107

Probing the Stereospecificity and Chemospecificity of Polyketide Thioesterases

Argyropoulos, Panos 06 May 2014 (has links)
Macrocyclization is a synthetically challenging step in the total synthesis of natural products. The success of chemical approaches such as the Corey-Nicolaou, Yamaguchi and Keck macrolactonization is heavily based on the confirmation and stereochemistry of the substrate. While there have been some advances in computational modeling, it has been difficult to predict whether the above-mentioned reactions will work. We have begun characterizing polyketide thioesterase catalytic activity and substrate tolerance to find more efficient and dependable routes towards macrolactonization and macrolactamization.
108

The Use of Catellani-type Reactions for the Synthesis of Heterocycles and Stereoselective Reactions of Arynes

Candito, David 10 December 2012 (has links)
This work can be separated into two parts; the first will encompass chapters one and two, which discuss the use of the Catellani reaction to access different heterocycles. The second part will encompass the remaining chapters and deals with stereoselective reactions of arynes. Chapter one outlines the development of a general and high yielding synthesis of the phenanthridine nucleus via a palladium-catalyzed domino reaction of aryl iodides with N-H or N-trimethylsilyl imines. This strategy was applied in a succinct synthesis of benzo[c]phenanthridine alkaloids nitidine and NK109. Chapter two details the development of a Catellani-type reaction of aryl iodides with 2H-azirines. Conditions were found to selectively provide access to either the indole nucleus or unusual dihydroimidazoles. The yields of the products ranged from moderate to good, however, the success of the reaction is highly dependent on the structure of the 2H-azirine. A mechanism was proposed involving oxidative addition of the azirine to generate an azaalyl intermediate. In Chapter three the foray into the area of aryne chemistry begins. A stereoselective nickel- catalyzed [2+2+2] cycloaddition of 1,6-enynes with aryne intermediates was developed. Attempts were made at the development of an asymmetric variant of the reaction, however, only low enantiomeric excess was observed. Good diastereoselectivity could be obtained when an allylic substituent was present. The yields of the products ranged from moderate to excellent. However, the yields vary greatly and in a number of cases the reactions were unsuccessful. Finally, in chapter four the development of a general and high yielding annulation strategy for the synthesis of various carbo- and heterocycles, based on an intramolecular aryne ene reaction is described. It was found that the geometry of the olefin is crucial to the success of the reaction and it that regioselective hydrogen migration occurs. Furthermore, the electronic nature of the aryne was found to be important to the success of the reaction. Deuterium labeling studies and DFT calculations provided insight into the reaction mechanism. The data suggested a concerted asynchronous transition state, resembling a nucleophilic attack on the aryne. This strategy was successfully applied to the formal synthesis of the ethanophenanthridine alkaloid (±)-crinine. In a similar vein, preliminary results demonstrating an intramolecular, formal [2+2] cycloaddition of an aryne with an S, O-ketene acetal to give interesting tricyclic benzocyclobutene products are disclosed.
109

Isolation, Synthesis and Structure-Activity Relationship Study of Anticancer and Antimalarial Agents from Natural Products

Dai, Yumin 18 November 2013 (has links)
The Kingston group's engagement in an International Cooperative Biodiversity Group (ICBG) program and a collaborative research project established between Virginia Tech and the Institute for Hepatitis and Virus Research (IHVR) has focused on the search for bioactive natural products from tropical forests in both Madagascar and South Africa. As a part of this research, a total of four antiproliferative extracts were studied, leading to the isolation of fourteen novel compounds with antiproliferative activity against the A2780 human ovarian cancer line. One extract with antimalarial activity was studied, which led to the isolation of two new natural products with antiplasmodial activity against a drug-resistant Dd2 strain of Plasmodium falciparum. The plants and their secondary metabolites are discussed in the following order: two new antiproliferative acetogenins from a Uvaria sp. (Annonaceae); two new antiproliferative calamenene-type sesquiterpenoids from Sterculia tavia (Malvaceae); two new antiproliferative triterpene saponins from Nematostylis anthophylla (Rubiaceae); six new antiproliferative homoisoflavonoids and two new bufatrienolides from Urginea depressa (Asparagaceae); and two new antiplasmodial anthraquinones from Kniphofia ensifolia (Asphodelaceae). The structures of all these compounds were determined by analysis of their mass spectrometric, 1D and 2D NMR, UV and IR spectroscopic and optical rotation data. Other than structural elucidation, this work also involved bioactivity evaluations of all the isolates, as well as total synthesis of the two antiproliferative sesquiterpenoids, and a structure-activity relationship (SAR) studies on the antiplasmodial anthroquinones. / Ph. D.
110

Total synthesis of ancistrotanzanine A.

Brusnahan, Jason Stewart January 2010 (has links)
This thesis describes the first total synthesis of ancistrotanzanine A, a member of the naphthylisoquinoline class of natural products. In Chapter 1 the synthetic challenges presented by the naphthylisoquinoline alkaloids are discussed and strategies that have been adopted in previous syntheses of naphthylisoquinoline alkaloids overviewed. Chapter 2 describes the preparation of the key 5,3'-biaryl linkage via the Pinhey-Barton reaction. Studies into forming the linkage atropselectively were investigated using chiral hydrobenzoin acetal auxiliaries. This was found to have limited success with an atropisomeric ratio of 65:35 obtained. Changing the base from the achiral pyridine to the chiral brucine was also investigated and found to give no enhancement in the diastereoselectivity. From the results presented in Chapter 2, it was concluded that hydrobenzoin acetal auxiliaries were not appropriate for the diastereoselective synthesis of the key biaryl linkage of ancistrotanzanine A. As the chiral acetal strategy outlined in Chapter 2 failed to yield an atropselective process, efforts were re-focused on a new approach to the naphthylisoquinolines. In Chapter 3, an overview of all the methods available for the synthesis of chiral 3,4-dihydroisoquinolines is provided. From this, it was decided to apply the alkylation of o-tolylnitriles with chiral sulfinimines, as originally developed by Davis, to the synthesis of naphthylisoquinolines. Synthesis of the o-tolylnitrile lead reagent was readily achieved, but it was found that the amount of lead tetraacetate had to be carefully controlled to avoid side-reactions in the Pinhey-Barton reaction. After careful optimisation, the key 5,3'-biaryl linkage was prepared in high yield. Application of the Davis methodology to the MOM protected biaryl failed, with no reaction resulting. After much experimentation, it was established that the reaction was very sensitive to steric hindrance. A successful reaction was finally achieved by changing the base to lithium diethylamide. However, it was found the diastereoselection of the alkylation was quite low when p-tolyl sulfinimine was used. The use of the t-butane sulfinimine meant that the diastereoselection was significantly improved, with a ratio of 85:15 being obtained. After 3 more steps, the total synthesis was completed and ancistrotanzanine A was obtained, as a 1:1 mixture of atropisomers. Efforts to separate the atropisomers formed failed and even the use of chiral HPLC failed to resolve the material. To complete the Chapter, two analogues of ancistrotanzanine A were prepared – the tetrahydroisoquinoline and the methoxy ether. Chapter 4 summarises the above results and discusses the future potential of this research. / Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 2010

Page generated in 0.071 seconds