Lewis acid-promoted cycloaddition reactions of aziridines and cyclopropanes

Abstract

I. Lewis Acid-Promoted Carbon-Carbon Bond Cleavage of Aziridines: Divergent Cycloaddition Pathways of the Derived Ylides The formation of azomethine ylides from aziridines under Lewis acidic conditions and the productive reactivity of these species are described. Efficient carbon-carbon bond cleavage of aziridines through treatment with stoichiometric or catalytic amounts of Zn(II) salts has been shown to occur under relatively mild conditions. Depending on the constitution of the dipolarophile, the resulting azomethine ylides undergo cycloadditions with electron-rich olefins in either a [3+2] or [4+2] manner producing pyrrolidine or tetrahydroquinoline derivatives. R CO2R' N CO2R' ZnX2 R N O OR' R'O O Ar ZnXn N Ar R CO2R' CO2R' [3+2] [4+2] H2C EDG Y N R Y CO2R' CO2R' Z Z n n EDG R = Aryl or H X = Cl or OTf II. Heterocycle Synthesis Via [3+2] Reactions of Cyclopropanes A review of the literature concerning the use of cyclopropanes in the one-step synthesis of five-membered heterocycles is presented. Methodology for the convenient, stereoselective preparation of substituted ?-lactones, ?-lactams, tetrahydrofurans, pyrrolidines, and various other heterocycles from simple cyclopropane precursors is described. Lewis acid-promotion/catalysis, as well as thermal methods for the ring cleavage of cyclopropanes are discussed. Where possible, proposed models are given for the stereochemical outcome of these annulation reactions. III. Enantiospecific and Diastereoselective Tin-Catalyzed Cycloaddition Reactions of Aldehydes and Donor-Acceptor Cyclopropanes: The Synthesis of 2,5- Disubstituted Tetrahydrofurans The diastereoselective synthesis of a variety of cis-2,5-disubstituted tetrahydrofurans via Sn(II)- or Sn(IV)-catalyzed cycloaddition reactions of donor-acceptor (D-A) cyclopropanes with aldehydes is described. In some cases, the use of enantioenriched cyclopropane has been shown to afford optically active tetrahydrofuran derivatives with excellent stereochemical transfer. Evidence is presented supporting an SN2 reaction in which the aldehyde participates as a nucleophilic species. R CO2Me CO2Me R'CHO Sn(OTf)2 or SnCl4 (cat.) R O R' CO2Me MeO2C + R = Aryl, heteroaryl, alkenyl, alkyl R' = Aryl, heteroaryl, alkenyl, alkynyl, alkyl IV. Application of the Tin-Catalyzed Cyclopropane/Aldehyde Cycloaddition Reaction: Progress Towards the 2-Oxabicyclo[4.3.0]nonane Core of the Cladiellin Diterpenes Efforts towards the construction of the hydroisobenzofuran core common to the cladiellin diterpenes are discussed. The use of a model system in the catalytic cyclopropane/aldehyde cycloaddition manifold has been explored. The stereochemical results of this successful reaction are discussed in the context of possible extension to a more sophisticated system. Synthetic efforts herein have disclosed a potential straightforward route for the preparation electron-poor alkenylidene cyclopropanes via an unprecedented 3- exo-dig cyclization pathway