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  • 1
    Digitale Medien
    Digitale Medien
    Synthesis of Optically Active 2-Siloxycyclopropanecarboxylates by Asymmetric Catalysis, IV. - Influence of Nucleophilicity of Silyl Enol Ethers on Stereoselectivities in Asymmetric Cyclopropanation Reactions (1997)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 521-526 
    Details
    ISSN: 0947-3440
    Schlagwort(e): Cyclopropanecarboxylates, 2-siloxy ; Silyl enol ethers ; Diazoacetates ; [2+1] Cycloadditions ; Asymmetric catalysis ; Copper ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: By variation of the aryl substituents of silyl enol ethers 1a-e and 1f-k, the influence of their nucleophilicity on the stereochemical outcome of cyclopropanation reactions was studied. Using the neutral Schiff-base catalyst 2 · Cu(OAc)2, there was only a weak effect. On the other hand, with the cationic bisoxazoline complex 3· CuOTf a remarkable increase in the enantioselectivity was observed with more strongly electron-attracting substituents such as a trifluoromethyl or nitro group.
    Zusätzliches Material: 6 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jlac.199719970312
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  • 2
    Digitale Medien
    Digitale Medien
    Mukaiyama Reactions of a Silyl Enol Ether with Heteroatom-Substituted β-Formyl EstersDedicated to Prof. Dr. Ekkehard Winterfeldt on the occasion of his 65th birthday. (1997)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1997 (1997), S. 2215-2220 
    Details
    ISSN: 0947-3440
    Schlagwort(e): β-Formyl esters ; Aldol reactions ; Silyl enol ethers ; γ-Lactones ; Chelates ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Heteroatom-substituted aldehydes 3, 4, and 5 were subjected to Mukaiyama reactions with silyl enol ether 10. The reaction of the new trifluoromethyl-substituted aldehyde 3 with 10 afforded γ-lactone 11 with excellent trans selectivity. In contrast, the known benzyloxy-substituted aldehyde 4 and 10 provided γ-lactone 13 without any selectively in the presence of BF3 as the promoting Lewis acid; use of TiCl4 induced exclusive cis selectivity. Carbamoyl-substituted aldehyde 5 and silyl enol ether 10 were combined under different conditions, although good diastereoselectivity could not be achieved. The results obtained are discussed in terms of the electronic effects of the substituents and their ability to form chelates.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jlac.199719971107
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  • 3
    Digitale Medien
    Digitale Medien
    Model studies of the reduction of 3-phenyl-6H-1,2-oxazines, chemo- and stereoselectivity: synthesis of amino alcohols, amino acids, and related compounds (1992)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 125 (1992), S. 2243-2248 
    Details
    ISSN: 0009-2940
    Schlagwort(e): 1,2-Oxazines ; Hydrogenation, catalytic ; Amino alcohols ; γ-Amino acids ; Pyrroles ; γ-Lactams ; Chemistry ; Inorganic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: While palladium-catalyzed hydrogenation of 3-phenyl-6H-1,2-oxazine 1 produces primary amine 5 in a nitrogen-transposition reaction, the reductions of the related 1,2-oxazines 2, 10, and the 1,2-oxazin-6-one 3 afford the expected amino alcohols 4, 11, and the γ-amino acid 6, respectively, with low diastereoselectivites. In the presence of acetic acid 3 is reductively converted into γ-keto carboxylic acid 9 and 1 into the γ-lactam derivative 12 probably by a ring contraction to a nitrone intermediate. Raney nickel as the catalyst is able to transform 1,2-oxazine 7 bearing an exo-methylene unit into 3,4-dihydro-2H-pyrrole 13. The reaction of 6H-1,2-oxazine 1 with aluminium amalgam produces pyrrole 14 in moderate yield. Treatment of 1 with sodium in 2-propanol brings about its transformation into pyrrolidine derivative 15 together with pyrrole 14 and amino alcohol 4 as minor products. The chemoselectivity and stereoselectivity of these reductions are discussed including mechanistic proposals for the multistep processes involved.
    Zusätzliches Material: 1 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/cber.19921251012
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  • 4
    Digitale Medien
    Digitale Medien
    Synthese optisch aktiver 2-Siloxycyclopropancarbonsäureester durch asymmetrische Katalyse, II[1]. Einfluß der Silylenolether-Struktur auf die enantioselektive Cyclopropanierung mit Diazoessigsäure-methylester (1993)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 126 (1993), S. 2727-2732 
    Details
    ISSN: 0009-2940
    Schlagwort(e): 2-Siloxycyclopropanecarboxylates ; Silyl enol ethers ; [2 + 1] Cycloaddition ; Asymmetric catalysis ; (Schiff base)copper complexes, chiral ; Chemistry ; Inorganic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Synthesis of Optically Active Siloxycyclopropanes by Asymmetric Catalysis, II[1]. - Influence of the Silyl Enol Ether Structure on the Enantioselective Cyclopropanation with Methyl DiazoacetateOn the basis of reactions of (Z)-1-phenyl-1-(trimethylsiloxy)-1-propene (1a) with methyl diazoacetate (2) in the presence of different chiral (Schiff base) copper catalysts 4-Cu we extended our study to the silyl enol ethers 1b-1j. The highest enantiomeric excesses were obtained in most cases with catalyst 4b-Cu. Whereas cyclopropanations of 1-aryl- and 1-alkenyl-substituted silyl enol ethers 1b-1h gave moderate to good enantioselectivities (up to 80%ee), alkyl-substituted olefins 1i and 1j provided less satisfactory results. By deprotonation and alkylation of 3c, 3h, and 3i it could be established that for these cyclopropanes the predominating absolute configuration of cis- and trans-cyclopropanes is equal at C-1, but consequently opposite at C-2. The influence of the silyl enol ether structure on the enantioselectivities achieved is briefly discussed.
