Browsing by Subject "Stereochemistry"
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Item Approaches to the synthesis of gnididione(Texas Tech University, 1983-05) Erickson, Ronald HartwickNot availableItem Baylis-Hillman-type C-C bond formation and olefin diamination reaction(Texas Tech University, 2003-12) Pei, WeiThis thesis described new methodologies for Baylis-Hillman-type C-C bond formation and olefin diamination reaction. The Baylis-Hillman-type reaction was developed using a,â-unsaturated cycloketone and S-ethyl thioacrylate as the Michael-type acceptors with diethylaluminum iodide alone as the promoter. Direct diamination of enones was developed using N,N-dichloro-2-nitrobenzenesulfonamide (2-NSNCI2) as the electrophilic nifrogen source. The Baylis-Hillman-type reaction between a,â-unsaturated cycloketone or a,â-unsaturated S-ethyl thioacrylate and aldehydes has been achieved using diethylaluminum iodide as the promoter without the direct use of any Lewis bases. The reaction showed a good scope of substrates, and aldehydes which failed to give desired Baylis-Hillman products under our previous TiCl4-based conditions can be successfully used as the electrophiles to give good yields. The reactions proceeded to completion at 0 °C in CH2CI2 within 7-24 hours without the protection by inert gases. These processes involve the conjugate addition of Et2AlI to a,â-unsaturated substrates followed by carbonyl coupling. Releasing CH3CH3 is believed to act as a strong driving force and make the reactions irreversible. New electrophilic imidazolination reaction of enones was developed using 2-NsNCl2 as the electrophilic nitrogen source without any catalyst. It showed faster reaction rates and higher chemical yields than the reaction using analogous nitrogen source, 4-TsNCl2, under similar conditions. Molecular sieves and temperature played important roles to control the formation of desired products, l-o-nitrobenzenesulfonyl-3-dichloromethyl- 4,5-imidazolines and l-o-nitrobenzenesulfonyl-3-trichloromethyl-4,5-imidazolines. A [2+3] cycloaddition reaction was proposed as one step for the reaction to explain the regio- and stereoselectivity. a,â-Differentiated 1,2-vicinal diamines have been efficiently synthesized with good to excellent yields (87 - 96%) by using the new electrophilic imidazolination reaction of alkenes. The hydrolysis of imidazolines was performed by treatment with 6 A'' HCl in THF at 70 °C or in CHCI3 at 50 °C without epimerization.Item Stereochemistry of base promoted β-elimination from dihalocycloalkanes and 2-monosubstituted norbornanes(Texas Tech University, 1978-12) Lee, Jong GunElimination reactions are one of the four fundamental classes of reactions of organic compounds. The most frequently encountered type of elimination is β-elimination which involves the heterolytic cleavage of two bonds and the formation of a new multiple bond linkage. This process can be accomplished by a variety of mechanistic possibilities ranging from multistep reactions to concerted processes with different electron distributions and geometries. Three basic mechanisms of 3-elimination reactions have been recognized. These mechanisms differ chiefly in the timing of the carbonhydrogen and carbon-leaving group cleavage and are termed El, ElcB, and E2 mechanism.Item Studies directed towards the total synthesis of the natural product diazonamide A(2001-12) Kreisberg, Jennifer Diane; Magnus, Philip D.This dissertation is devoted to our synthetic studies aimed at the total synthesis of the natural product diazonamide A. Chapter 1 serves as an introduction to the diazonamides with a discussion of related marine cyclopeptides, previous synthetic work, and our retrosynthetic analysis. In Chapter 2, the development of a methodology for constructing benzofuran-2-ones related to the IHG-rings of diazonamide A using the Pummerer reaction is presented. Chapter 3 describes the preparation of bis-benzylic radicals and an examination of their chemical properties. A new methodology for the synthesis of 2,4-disubstituted oxazoles is reported in Chapter 4. Chapter 5 details the synthesis of several advanced model systems including the completion of the IHGCDEFrings. Finally, Chapter 6 contains a description of the experiments performed along with the relevant analytical data.Item Studies on the synthesis and stereochemistry of occidenol(Texas Tech University, 2001-05) Ajlouni, Abdulaziz M.Not availableItem Studies on the synthesis of dihydrodamsin(Texas Tech University, 1970-08) Ramsey, Herman DouglasNot availableItem Substitution reactions of 5[alpha]-Cholestan-6[alpha]-ols(Texas Tech University, 1976-05) Lundberg, Robert DwightThe primary objective of the present study was to reexamine the original reaction of 5a-cholestan-6a-ol with phosphorus pentachloride using modern techniques to determine whether 6a-chloro-5a-cholestane is the sole product, or whether the epimeric 63-chloro-5a-cholestane is also formed and, if so, to what extent. A secondary objective was the synthesis of 3a-halogen substitued 5a-cholestan-6a-ols, the 3a-substituent of which might be capable by steric interaction with the 6a-halophosphonate moiety thus influencing the steric course of the reaction. The 3a,6a-distance is ca. 5A, and for suitably sized 3a-substituents is capable of being spanned by the large phosphorus atom of a 6a-halophosphonate.Item Synthetic approaches toward dihydrodamsin and gnididione(Texas Tech University, 1980-08) Ramsey, Herman DouglasNot availableItem Theoretical and experimental studies on the reactivities of conjugated ketenes(Texas Tech University, 1998-08) Ham, SihyunThe reactivity of imidoylketene was examined using ab initio molecular orbital theory. MP4(SDQ)/6-31G*//MP2/6-31G* calculations on the conformations of imidoylketene as well as transition states for several of its reactions show parallels between the reactivity of imidoylketene and its oxygen analog formylketene. All reactions proceed via concerted, planar (or nearly so) transition structures regardless of the number of electrons involved. Calculated activation energies are remarkably lower than those for a pericyclic process, as expected from the case of formylketene. The reactions are interpreted in light of their pseudopericyclic orbital topology. N-Propylacetacetimidoylketene was produced by the solution pyrolysis of t-butyl N-propyl 3-amino-2-butenoate. Selectivities of acetimdoylketene toward various polar reagents were measured for the first time in a series of competitive trapping reactions. Significant steric and electronic discriminations of this ketone were observed, suggesting further synthetically useful applications. These experimental reactivity trends indirectly provide support for the planar, pseudopericyclic transition structures predicted by ab initio calculations. The mechanism of the reactions of nitrosoketene to form cyclic nitrones (which leads stereoselective synthesis of a-amino acids) was investigated using ab initio molecular orbital theory (MP4(SDQ)/6-31G*//MP2/6-31G* + ZPE). The direct [3+2] cycloadditions of nitrosoketene with ketones are calculated to be favored over the alternative [4+2] pathway via concerted, asynchronous, pseudopericyclic transition states. The detailed conformations and the reactivity of nitrosoketene toward sterically and electronically different ketones render useful information of the synthetic route for the biologically important reactions. Transition structures for a series of eight cheletropic decarbonylations were optimized at the MP4(SDQ)/D95**//MP2/6-3lG* + ZPE level. Dramatic differences in activation energies and in exothermicities are discussed in terms of the molecular orbital topology. A fundamental question regarding pseudopericyclic orbital overlap is addressed, specifically, how many and what type of orbital orthogonahties in the reaction sites are needed for a reaction to be pseudopericyclic. Generalizations regarding the characteristics of the pseudopericyclic reactions are made to provide a better understanding of the "allowedness" and "favoredness" of the orbital topologies.