New chemistry of donor-acceptor cyclopropanes
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Donor-Acceptor (DA) cyclopropanes are considered to be well-understood compounds with predictable chemistry. This dissertation focuses on our recent advances and synthetic applications of DA cyclopropanes principally involving 1,3-dipole intermediates revealed by reactions with Lewis acids. Particular emphasis is placed on reactions involving sugar-derived substrates. Chapter one is an overview of the literature preparations and synthetic applications of DA cyclopropanes. It begins with general DA cyclopropanes but is mostly focused on carbohydrate-derived cyclopropanes. Chapter two describes our intramolecular glucal cyclopropanation approach toward lactonized cyclopropanes 2.5a-c, their transformations under mild conditions, and their application to the quasi formal synthesis of the natural product xylobovide 2.34a. Chapter three investigates the different chemical behaviors of glucal-derived lactonized cyclopropane 2.5a under the influence of a variety of different Lewis acids. Use of boron trifluoride etherate (BF3•OEt2) leads to regioselective mono-desilylation of the di-tert-butylsilylene ether to 2.27; trimethylsilyl trifluoromethanesulfonate (TMSOTf) results in anhydro-sugar 3.8 formation; and titanium tetrachloride (TiC4) facilitates stereoselective allylation and glycosidation. Chapter four discuses our recently discovered formal [3+2] dipolar cycloaddition reactions of DA cyclopropanes with a wide variety of dipolarophiles including silyl enol ethers, imines, aldehydes and nitriles. The extension of nitrile cyclization to general noncarbohydrate derived DA cyclopropanes leads to the discovery of a novel synthetic methodology toward pyrroles, bipyrroles and thienylpyrroles. Finally, the [3+2] cycloaddition has also been extended to heterocycles including pyridines, quinolines and indoles. Chapter five summarizes the investigation of regio-selective mono-desilylation of di-tert-butylsilylene ethers as a new protocol for selective hydroxy protection. Under optimized conditions, the reaction demonstrates high regioselectivity as well as strong functional group compatibility, and proves promising as a new protocol for selectively manipulating 1,3-diols. Chapter six provides the experimental procedures and characterization of all the new compounds.