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  • 1
    Electronic Resource
    Electronic Resource
    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
    Keywords: β-Formyl esters ; Titanium enolates ; Lithium enolates ; Silyl enol ethers ; Chelate control ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19961290218
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  • 2
    Electronic Resource
    Electronic Resource
    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
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jlac.199719970312
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    Paper (German National Licenses)
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