SciCombinator

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Concept: Cyclopropane

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No transition metal needed: A calcium-catalyzed cycloisomerization yields highly substituted cyclopropanes with excellent diastereoselectivity (see scheme; Nu=nucleophile, Ts=p-toluenesulfonyl). The reaction is based on the equilibrium of a homoallenyl cation with its cyclopropane congener, which is known for gold-catalyzed reactions. According to mechanistic analysis the carbocation cascade is concerted asynchronous, and hence biomimetic.

Concepts: Cyclopropane

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An investigation of the intramolecular cyclopropanation reactions of α-diazo-β-ketonitriles is reported. These studies reveal that α-diazo-β-ketonitriles exhibit unique reactivity in their ability to undergo arene cyclopropanation reactions; other similar acceptor-acceptor-substituted diazo substrates instead produce mixtures of C-H insertion and dimerization products. α-Diazo-β-ketonitriles also undergo highly efficient intramolecular cyclopropanation of tri- and tetrasubstituted alkenes. In addition, the α-cyano-α-ketocyclopropane products are demonstrated to serve as substrates for SN2, SN2', and aldehyde cycloaddition reactions.

Concepts: Alkyne, Alkene, Functional groups, Cycloaddition, Aldehyde, Wittig reaction, Elimination reaction, Cyclopropane

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The borylation of cyclopropanes catalyzed by the combination of (η6-mes)IrBpin3 or [Ir(COD)OMe]2 and a phenanthroline derivative is reported. The borylation occurs selectively at the methylene C-H bonds of the cyclopropane ring over methine or methyl C-H bonds. High diasteroselectivities were observed from reactions catalyzed by the combination of iridium and 2,9-Me2phenanthroline. The cyclopropylboronate esters that are generated are versatile synthetic intermediates that can be converted to trifluoroborate salts, boronic acids, cyclopropylarenes, cyclopropylamines, and cyclopropanols.

Concepts: Acid, Ammonia, Acetic acid, Functional groups, Cyclopropane

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A tandem reaction between N'-(2-alkynylbenzylidene)hydrazide and cycloprop-2-ene-1,1-dicarboxylate co-catalyzed by silver triflate and tris(triphenylphosphine)rhodium chloride is reported. The reaction proceeds through 6-endo-cyclization, [3 + 2] cycloaddition, cyclopropane opening, and aromatization, leading to pyrazolo[5,1-a]isoquinolines in moderate to good yields.

Concepts: Chemical reaction, Catalysis, Enzyme catalysis, Ozone, Cyclopropane

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Herein, we report the enantio- and diastereoselective formation of trans-iodo- and trans-chlorocyclopropanes from α-iodo- and α-chlorozinc carbenoids by using a dioxaborolane-derived chiral ligand. The synthetically useful iodocyclopropane building blocks were derivatized by an electrophilic trapping of the corresponding cyclopropyl lithium species or a Negishi coupling to give access to a variety of enantioenriched 1,2,3-substituted cyclopropanes. The synthetic utility of this method was demonstrated by the formal synthesis of an HIV-1 protease inhibitor. In addition, the related stereoselective bromocyclopropanation was also investigated. New insights about the relative electrophilicity of haloiodomethylzinc carbenoids are also presented.

Concepts: HIV, Stereochemistry, Chirality, Utility, Tartaric acid, Electrophile, Electrophilic addition, Cyclopropane

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With the use of benzonitrile-stabilized Au(I) catalyst [Au(IPr)(NCPh)]SbF(6) (Ic; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), a spectrum of reactivity is observed for propargyl ester 4 a with cyclic vinyl ethers, ranging from exclusively [3C+2C] cycloaddition reactions to exclusively cyclopropanation depending only on the structure of the substrate. Some initially formed cyclopropanation products rearrange into the corresponding formally [3C+2C] cycloaddition products after treatment with fresh Au(I) complex at 80 °C. Vinylcyclopropanes formed from dihydrofuran and dihydropyran resisted such rearrangement, even in the presence of fresh Au(I) catalyst at elevated temperature. This study addresses an important mechanistic question concerning whether the five-membered-ring products were produced by a direct [3C+2C] cycloaddition reaction or by a sequential cyclopropanation/ring-expansion reaction. A dual pathway is proposed for the Au(I) -catalyzed reactions between propargyl esters and cyclic vinyl ethers. The different behavior among vinyl cyclic ethers is attributed to the difference in the polarization of the π bond. Highly polarized bonds appear to undergo the cycloaddition reaction whereas less polar π-bonds produce cyclopropanes.

Concepts: Alcohol, Chemical reaction, Carbonyl, Functional groups, Cycloaddition, Polarization, Cyclopropane

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Carbocyclic nucleosides are nucleoside analogues in which the furanosidic moiety has been replaced by a carbocycle. Several members of this family have been isolated from natural sources and include a 5-membered ring carbocycle. The aim of this review is to examine critically the different methodologies for the enantioselective construction of 3- to 6-membered rings, with a particular focus on 5-membered rings and their modifications. The procedures for bonding the heterocyclic moiety and the carbohydrate are treated separately. The methods for synthesising the carbocyclic moiety mainly focus on the construction of the cycle, although precise details about the functionalisation are provided in some cases. The selected methods aim to provide an overview of the synthesis of carbocycles related to the synthesis of carbocyclic nucleosides. The methods of synthesis of 5-membered rings are classified into two types: methods in which the cyclopentane ring is formed by ring closing reactions (C[double bond, length as m-dash]C and C-C formation) and methods that start from preformed 5-membered rings, based mainly on cycloaddition reactions. With respect to the methods of synthesis of 3-, 4- and 6-membered ring carbocyclic nucleosides, a selection of the more relevant enantioselective procedures is presented in a systematic manner.

Concepts: Natural selection, Chemical reaction, Nucleoside, Cyclopropane

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The Staudinger ketene-imine [2 + 2] cycloaddition reaction for conversion of α-heteroatom-substituted exocyclic imines to C4 heterocyclic spiro-β-lactams has rarely been investigated due to their instability. Herein, we describe the Staudinger reaction between ketenes and α-selenium-substituted exocyclic imines to synthesize C4 spiro-β-lactams.

Concepts: Cycloaddition, Cyclopropane, Cycloadditions, Hermann Staudinger, Ketene

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The Pd-catalyzed, Ag(I)-mediated intramolecular direct arylation of cyclopropane C-H bonds is described. Various spiro 3,3'-cyclopropyl oxindoles can be obtained in good to excellent yields from easily accessible 2-bromoanilides. The kinetic isotope effect was determined and epimerization studies were conducted, suggesting that the formation of a putative Pd-enolate is not operative and that the reaction proceeds via a C-H arylation pathway.

Concepts: Isotope, Kinetic isotope effect, Cyclopropane

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The stereoselective total synthesis of garsubellin A is described. The total synthesis was achieved through the stereoselective construction of a bicyclo[3.3.1]nonane derivative via a three-step sequence: intramolecular cyclopropanation, formation of a germinal dimethyl group, and regioselective ring opening of cyclopropane. To complete the total synthesis of garsubellin A, chemo- and stereoselective hydrogenation to generate the C8 stereogenic center is followed by the formation of the fused tetrahydrofuran ring by a regioselective epoxide-opening reaction with C3 ketone, and finally cross metathesis to construct two prenyl groups.The Journal of Antibiotics advance online publication, 6 February 2013; doi:10.1038/ja.2012.125.

Concepts: Stereochemistry, Total synthesis, Mathematical analysis, Vector space, Alkane, Stereocenter, Construction, Cyclopropane