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  • Inorganic Chemistry  (22)
  • Silyl enol ethers  (4)
  • Pyrroles  (3)
  • Regioselectivity  (3)
  • 6H-1,2-Oxazines  (2)
  • Wiley-Blackwell  (28)
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  • Wiley-Blackwell  (28)
  • 1
    ISSN: 0947-3440
    Keywords: β-Formyl esters ; Aldol reactions ; Silyl enol ethers ; γ-Lactones ; Chelates ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0947-3440
    Keywords: Cyclopropanecarboxylates, 2-siloxy ; Silyl enol ethers ; Diazoacetates ; [2+1] Cycloadditions ; Asymmetric catalysis ; Copper ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 6 Tab.
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  • 3
    ISSN: 0170-2041
    Keywords: 6H-1,2-Oxazines ; Methoxyallene ; Nitroso alkene ; Hetero Diels-Alder reaction ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Regioselective hetero Diels-Alder reactions of in-situ generated nitroso alkenes 2-4 with methoxyallene derivatives 1 provide 4H-1,2-oxazines 5-7 with an exo-methylene group at C-5. These primary cycloadducts are smoothly transformed into conjugated 6H-1,2-oxazines 11-13 by base or acid catalysis. The bicylic nitroso alkene 17 and methoxyallene (1a) combine to give the tricyclic 1,2-oxazine 18, thus demonstrating that an exo-transition state is favoured in this [4+2] cycloaddition. The reaction of the sterically hindered methoxyallene 1h with nitrosostyrene affords the cycloadduct 22 in low yield. 22 is very likely formed by a two-step Diels-Alder reaction via a zwitterion. Allene derivatives lacking a methoxy group are not sufficiently reactive towards nitroso alkenes and do not provide 1,2-oxazines.
    Additional Material: 8 Tab.
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  • 4
    ISSN: 0170-2041
    Keywords: 1,2-Oxazines, transformations of ; Pyrroles ; Molybdenum, hexacarbonyl- ; 1,4-Dicarbonyl compounds ; Oximes ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 6 Tab.
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  • 5
    ISSN: 0170-2041
    Keywords: Pyrenophorin ; (Z)-Jasmone ; 1,2-Oxazine ; Pheromones ; Pyrroles ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
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  • 6
    ISSN: 0170-2041
    Keywords: 6H-1,2-Oxazines ; γ-Lactams ; Lewis acids ; Ring contraction ; Phosphite, trimethyl ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Unexpected Formation of γ-Lactams from 6H-1,2-Oxazines Promoted by Lewis AcidsThe reaction of 6H-1,2-oxazine 1 with trimethylsilyl cyanide in the presence of titanium tetrachloride provides α,β-unsaturated γ-lactam 3. Similarly, Lewis acid-promoted addition of trimethyl phosphite towards 1 affords a mixture of γ-lactams 8 and 9. The structure of 9 was established by X-ray analysis. A plausible mechanism for the formation of these γ-lactams by Lewis acid-induced ring contraction is proposed.
    Additional Material: 1 Ill.
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  • 7
    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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  • 8
    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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  • 9
    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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  • 10
    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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