Browsing by Author "Shi, Yian, advisor"
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Item Open Access Asymmetric epoxidation of various olefins catalyzed by fructose- and glucose-derived ketones(Colorado State University. Libraries, 2009) Wong, On Lo Andrea, author; Shi, Yian, advisorNumerous laboratories have studied dioxirane- and oxaziridinium-mediated epoxidations during the past two decades. Many chiral ketone and iminium salt catalysts, bearing a wide variety of structural features, have been investigated. Out of the systems studied a fructose-derived ketone has been proven to be one of the most general and practical catalysts. This catalyst epoxidizes trans- and trisubstituted olefins in good yield and enantioselectivity, and it has been employed in the syntheses of many complex molecules. In efforts to expand the substrate scope a series of glucose-derived, oxazinolidine-bearing ketones were reported to be excellent catalysts for the epoxidation of conjugated cis-olefins. The stereodifferentiation in the epoxidation transition state originates from the attraction between the N-substituent of the oxazolidinone and the Rπ substituent on the olefin. The existence of this interaction was supported by the observation that 6-substituted chromenes were epoxidized with higher enantioselectivities than 8-substituted chromenes. Using this glucose-derived ketone system, substituted chiral styrene oxides could be obtained in 80-92% ee. Fluoroolefins were investigated as epoxidation substrates with several fructose- and glucose-derived ketone catalysts. A fluorine substituent was found to improve enantioselectivity in some cases but was detrimental to enantioselectivity in others. The substrate scope of a diacetate-containing ketone was expanded. High enantioselectivities were obtained for the epoxidation of trans- and trisubstituted olefins, and cis-olefins bearing a bulky substituent. The optical rotations of the resulting cis-epoxides were opposite to those obtained using glucose-derived ketones. 1,1-Disubstituted terminal olefins were epoxidized in good enantioselectivities with a glucose-derived morpholinone ketone. From the absolute configuration of the resulting epoxides, the major transition state appears to be a planar-like transition state. Also studied was a glucose-derived dimethylmorpholinone ketone that has the combined features of several of the previously studied ketones. This catalyst epoxidizes trans- and trisubstituted olefins in high enantoselectivities, but compared to the oxazolidinone-containing ketones gives slightly lower enantioselectivities with cis- and 1,1-disubstituted olefin substrates. Lastly, the epoxidation transition state model was studied using 18O-labeled ketone catalysts, and the results support the currently accepted transition state model.Item Open Access Chiral ketone-catalyzed asymmetric epoxidation of cis-olefins(Colorado State University. Libraries, 2007) Burke, Christopher Patrick, author; Shi, Yian, advisorEpoxides are invaluable intermediates in organic synthesis and are present in many biologically active compounds. As such, there is great interest in the synthesis of chiral non-racemic epoxides. One of the most convenient methods for their synthesis is the catalytic asymmetric epoxidation of olefins, and much work has been done in this area. Our group has been interested in epoxidation of unfunctionalized olefins using chiral dioxiranes generated from chiral ketones and Oxone or other oxidant.Item Open Access Oxidation of unfunctionalized olefins involving three-membered heterocycles and its related applications(Colorado State University. Libraries, 2008) Wang, Bin, author; Shi, Yian, advisorTypically three-membered heterocycles are highly strained molecules. They can exist as stable functional groups or very reactive intermediates. This dissertation discusses three types of three-membered heterocycles including dioxiranes, epoxides, and diaziridines. Dioxirane, a three-membered ring peroxide, is a very powerful oxidant which transfers an oxygen atom to a variety of functional groups including heteroatoms, π-bonds, X-H δ-bonds (X = C or Si), and organometallic compounds. Our group has been interested in the asymmetric epoxidation of unfunctionalized olefins using chiral dioxiranes generated from chiral ketones and Oxone. Asymmetric epoxidation produces chiral epoxides, a very useful three-membered heterocycle, which can be opened and rearranged to form more complex chiral molecules. A glucose-derived ketone with an oxazolidinone moiety has been employed in asymmetric epoxidation of conjugated tri- and tetrasubstituted olefins. The asymmetric epoxidation and subsequent epoxide rearrangement produced the enantioenriched aryl-substituted epoxides, cyclopentanones, cyclobutanones, and γ-butyrolactones in good yields and enantioselectivities. In addition to the above chiral products, chiral allylic alcohols can also be produced via asymmetric epoxidation catalyzed by fructose-derived ketone and base-mediated epoxide isomerization in good yields, high enantioselectivities, and high stereoselectivities. It was proposed that the isomerization of acyclic silyl epoxides to give the (Z)-allylic alcohols proceeds through an unusual silicon-assisted E1cb mechanism based on