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  • 1
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Preparation of Vinylsulfonates from Trimethylsilyl Enol Ethers  -  Synthetic Consequences of a Remarkable Cation EffectThe C/O-selectivity observed in the sulfonylation of the enolate ion 5 with benzenesulfonyl fluoride depends strongly on the nature of the gegenion. Li⊕ yields the β-oxosulfone 3 by C-sulfonylation exclusively. The fraction of O-Sulfonylation is increased with the size of the cation, yielding the vinylsulfonates 6b exclusively in the presence of Cs⊕ or quaternary ammonium ions. From this behaviour a regio- and stereoselective synthesis of vinylsulfonates is developed starting from the corresponding trimethylsilyl enol ethers.
    Notes: Die C/O-Selektivität der Sulfonylierung des Enolat-Ions 5 mit Benzolsulfonylfluorid zeigt einen starken Einfluß des Gegenions. Während mit Li⊕ ausschließlich C-Sulfonylierung zum β-Oxosulfon 3 erfolgt, wächst mit der Größe des Kations der Anteil an O-Sulfonylierung, bis mit Cs⊕ oder quartären Ammonium-Ionen ausschließlich Enolsulfonsäureester 6b entstehen. Dieses Verhalten wird für eine regio- und stereoselektive Synthese von Enolsulfonsäureestern aus den entsprechenden Trimethylsilyl-enolethern genutzt.
    Additional Material: 2 Tab.
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  • 2
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Several phenyl-substituted cyclopropane derivatives were investigated regarding rate and regiochemistry of their hydrogenolysis. The following reactivity sequence has been established: 1〈2〈4〈7〈9. Substitutent effect are discussed and compared to those reported for monoactivated cyclopropanes.
    Notes: Einige phenylsubstituierte Cyclopropane wurden hinsichtlich der Geschwindigkeit und Regiochemie ihrer Hydrogenolyse untersucht. Dabei wurde die folgende Reaktivitätsfolge ermittelt: 1〈2〈4〈7〈9. Die Substituenteneffekte werden diskutiert und mit denen bei monoaktivierten Cyclopropanen verglichen.
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  • 3
    ISSN: 0009-2940
    Keywords: Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Alkyl-substituted methyl 2-(trimethylsiloxy)cyclopropanecarboxylates 8a - 8f are opened by hydrogen in the presence of palladium on carbon affording 4-oxoalkanoci esters 10a - 10e by desilylation. 2-Phenyl-substituted cyclopropanes 8g - 8j, however, provide 4-phenylbutanoic esters 13g - 13j under these conditions. Formation of 13j is not stereoselective. Here the primary cyclopropane cleavage is followed by hydrogenolysis of the benzylic C - O bond. This subsequent reaction can be suppressed in the case of 8g by poisioning the catalyst with triethylamine; otherwise the tert-butyldimethylsiloxy compounds 11g - 11i have to be used. Butanoic esters 14h and 14i, respectively, are formed as mixtures of diastereomeres, which is another indication for nonstereoselective hydrogenolysis of cyclopropanes. Whereas slow desilylation by hydrogen was observed with the 3-phenyl-substitued cyclopropane derivative 8k, the vinylcyclopropane 8m in part suffers cleavage of the 1 - 3 bond of the three-membered ring. Regio- and stereoselectivity of the hydrogenolysis of the donor-acceptor-substituted cyclopropanes 8 and 11 are discussed.
    Notes: Die alkylsubstituierten 2-(Trimethylsiloxy)cyclopropancarbonsäureester 8a - 8f werden von Wasserstoff in Gegenwart von Palladium auf Kohlenstoff durch Desilylierung zu den γ-Oxocarbonsäureestern 10a - 10e geöffnet. Dagegen ergeben die 2-phenylsubstituierten Cyclopropane 8g - 8j unter diesen Bedingungen die 4-Phenylbutansäureester 13g - 13j. Die Bildung von 13j erfolgt stereounselektiv. Der primären Cyclopropanspaltung schließt sich hier eine Hydrogenolyse der benzylischen C - O-Bindung an. Diese Folgereaktion kann bei 8g durch Vergiftung des Katalysators mit Triethylamin, in anderen Fällen durch Einsatz der tert-Butyldimethylsiloxy-Verbindungen 11g - 11i verhindert werden. Die Butansäureester 14h bzw. 14i fallen als Diastereomerengemische an, was wiederum als Beleg für stereounselektive Cyclopropanhydrogenolyse gewertet wird. Während das 3-phenylsubstituierte Cyclopropanderivat 8k lediglich unter langsamer Desilylierung mit katalytisch erregtem Wasserstoff reagiert, findet man bei dem Vinylcyclopropan 8m auch teilweise eine Spaltung der 1 - 3-Bindung des Dreiringes. Regio- und Stereo-selektivität der Hydrogenolyse der Donor-Acceptor-substituierten Cyclopropane 8 und 11 werden diskutiert.
    Additional Material: 4 Tab.
