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  • General Chemistry  (46)
  • [2 + 1] Cycloaddition  (4)
  • Regioselectivity  (3)
  • Transition structures  (3)
  • Vinylcyclopropanes, donor-acceptor-substituted  (3)
  • Alkylation  (2)
  • Amino alcohols  (2)
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  • 1
    ISSN: 0947-3440
    Keywords: Vinylcyclopropanes, donor-acceptor-substituted ; Cyclopentenes ; Rearrangement ; 1,3-Zwitterions ; Solvent effect ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Donor-acceptor-substituted vinylcyclopropanes such as 1a, 3a, 5a, 5b, 7a, 9a, 9b, and 12a smoothly undergo thermal ring enlargement to functionalized cyclopentene derivatives 2, 4, 6, 8, 10, and 13 at relatively low temperatures (150-190°C) in good yields. This vinylcyclopropane-cyclopentene rearrangement proceeds with low (1a, 3a), moderate (9a, b) or high (7a, 12a) stereoselectivity. In the case of 7a and 12a almost complete retention of configuration with respect to the migrating (methoxy) (phenyl)-substituted carbon atom was observed. The definite mechanistic interpretation of these results is complicated by the stereoisomerization of the starting donor-acceptor-substituted vinylcyclopropanes which proceeds as competing reaction during ring enlargement. Nevertheless, all observations together, in particular the recorded solvent effects, support a stepwise mechanism of the rearrangement with a highly stabilized 1,3-zwitterionic species as common intermediate for cyclopropane stereoisomerization and ring enlargement.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0570-0833
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    ISSN: 0570-0833
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 0947-6539
    Keywords: asymmetric catalysis ; cycloadditions ; cyclopropanecarboxylates ; diazoacetates ; silyl enol ethers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The scope and limitations of asymmetric syntheses of 2-siloxycyclopropanecarboxylates 3 were examined by combination of structurally typical silyl enol ethers and methyl diazoacetate with various chiral catalysts. It was found that the Schiff base complex 5ṁCu(OAc)2 and the bisoxazoline complex 6ṁCuOTf gave the highest stereoselectivities (e.g., 72% ee for cis-3d with 5ṁCu(OAc)2 as catalyst or 76% and 73% ee for cis- and trans-3a, respectively, with 6-CuOTf as catalyst). High enantioselectivities or diastereoselectivities were obtained. Other catalysts based on copper, rhodium or ruthenium complexes afforded significantly lower values. In further investigations, Evans's bisoxazoline complex 6-CuOTf proved to be limited to 1,1-disubstituted silyl enol ethers (e.g. 1a and 1e) for high enantiomeric excesses (up to 〉 95%, poor diastereoselectivities) and to 1,2-substituted enol ethers (e.g. 1e) for high diastereoselectivities (cis:trans 〈 3:97, eetrans = 49%).
    Additional Material: 4 Ill.
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  • 5
    ISSN: 1434-193X
    Keywords: Large ring compounds ; Intramolecular Michael addition ; Cesium fluoride ; 2-Siloxy-cyclopropanecarboxylates ; Alkylations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Starting from siloxycyclopropyl-substituted dimethyl malonates 1 and 2 a chain elongation was performed to give new malonates 8, 9, 10, and 18 in reasonable overall yield. Further elongation by five carbon atoms transforms 10 into 15. Precursor 21 was prepared by alkylation of cyclopropanecarboxylate 20 with dibromide 17 and subsequent treatment with sodium dimethyl malonate. Compounds 8, 9, 10, 15, 18, and 21 were subjected to cesium fluoride under high dilution which induced a ring-opening/ring-closure cascade reaction giving cyclopentadecanone derivatives 11, 12, 13, cycloeicosanone derivative 16, cyclotetradecanone derivative 19 and cyclononane derivative 22 (together with 23). The efficiency of this reaction including an intramolecular Michael addition is discussed. The sequence allows efficient synthesis of medium and large carbocycles by a highly flexible building block system involving in principle unlimited repetition of construction steps. An X-ray analysis of 19 reveals its crown-like conformation which is slightly distorted to a more planar skeleton compared with a ten-membered ring analog 24.
    Additional Material: 1 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2373-2381 
    ISSN: 1434-193X
    Keywords: 4,4′-Bipyridine ; NMR spectroscopy ; Pyridinophanes ; X-ray crystal structure determination ; Macrocycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Syntheses of nine novel phanes derived from 4,4′-bipyridine incorporating new spacer elements are described together with their structural and spectroscopic properties. Starting from the previously unknown pyridine-bridged bis(4,4′-bipyridinium) dication 2 and various bis(bromomethyl)arenes, macrocycles 3-7 were prepared in a simple heterogeneous liquid-liquid reaction in satisfactory yields. In complementary experiments starting from bis(4,4′-bipyridinium) compounds 12-14 with substituted xylenes as spacers, ring closure was performed with 2,6-bis(bromomethyl)pyridine (1) to furnish compounds 16-18 in low yield. Temperature-dependent NMR-spectroscopic studies of the macrocycles were performed to investigate their conformational behaviour. The X-ray crystal structures for phanes 3,4 and 6 illustrate the preferred anti conformation of the macrocycles in the solid state. Consecutive reactions at the pyridine bridge were not successful; however, anisolophane 7 was converted into nitroanisolophane 11 by simple nitration of the anisolo spacer in excellent yield.