deuterium-labeling experiments and other observations. A practical synthesis of a diacetate chiral ketone was developed. The application of this diacetate ketone in asymmetric epoxidation of trans- and trisubstituted olefins, as well as its related mechanism, are discussed. A ketone with two oxazolidinone rings proved to be a robust catalyst for asymmetric epoxidation. The catalyst loading can be reduced to 1 mol %, and high enantioselectivities can still be achieved. A glucose-derived ketone with a lactam ring has been employed in the epoxidation of 1,1-disubstituted olefins with good enantioselectivities. Studies indicated that the epoxidation of 1,1-disubstituted olefins with the lactam ketone proceeds mainly via a planar-like transition state. The α, α-dimethyl substituted lactam ketone shows different reactivity from those without substitutions. It is an effective catalyst for asymmetric epoxidation of trans- and trisubstituted olefins. A study on the structural effect of ketone catalysts on asymmetric epoxidation revealed that the nitrogen atom in the spiro ring of the oxazolidinone-containing ketone is an important structural element in asymmetric epoxidation of cis-olefins. N,N'-di-t-butylthiadiaziridine 1,1-dioxide, a nitrogen analogue of dioxiranes, was explored as a nitrogen source for Pd-catalyzed dehydrogenative diamination of unfunctionalized olefin. The diamination is likely to proceed via Pd-catalyzed allylic amination and subsequent cyclization. This diamination is mechanistically distinct from the previously studied process using di-t-butyldiaziridinone as nitrogen source, thus resulting in different regioselectivity.Item Open Access The development and application of metal-catalyzed diamination reactions(Colorado State University. Libraries, 2014) Cornwall, Richard G., author; Shi, Yian, advisor; Kennan, Alan, committee member; McNaughton, Brian, committee member; Henry, Charles, committee member; Chatterjee, Delphi, committee memberNitrogen-rich molecules are of great interest in chemistry and incorporation of nitrogen into molecules is an on-going active field of study. In particular, vicinal diamines are important functional moieties that are found throughout biologically active molecules and natural products as well as highly effective chiral control agents in organic synthesis. There has been much effort directed toward the efficient synthesis of vicinal diamines; however the development of a direct route has proven to be challenging. This dissertation discusses the application of diamination products from existing methods to synthesize biologically active motifs, as well as the development of new metal-catalyzed diamination methods for the synthesis of biologically interesting motifs from readily available starting materials. The β, γ-diamino acid motif is an area of active research because of its prevalence in biologically active molecules and its use in peptide library syntheses. Cyclization of β, γ-diamino acids give the closely related 4-aminopyrrolidinones. These five-membered amino lactams have been reported to potentiate insulin activity when incorporated into hypoglycemic peptide analogues and made the analogues more stable towards physiological degradation. Current methods for the synthesis of these compounds require multi-step procedures and rely heavily on commercially available amino acids as starting materials, thus limiting the structural variability for biological studies. Using a diamination method discovered in our lab, 4-aminopyrrolidinones were efficiently synthesized in 40% overall yield, over five steps from readily available terminal olefins or conjugated dienes, providing a comparable process in the synthesis of these compounds. As part of our ongoing efforts to study the mechanism of metal-catalyzed diaminations using diaziridinone as nitrogen source, it was found that regioselectivity in the diamination of conjugated dienes could be controlled using Cu(I) as catalyst and varying reaction conditions. An alternative nitrogen source, thiadiaziridine 1,1-dioxide, which has shown to display interesting reactivity, was chosen to further investigate the Cu(I)-catalyzed regioselective diamination. Upon varying reaction conditions with Cu(I) catalysts, regioselective diamination occurred for various conjugated dienes and allowed direct access to a range of diverse cyclic sulfamides which have interesting biological potential. With the racemic synthesis of cyclic sulfamides, it was of interest to obtain these compounds asymmetrically, as their biological properties are of value and current methods for their asymmetric synthesis do not allow much variation in substitution patterns. Using Pd2(dba)3 and a chiral phosphoramidite ligand, a variety of chiral cyclic sulfamides were synthesized in moderate to high yields and with ee's greater than 90%, providing direct access to these valuable compounds in one step from readily available conjugated diene substrates. Lastly, N,N-Di-tert-butyl thiadiaziridine 1,1-dioxide has been found to be a versatile reagent for interesting reactivity. Other uses of this reagent include the Pd(II)-catalyzed terminal diamination of conjugated dienes, diamination of allenes, and the Pd-catalyzed oxidation of alcohols to form α, β-unsaturated compounds.