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  • 4
    ISSN: 0009-2940
    Keywords: γ-Lactones, di- and tetrasubstituted ; Chelates, seven-membered ; Allylsilane additions, Lewis acid promoted ; Felkin-Anh model ; Aldehydes, chiral ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Ein chelatkontolliere Weg zu di-und tetasubstituierten γ-Lactonen - Stereoselektivität von Lewis-Säre-indzierten Additionen an chirale β-Formylcarbonsäure-methylesterDie chiralen β-Formylcarbonsäure-methylester 1a-1h liefern mit Allyltrimethylsilan/TiCl4 disubstituierte und tetrasubstituierte γ-Lactone 3a-3h mit mäßigen bis exzellenten Diastereoselektivitäten. Andere Lewis-Säuren sind weniger selektiv. Die Bildung von siebengliedrigen Chelaten 1 · TiCl4 wurde in einigen Fällen NMR-spektroskopisch eindeutig nachgewiesen. Trichlormethyltitan zeigt ein ähnliches Selektivitätsmuster wie die Reagentienkombination Allylsilan/TiCl4. Die Aldehyde 1a-1d, 1g und 1h werden hauptsächlich in trans-γ-Lactone umgewandelt, während 1f unter geeigneten Bedingungen im Überschuß das cis-γ-Lacton 3f liefert. Diese Ergebnisse werden unter Berücksichtigung des Felkin-Anh-Modells und der Chelatbildung diskutiert. Außerdem werden weitere Modellreaktionen mit den einfacheren chiralen Aldehyden 5 und 7 sowie mit den Acetalen 9a/b und 11a/b vorgestellt. Diese unterstreichen die Bedeutung der Chelatkontrolle bei den Additionen an die β-Formylester 1.
    Notes: On reaction with allyltrimethylsilane/TiCl4, chiral methyl β-for-mylcarboxylates 1a-1h provide disubstituted and tetrasubstituted γ-lactones 3a-3h with moderate to excellent diastereoselectivities. Other Lewis acids are less selective. The formation of seven-membered ring chelates 1 · TiCl4 has been proven unambiguously by NMR spectroscopy in several cases. Trichloromethyltitanium displays a selectivity pattern similar to the reagen combination allylsilane/TiCl4. Aldehydes 1a-1d, 1g, and 1h give primarily trans-γ-lactones, whereas under appropriate conditions 1f affords an excess of cis-γ-lactone 3f. These results are discussed with regard to the Felkin-Anh model and to chelate formation. Model studies with simpler chiral aldehydes 5 and 7 as well as with acetals 9a/b and 11a/b are presented and discussed. They emphasize the importance of chelate control in additions to β-formyl esters 1.
    Additional Material: 9 Tab.
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  • 5
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines ; Deprotonation, stereoselective ; Substitution with retention of configuration ; Ring opening, reductive ; Amino alcohols ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 6-(trimethylsilyl)methyl-substituted 1,2-oxazine 1 can smoothly be deprotonated with n-butyllithium at C-4 to give a lithiated species which reacts with a variety of electrophiles to provide the new 1,2-oxazines 5 - 16 in good yields. Besides the preparative aspect of these transformations, the high stereoselectivity of many reactions is also interesting from a mechanistic point of view. By deprotonation of the 4-deuterated compound 5a it has been proven that n-butyllithium removes exclusively the proton (or deuteron) cis to the 6-CH2SiMe3 group. Also, in most cases the reaction of lithiated 1 with electrophiles occurs with overall retention of configuration to afford preferentially cis-1,2-oxazines (series a). A mechanistic proposal for this highly stereoselective deprotonation process, which seems to be governed by the 1,2-oxazine oxygen, is discussed including a comparison with a recently reported ab initio calculation dealing with oxime ethers. Similar deprotonation/substitution reactions are described for 1,2-oxazines 14, 2, 3, and 4. Possibly due to a differing carbanion structure a deviating behavior is observed in some cases. Several acidinduced and reductive ring-opening reactions of 1, 6a, 8a, and 14a demonstrate the potential of 4-substituted 1,2-oxazines for the stereoselective synthesis of polyfunctionalized compounds.
    Additional Material: 11 Tab.
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  • 6
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines ; Hydrogenation, catalytic ; Amino alcohols ; γ-Amino acids ; Pyrroles ; γ-Lactams ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 1 Tab.
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  • 7
    ISSN: 0009-2940
    Keywords: Cyclopropanecarboxylates, 2-silyloxy, methyl ; [2 + 1] Cycloaddition ; Enantioselectivity ; Asymmetric catalysis ; Copper-Schiff base complexes, chiral ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of Optically Active Siloxycyclopropanes by Asymmetric Catalysis, I. - Influence of the Catalyst on the Cyclopropanation of (Z)-1-Phenyl-1-(trimethylsiloxy)prop-1-eneThe cyclopropanation of silyl enol ether 1a with methyl diazoacetate (2a) and diazoesters 2b-d in the presence of optically active copper salicylimine complexes 4-Cu was systematically studied. Up to 88% enantiomeric excess in products 3 were obtained by employing the appropriate reaction conditions and the optimal catalyst ligands. Thus, for the first time respectable optical yields were achieved in asymmetric cyclopropanations of silyl enol ethers with easily available copper-Schiff base catalysts. The ring opening of the separated diastereomers of 3a employing nBu4NF provided methyl γ-oxocarboxylate 6a in good optical purity. This demonstrates that this process occurs without racemization and also that 3a is formed with the same absolute configuration at C-1.
