Browsing by Subject "Cycloaddition"

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    The Hexadehydro-Diels–Alder (HDDA) Reaction-Enabled Bottom-up Synthesis of Elaborated Polycyclic Aromatics
    (2019-05) Xiao, Xiao
    Polycyclic arenes are an important class of organic molecules with promising semiconducting properties. Their relatively low cost, band-gap tunability, and ease of fabrication render them suitable for a host of applications for the next-generation optoelectronics devices. The biggest challenge to realize the full potential of organic semiconductors is the chemical synthesis of atomically precise polycyclic aromatics. The current strategies to assemble these materials heavily rely on transition-metal catalyzed cross-coupling reactions of prefunctionalized arenes, which in a way limit the vision of material scientists when search for potential compounds. This thesis describes a complementary synthetic approach to polycyclic aromatic products from polyynes via the hexadehydro-Diels–Alder (HDDA) reaction. The HDDA reaction is a variant of the classic Diels–Alder reaction, which generates pristine benzyne intermediates purely thermally. This mechanistically intriguing transformation also has served as a great platform for many discoveries of fundamentally new reactivities. Here multiple aspects of the HDDA reaction are discussed: (1) reaction of perylenes with HDDA-benzynes and photochemical HDDA reactions, (2) accessing other reactive intermediates via HDDA-generated benzynes, (3) copper(I)- and BF3- catalyzed trapping reactions of benzynes, (4) rapid construction of polyacenes via the domino HDDA reaction, and (5) a cascade strategy for using classically generated benzynes as in situ diynophiles for accessing HDDA-naphthynes.
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    Regioselective Synthesis of Isoxazoles by Hypervalent Iodine(III) Reagent Mediated Oxidative Cycloaddition
    (2019-08) Jarvi, Melissa
    Isoxazole is a five membered heterocyclic compound containing oxygen and nitrogen atoms in the 1,2 positions. Isoxazole rings are found in natural products, such as ibotenic acid and muscimol. The isoxazole structure is incorporated in a variety of pharmaceutical agents. Substitution on different positions on the nucleus of the isoxazole results in various pharmacological effects. Specifically, isoxazole compounds are used in various types of pharmaceutical compounds to treat bacterial infections and pain relief. Oxidative cycloaddition of aldoximes with unsaturated substrates provides an efficient approach to the formation of heterocyclic compounds. More specifically, various isoxazoles can be easily prepared by the oxidative cycloaddition of aldoximes with unsaturated substrates in the presence of hypervalent iodine(III) reagents. Oxidation of aldoximes by hypervalent iodine(III) compounds produces nitrile oxides which further react with respective unsaturated substrates through a 1,3-dipolar cycloaddition reaction resulting in various isoxazole products. This technique is an efficient process that utilizes green chemistry. Here, we report an efficient synthetic approach for preparation of various regioselective 3,4-substituted isoxazoles and 3,4,5-substituted isoxazoles using [hydroxy(tosyloxy)iodo]benzene, commonly known as Koser’s reagent. To our knowledge, this is the first time the 3,4-substituted isoxazole has been prepared. The oxidative cycloaddition proceeds at room temperature resulting in moderate to high yields. Structures of several isoxazole derivatives were established by X-ray crystallography. These isoxazole derivatives are stable compounds that have the potential to be useful for biologically active molecules and pharmaceutical reagents.