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Cyclization

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Cyclization - Free Register to Access Experts & Abstracts

Hans-ulrich Reissig - One of the best experts on this subject based on the ideXlab platform.

  • lithium bromide water as additives in dearomatizing samarium ketyl hetero arene Cyclizations
    Chemistry: A European Journal, 2015
    Co-Authors: Christoph Bentz, Hans-ulrich Reissig
    Abstract:

    New conditions for dearomatizing samarium-ketyl (hetero)arene Cyclizations are reported. In many examples of these samarium diiodide-mediated reactions, lithium bromide and water can be used as additives instead of the carcinogenic and mutagenic hexamethylphosphoramide (HMPA). The best results were obtained for the Cyclizations of N-acylated indole derivatives delivering the expected indolines in good yields and excellent diastereoselectivities. A new type of Cyclization delivering indolyl-substituted allene derivatives is also described. The scope and limitations of the lithium bromide/water system are discussed.

  • towards the core structure of strychnos alkaloids using samarium diiodide induced reactions of indole derivatives
    Chemistry: A European Journal, 2013
    Co-Authors: Christine Beemelmanns, Steffen Gross, Hans-ulrich Reissig
    Abstract:

    This report describes the development of a first and second generation approach towards the synthesis of the ABCEG pentacyclic core structure of Strychnos alkaloids. First, we discuss a sequential approach applying a series of functional group transformations to prepare suitable precursors for Cyclization reactions. These include attempts of samarium diiodide-induced Cyclizations or a Barbier-type reaction of a transient lithium organyl, which successfully led to a tetracyclic key building block earlier used for the synthesis of strychnine. Secondly, we account our first steps towards the development of an atom-economical samarium diiodide-induced cascade reaction using “dimeric” indolyl ketones as Cyclization precursors. In this context, we discuss plausible mechanisms for the samarium diiodide-induced cascade reaction as well as transformations of the obtained tetracyclic dihydroindoline derivatives.

  • New samarium diiodide-induced Cyclizations
    Pure and Applied Chemistry, 2011
    Co-Authors: Christine Beemelmanns, Hans-ulrich Reissig
    Abstract:

    Samarium diiodide (SmI 2 ) smoothly promotes the Cyclizations of suitably substi- tuted carbonyl compounds with styrene subunits leading to benzannulated cyclooctenes. The intramolecular samarium ketyl addition to arene or hetarene moieties enables a new, efficient, and highly stereoselective entry to dearomatized products such as hexahydronaphthalenes, steroid-like tetra- or pentacyclic compounds, or dihydroindole derivatives. The usefulness of the developed SmI 2 -induced Cyclization method was demonstrated by the shortest formal total synthesis of the alkaloid strychnine.

Diego J Cardenas - One of the best experts on this subject based on the ideXlab platform.

  • titanocene iii catalyzed 6 exo versus 7 endo Cyclizations of epoxypolyprenes efficient control and synthesis of versatile terpenic building blocks
    Chemistry: A European Journal, 2013
    Co-Authors: Jose Justicia, Diego J Cardenas, Tania Jimenez, Delia Miguel, Rafael Contrerasmontoya, Rachid Chahboun, Enrique Alvarezmanzaneda, Daniel Colladosanz, Juan M Cuerva
    Abstract:

    In this article, a complete study on the selectivity of titanocene(III) Cyclization of epoxypolyprenes is presented. The requirements for the formation of six- or seven-membered rings during these Cyclizations are determined, taking into account the different substitution pattern in the epoxypolyprene precursor. Thus, a complete selectivity to 6-exo or 7-endo Cyclization process has been achieved, yielding mono-, bi-, and even tricyclic compounds, constituting a new and efficient access to this type of derivative. Additionally, this procedure opens the possibility to prepare excellent building blocks for the synthesis of polycyclic compounds with a trisubstituted oxygenated function, which is present in several natural terpenes.

  • 7 endo radical Cyclizations catalyzed by titanocene iii straightforward synthesis of terpenoids with seven membered carbocycles
    Journal of the American Chemical Society, 2005
    Co-Authors: Jose Justicia, Juan M Cuerva, Juan L Ollerlopez, Araceli G Campana, Enrique J Oltra, Elena Bunuel, Diego J Cardenas
    Abstract:

    We describe a novel procedure for the straightforward synthesis of seven-membered carbocycles via free-radical chemistry, based on titanocene(III)-catalyzed 7-endo-dig and 7-endo-trig Cyclizations. This procedure has proved to be useful for the chemical preparation of terpenoids with different skeletons containing cycloheptane rings, including the first total syntheses of dauca-4(11),8-diene (2), barekoxide (3), authentic laukarlaol (81), and a valparane diterpenoid (72), as well as a substantially improved synthesis of karahanaenone (1). We also provide theoretical and experimental evidence in support of a plausible mechanism, which may rationalize the preference for the unusual 7-endo Cyclization mode shown by radicals with substitution patterns characteristic of the linalyl, nerolidyl, and geranyl linalyl systems. In light of these chemical findings, we discuss the potential involvement of radical Cyclizations in the biosynthesis of some terpenoids containing seven-membered carbocycles.