    Additional Material: 3 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1995 (1995), S. 667-671 
    ISSN: 0947-3440
    Keywords: 1,2-Oxazines, bis(trimethylsilyl)amino-substituted ; Hydrogenolysis ; Deprotonation ; Alkylation ; Deuteration ; Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1,2-Oxazine 1 was converted into an unusual condensation product 5 by treatment with tetra-n-butylammonium fluoride. The hydrogenolysis of 1 with Pd/C as catalyst provided the expected primary amine 9, whereas the same reaction with the ethoxycarbonyl-substituted 1,2-oxazine 2 as starting material gave the 4-methylproline derivative 10 after N-protection. Deprotonation at C-4 of 1 required rather harsh conditions but cleanly afforded the corresponding lithiated intermediate 11. Treatment of 11 with electrophiles provided the C-4-substituted derivatives 12-16 in good to moderate yield, but generally with very high diastereoselectivity. The overall substitution process preferentially occurs with retention of configuration which is explained by assuming ion pair structure B for intermediate 11.
    Additional Material: 4 Tab.
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  • 8
    ISSN: 1434-193X
    Keywords: Intramolecular Diels-Alder reaction ; 1,7,9-Decatrien-3-ones ; α-Eudesmol ; Lewis acids ; Chelates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Trienones 1-3 were subjected to Lewis acid-promoted intramolecular Diels-Alder reactions. It was shown that with monocoordinating Lewis acids the endo selectivity of the cycloaddition was generally high. The preference for either of the two possible endo products cis-a and cis-b, however, was shown to be highly dependent on the nature of the Lewis acid, and on the substitution pattern of the trienone substrates. Lewis acids with two coordination sites furnished predominantly the exo product trans-b via seven-membered ring chelate intermediates. The chelate-controlled intramolecular Diels-Alder reaction was then utilised as the key step in a stereocontrolled synthesis of the sesquiterpene α-eudesmol. This reaction mode, performed on a model trienone, also paved the way for the synthesis of the pharmaceutically important natural product dihydromevinolin.
    Additional Material: 6 Tab.
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  • 9
    ISSN: 1434-193X
    Keywords: Natural products ; Intramolecular Diels-Alder reactions ; 2-Siloxycyclopropanecarboxylates ; 1,7,9-Decatrien-3-ones ; Octalones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Starting from enones 7 and 8 siloxydienes 9 and 12 were synthesized. Cyclopropanation and subsequent alkylation with sorbyl bromide provided tetrasubstituted siloxycyclopropanes 15 and 16 in good overall efficiency. Their ring cleavage with fluoride reagents gave 1,7,9-decatrienones 5 and 17; the latter was converted into the protected compound 6. The thermal intramolecular Diels-Alder reactions of 5 and 6 were compared with those of related trienones 3 and 4. Octalones 1, 2, 21, and 22 were formed as mixtures of three or four diastereomers. Thus, for the stereoselective construction of natural products such as α-eudesmol or dihydromevinolin alternative reaction conditions for the cycloaddition step have to be developed.
    Additional Material: 5 Tab.
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  • 10
    ISSN: 1434-193X
    Keywords: Rearrangements ; Density functional calculations ; Substituent effects ; Transition structures ; Small-ring compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Two possible types of competing [1,3] rearrangements of divinylcyclopropane derivatives and of the hetero-analogous compounds vinylcyclopropanecarbaldehyde and vinylcyclopropanecarbthioaldehyde to five-membered ring compounds have been investigated theoretically by (U)DFT (Becke3LYP/6-31G*) methodology. Although both of the rearrangement pathways correspond to a reaction of the monosubstituted cyclopropane substructure, i.e. a reaction of vinylcyclopropane or of cyclopropanecarb(thio)aldehyde, new features in the reactivity are seen due to the combination of two substituents. Reaction parameters such as reaction and activation energies of the [1,3]-sigmatropic rearrangements have been calculated and compared. Substituent effects of hydroxy and formyl (donor and acceptor) groups in specific positions have also been considered. On the basis of these calculations, the [1,3] rearrangement of vinylcyclopropanecarbaldehydes to dihydrofurans can be expected to be kinetically favored over their conversion to cyclopentenes, while formation of the latter should be favored thermodynamically. This prediction is borne out by the experimental findings. Depending on the substitution pattern, the [1,3]-sigmatropic rearrangement of vinylcyclopropanecarbaldehydes may compete with the [3,3]-sigmatropic rearrangement. For the other investigated structures, the [1,3] rearrangement is strongly disfavored.Supporting information for this article is available on the WWW under -http://www.wileY-Vch.de/contents/jc_2046/1999/98456_s.pdf or from the author.
    Additional Material: 9 Ill.
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