    Zusätzliches Material: 4 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/cber.19931261223
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  • 5
    Digitale Medien
    Digitale Medien
    Chelate-Controlled Additions of Titanium and Lithium Enolates to Chiral β-Formyl Esters - Diastereofacial and Simple Diastereoselectivity (1996)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 129 (1996), S. 227-232 
    Details
    ISSN: 0009-2940
    Schlagwort(e): β-Formyl esters ; Titanium enolates ; Lithium enolates ; Silyl enol ethers ; Chelate control ; Chemistry ; Inorganic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The aldol-type additions of metal enolates 2-Met derived from pinacolone to the chiral β-formyl carboxylate 1a were optimized. The highest trans:cis ratio (86:14) of the products 3 was obtained when the trichlorotitanium enolate 2-TiCl3 was combined with 1a precomplexed with one equivalent of TiCl4. The lithium enolate 2-Li is rather unselective. The simple diastereoselectivity of prochiral enolates 4-Met was first examined with achiral β-formyl carboxylates 1b and 1c. Appropriate reagents made products with high anti or with high syn selectivity available when the unbranched aldehyde 1b was the electrophile. In contrast, the sterically more hindered aldehyde 1c provided syn products with all enolates 4-Met employed. Finally, chiral aldehyde 1a was combined with prochiral enolates 4-Met. Conditions could be found which furnished either the trans/anti or the trans/syn product 7 with good selectivity. The results are discussed and compared with reactions of related metal enolates with aldehydes capable of chelate formation.
    Zusätzliches Material: 2 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/cber.19961290218
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  • 6
    Digitale Medien
    Digitale Medien
    Transition metal- and acid-induced transformations of 6-siloxy-substituted 5,6-dihydro-4H-1,2-oxazines: Preparation of pyrroles, nitrones, 1,4-dicarbonyl compounds and derivatives thereof (1990)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1990 (1990), S. 469-474 
    Details
    ISSN: 0170-2041
    Schlagwort(e): 1,2-Oxazines, transformations of ; Pyrroles ; Molybdenum, hexacarbonyl- ; 1,4-Dicarbonyl compounds ; Oximes ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: A variety of 6-siloxy-substituted 5,6-dihydro-4H-1,2-oxazines (abbreviation: 1,2-oxazines) 1, 3 could be transformed into di- and trisubstituted pyrroles 2, 4 by means of molybdenum hexacarbonyl. The mechanism of this deoxygenating ring contraction is discussed. With two bicyclic 1,2-oxazines an acid-catalyzed fragmentation affording α-methylenecycloalkanones 7 has been observed, while other 1,2-oxazines rearrange in methanolic acid to give nitrones 9, 10. The desilylation of 6-siloxy-substituted 1,2-oxazines 1, 3 employing NEt3 · 3 HF is a very general and smooth process providing 6-hydroxy-1,2-oxazines 11, 12 or their corresponding acyclic tautomers 13, 14 in high yields. For two examples of 11 deoximations by use of formalin could be achieved with moderate efficiency giving 1,4-dicarbonyl compounds 15.
    Zusätzliches Material: 6 Tab.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jlac.199019900189
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  • 7
    Digitale Medien
    Digitale Medien
    6-Siloxy-Substituted 5,6-Dihydro-4H-1,2-oxazines as Key Building Blocks for Natural Products (1992)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1992 (1992), S. 709-714 
    Details
    ISSN: 0170-2041
    Schlagwort(e): Pyrenophorin ; (Z)-Jasmone ; 1,2-Oxazine ; Pheromones ; Pyrroles ; Chemistry ; Organic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Methyl (E)-5-bromo-4-oxo-2-pentenoate (6) and the corresponding oxime 5 were prepared in high yields by starting from methyl levulinate (3). The novel nitrosoalkene 8, generated in situ from oxime 5, smoothly added to silyl enol ether 7 to give 1,2-oxazine 9 and nitrone 10 as byproduct. Methods are described transforming intermediate 9 into unsaturated diketo ester 11, or oximes 13, 15, and 18, which may serve as precursors of pyrenophorin. Reductive ring cleavage of 1,2-oxazine 9 with Raney nickel efficiently afforded the saturated diketo ester 19. Its intramolecular aldol reaction gave cyclopentenone derivative 20, which is a known precursor of (Z)-jasmone. Cycloaddition of methyl α-nitrosoacrylate to silyl enol ether 22 provided 1,2-oxazine 23. This compound was directly converted into the ant-trail pheromone 24 by hexacarbonylmolybdenum-promoted ring contraction.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1002/jlac.1992199201119
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