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  • 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.
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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.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 0170-2041
    Keywords: Amino acids ; Peptides ; Cyclopropanes, silylamino- ; Condensation, fluoride-induced ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The N-silylated methyl 2-aminocyclopropanecarboxylate 1 can be incorporated into a dipeptide via a CsF-mediated condensation reaction with N-tosylated phenylalanine chloride 5. Due to its instability the corresponding free β-amino acid 4 could not be isolated up to now, and peptides containing this structural element have previously not been accessible. After desilylation, the aminomethyl-substituted cyclopropane or cyclopropene derivatives trans- 10, 12, and 13 also provided N-protected dipeptides in good yields.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 0170-2041
    Keywords: Amino acids ; GABA analogues ; Aminocyclopropanes ; Enamines, N-silylated ; [2 + 1] Cycloaddition ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An Efficient Route to GABA-Analogous Amino Acids: Cyclopropanation of N-Silylated Allylamines and EnaminesN-Silylated allylamines 1 are effectively transformed into methyl cyclopropanecarboxylates 2 by methyl diazoacetate under Rh2(OAc)4 catalysis. Derivatives 2a and 2b are smoothly converted into trans-substituted amino acids 6a and 6b, respectively, and to bicyclic γ-lactams 5a and 5b. The pharmacologically interesting γ-aminobutyric acid (GABA) analogue trans-6a is now available in few steps. Photochemical and thermal Fe(CO)5-induced hydrogen shift converts allylamine derivatives 1 into N-silylated enamines 7. While enamine (E)-7a can be cyclopropanated with methyl diazoacetate under Cu(acac)2 catalysis to afford the desired cyclopropane derivatives 8a in good yield, the other enamines are rather unreactive towards the carbenoid. Use of an optically active catalyst provides 8a with an ee of 56% (cis) and 20% (trans). Acid-induced ring cleavage of 8a gives the β-formyl ester 10a, and reduction of 8a followed by desilylation provides the aminocyclopropane 14 in good overall yield, thus demonstrating that cyclopropanes like 8a can serve as useful synthetic intermediates.
    Additional Material: 1 Tab.
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  • 5
    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.
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  • 6
    ISSN: 0009-2940
    Keywords: 2-Siloxycyclopropanecarboxylates ; Silyl enol ethers ; [2 + 1] Cycloaddition ; Asymmetric catalysis ; (Schiff base)copper 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, 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.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
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