  • Reaction of Enol Ethers with Alkynes Catalyzed by Transition Metals: 5exo‐dig versus 6endo‐dig Cyclizations via Cyclopropyl Platinum or Gold Carbene Complexes
    Chemistry: A European Journal, 2003
    Co-Authors: Cristina Nevado, Diego J Cardenas, Antonio M Echavarren
    Abstract:

    The intramolecular reaction of enol ethers with alkynes in methanol is catalyzed by electrophilic Pt I I , Pd I I , and Au I I I chlorides and by a Cu 1 complex to give five- or six-membered rings bearing dimethyl acetals. The reaction takes place by an anti addition of the enol ether and the metal to the alkyne. The possible involvement of vinylidene complexes in this reaction is excluded. In addition to the usual 5-exo-dig (or 6-exo-dig) pathways, a 6-endo-dig pathway has also been found to take place with certain enynes. One case of 5-endo-dig Cyclization has also been found. A general scheme for the alkoxyCyclization of enynes catalyzed by transition metals based on DFT calculation of PtCl 2 and AuCl 3 complexes that includes exo and endo Cyclizations is presented.

  • Reaction of enol ethers with alkynes catalyzed by transition metals: 5-exo-dig versus 6-endo-dig Cyclizations via cyclopropyl platinum or gold carbene complexes
    Chemistry - A European Journal, 2003
    Co-Authors: Cristina Nevado, Diego J Cardenas, Antonio M Echavarren
    Abstract:

    The intramolecular reaction of enol ethers with alkynes in methanol is catalyzed by electrophilic Pt(II), Pd(II), and Au(III) chlorides and by a Cu(I) complex to give five- or six-membered rings bearing dimethyl acetals. The reaction takes place by an anti addition of the enol ether and the metal to the alkyne. The possible involvement of vinylidene complexes in this reaction is excluded. In addition to the usual 5-exo-dig (or 6-exo-dig) pathways, a 6-endo-dig pathway has also been found to take place with certain enynes. One case of 5-endo-dig Cyclization has also been found. A general scheme for the alkoxyCyclization of enynes catalyzed by transition metals based on DFT calculation of PtCl(2) and AuCl(3) complexes that includes exo and endo Cyclizations is presented.

David J Procter - One of the best experts on this subject based on the ideXlab platform.

  • Enantioselective Cyclizations and Cyclization cascades of samarium ketyl radicals
    Nature Chemistry, 2017
    Co-Authors: Nicolas Kern, Mateusz P Plesniak, Joseph J W Mcdouall, David J Procter
    Abstract:

    The rapid generation of molecular complexity from simple starting materials is a key challenge in synthesis. Enantioselective radical Cyclization cascades have the potential to deliver complex, densely packed, polycyclic architectures, with control of three-dimensional shape, in one step. Unfortunately, carrying out reactions with radicals in an enantiocontrolled fashion remains challenging due to their high reactivity. This is particularly the case for reactions of radicals generated using the classical reagent, SmI_2. Here, we demonstrate that enantioselective SmI_2-mediated radical Cyclizations and cascades that exploit a simple, recyclable chiral ligand can convert symmetrical ketoesters to complex carbocyclic products bearing multiple stereocentres with high enantio- and diastereocontrol. A computational study has been used to probe the origin of the enantioselectivity. Our studies suggest that many processes that rely on SmI_2 can be rendered enantioselective by the design of suitable ligands. Although samarium-mediated Cyclizations have the potential to generate significant molecular complexity, historically it has not proven possible to exert enantiocontrol through the use of a chiral ligand in complex product synthesis. Now, an enantioselective SmI_2-mediated radical Cyclization has been developed using a chiral aminodiol ligand. Desymmetrizing 5- exo ketyl-alkene Cyclizations and Cyclization cascades of unsaturated ketoesters deliver complex products and typically proceed with high enantioselectivity and diastereoselectivity.