    Additional Material: 7 Tab.
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  • 8
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines ; Bromination, radical ; SN2 reaction ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 5,6-Dihydro-4H-1,2-oxazines 1a-b, 2, and 3 are easily brominated at C-4 with N-bromosuccinimide/dibenzoyl peroxide in tetrachloromethane. The bromo substituent is incorporated with surprisingly high diastereoselectivity trans to the substituent at C-6. 4-Bromo-5,6-dihydro-4H-1,2-oxazines are useful reagents for substitution reactions with N-nucleophiles such as primary amines and azide ions. Inversion of configuration at C-4 provides derivatives of 4-amino-1,2-oxazines with uniform relative configuration. As a minor byproduct the dibromo adduct 7 is obtained by bromination of 3. The dehydrohalogenation of this compound allows the synthesis of the 4-bromo-6H-1,2-oxazine 12. The presented reaction sequence thus constitutes an “umpolung” reaction that allows the introduction of nucleophiles into a position of the oxazine ring that so far was accessible only for electrophiles. The diastereoselectivity of the bromination reaction is discussed.
    Additional Material: 4 Tab.
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  • 9
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines ; Hetero Diels-Alder reactions ; Nitroso alkene cycloaddition ; exo: endo Selectivity ; Stereospecificity ; Silyl enol ethers, relative reactivity of ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The mechanism of the hetero Diels-Alder reaction of nitroso alkenes 2 with silyl enol ethers and other olefins has been investigated. Using the bicyclic nitroso compound 2a a study of the exo/endo selectivity has demonstrated that the exo approach is preferred with the siloxyethene l a as dienophile. On the other hand, the siloxycyclopentene 1 c gives a mixture of cycloadducts 3 c with an excess of endo product (endo: exo = 82: 18). The stereospecificity of the nitroso alkene cycloaddition could be demonstrated with the stereochemically homogeneous silyl enol ethers 1 b and 1 d. Experiments with enol ethers 1 f and 1 g also occur stereospecifically. α-Nitrosostyrene 2b reveals surprisingly high kE/Z values when E/Z-isomeric olefins are compared in competition experiments. Also, a detailed reactivity scale of 2b including various structurally different silyl enol ethers and other typical dienophiles shows that a complex interplay of electronic and steric effects is operating. The large influence of steric effects is taken as evidence for a highly ordered transition state in the cycloaddition. All mechanistic details for the Diels-Alder reactions of nitroso alkenes 2 with (silyl) enol ethers are in strong accord with a concerted mechanism and exclude the involvement of zwitterions or diradicals as intermediates.
    Additional Material: 7 Tab.
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  • 10
    ISSN: 0009-2940
    Keywords: 1,2-Oxazines, lithiated ; Deprotonation ; Alkylation ; Deuteration ; Regioselectivity ; Calculations, MNDO ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 5,6-Dihydro-5-methylene-4H-1,2-oxazine 1 is smoothly converted by n-butyllithium into 1-Li which reacts with electrophiles such as D2O, carbonyl compounds, dimethyl sulfide, or an azo diester to give the γ-adducts 4a-4f. On the other hand, alkylation of 1-Li occurs exclusively at C-4 of the heterocycle and provides the α-adducts 3g and 3h. These reactions require the activation of 1-Li by tetramethylethylenediamine. Treatment with allyl bromide and methyl acrylate affords mixtures of regioisomers 3 and 4. 1,2-Oxazine 5 with a conjugated C = C bond is less acidic than 1 but is also converted into 1-Li, whilst compound 6, lacking the 6-methoxy group, is not deprotonated under standard conditions. The dianion of 1,2-oxazine 7 is generated by employing an excess of n-butyllithium. This dianion displays a similar regiochemical behavior as 1-Li. Deuterium is exclusively incorporated into the γ-position to give product 8, while methylation occurs at C-4 to produce 9. 1,2-Oxazine 3g with an additional 4-methyl group can also be metalated and affords γ-adducts 10 and 11 upon reaction with D2O or acetone. Treatment with methyl iodide gives a 3:1 mixture of regioisomers 12 and 13. Deuteration of 1,2-oxazines 14 and 16 bearing a 3-CF3 or 3-CO2Et substituent requires more severe deprotonation conditions to provide the γ-adducts 15 and 17 in moderate yields. MNDO calculations of neutral 1,2-oxazines, the corresponding carbanions, and the lithium compounds allow an insight into the structure and charge distribution of these species, and also an estimation of the relative acidities. The regioselectivity of reactions of 1-Li is discussed on the basis of these semiempirical calculations and comparison with related ambident nucleophiles.
    Additional Material: 1 Ill.
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