  • enantioselective Cyclizations and Cyclization cascades of samarium ketyl radicals
    Nature Chemistry, 2017
    Co-Authors: Nicolas Kern, Mateusz P Plesniak, Joseph J W Mcdouall, David J Procter
    Abstract:

    Although samarium-mediated Cyclizations have the potential to generate significant molecular complexity, historically it has not proven possible to exert enantiocontrol through the use of a chiral ligand in complex product synthesis. Now, an enantioselective SmI2-mediated radical Cyclization has been developed using a chiral aminodiol ligand. Desymmetrizing 5-exo ketyl-alkene Cyclizations and Cyclization cascades of unsaturated ketoesters deliver complex products and typically proceed with high enantioselectivity and diastereoselectivity.

Jose Justicia - One of the best experts on this subject based on the ideXlab platform.

  • titanocene iii catalyzed 6 exo versus 7 endo Cyclizations of epoxypolyprenes efficient control and synthesis of versatile terpenic building blocks
    Chemistry: A European Journal, 2013
    Co-Authors: Jose Justicia, Diego J Cardenas, Tania Jimenez, Delia Miguel, Rafael Contrerasmontoya, Rachid Chahboun, Enrique Alvarezmanzaneda, Daniel Colladosanz, Juan M Cuerva
    Abstract:

    In this article, a complete study on the selectivity of titanocene(III) Cyclization of epoxypolyprenes is presented. The requirements for the formation of six- or seven-membered rings during these Cyclizations are determined, taking into account the different substitution pattern in the epoxypolyprene precursor. Thus, a complete selectivity to 6-exo or 7-endo Cyclization process has been achieved, yielding mono-, bi-, and even tricyclic compounds, constituting a new and efficient access to this type of derivative. Additionally, this procedure opens the possibility to prepare excellent building blocks for the synthesis of polycyclic compounds with a trisubstituted oxygenated function, which is present in several natural terpenes.

  • 7 endo radical Cyclizations catalyzed by titanocene iii straightforward synthesis of terpenoids with seven membered carbocycles
    Journal of the American Chemical Society, 2005
    Co-Authors: Jose Justicia, Juan M Cuerva, Juan L Ollerlopez, Araceli G Campana, Enrique J Oltra, Elena Bunuel, Diego J Cardenas
    Abstract:

    We describe a novel procedure for the straightforward synthesis of seven-membered carbocycles via free-radical chemistry, based on titanocene(III)-catalyzed 7-endo-dig and 7-endo-trig Cyclizations. This procedure has proved to be useful for the chemical preparation of terpenoids with different skeletons containing cycloheptane rings, including the first total syntheses of dauca-4(11),8-diene (2), barekoxide (3), authentic laukarlaol (81), and a valparane diterpenoid (72), as well as a substantially improved synthesis of karahanaenone (1). We also provide theoretical and experimental evidence in support of a plausible mechanism, which may rationalize the preference for the unusual 7-endo Cyclization mode shown by radicals with substitution patterns characteristic of the linalyl, nerolidyl, and geranyl linalyl systems. In light of these chemical findings, we discuss the potential involvement of radical Cyclizations in the biosynthesis of some terpenoids containing seven-membered carbocycles.

Christine Beemelmanns - One of the best experts on this subject based on the ideXlab platform.

  • towards the core structure of strychnos alkaloids using samarium diiodide induced reactions of indole derivatives
    Chemistry: A European Journal, 2013
    Co-Authors: Christine Beemelmanns, Steffen Gross, Hans-ulrich Reissig
    Abstract:

    This report describes the development of a first and second generation approach towards the synthesis of the ABCEG pentacyclic core structure of Strychnos alkaloids. First, we discuss a sequential approach applying a series of functional group transformations to prepare suitable precursors for Cyclization reactions. These include attempts of samarium diiodide-induced Cyclizations or a Barbier-type reaction of a transient lithium organyl, which successfully led to a tetracyclic key building block earlier used for the synthesis of strychnine. Secondly, we account our first steps towards the development of an atom-economical samarium diiodide-induced cascade reaction using “dimeric” indolyl ketones as Cyclization precursors. In this context, we discuss plausible mechanisms for the samarium diiodide-induced cascade reaction as well as transformations of the obtained tetracyclic dihydroindoline derivatives.

  • New samarium diiodide-induced Cyclizations
    Pure and Applied Chemistry, 2011
    Co-Authors: Christine Beemelmanns, Hans-ulrich Reissig
    Abstract:

    Samarium diiodide (SmI 2 ) smoothly promotes the Cyclizations of suitably substi- tuted carbonyl compounds with styrene subunits leading to benzannulated cyclooctenes. The intramolecular samarium ketyl addition to arene or hetarene moieties enables a new, efficient, and highly stereoselective entry to dearomatized products such as hexahydronaphthalenes, steroid-like tetra- or pentacyclic compounds, or dihydroindole derivatives. The usefulness of the developed SmI 2 -induced Cyclization method was demonstrated by the shortest formal total synthesis of the alkaloid strychnine.