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  • 1
    Electronic Resource
    Electronic Resource
    Hetero-Diels-Alder cycloadditions of α,β-unsaturated acyl cyanides. Part 2Part 1: [5].. Reactions with N,N-dimethyluracils, a new route to 5-substituted uracil derivatives (1993)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 76 (1993), S. 1916-1927 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The [4 + 2] cycloadditions of 2-oxobut-3-enenitrile (1a), 2-oxopent-3-enenitrile (1b), and ethyl 4-cyano-4-oxobut-2-enoate (1c) with 1,3-dimethyluracil (2), 1,3, 6-trimethyluracil (9), or 1,3,5-trimethyluracil (16) were investigated. The reactions of 1a with 2 or with 9 lead to bicyclic adducts 3 and 10, respectively. These hexahydro-cis-pyranopyrimidines undergo ring opening under acidic conditions, restoring in 4 and 11, respectively, an uracil system comprising 2-hydroxybut-2-enenitrile as a side chain at C(5). The surprisingly stable enols tautomerize slowly to the corresponding acyl cyanides 6a and 13a, respectively. Reacting 1b or 1c with 2 and with 9 does not afford cycloadducts; instead the uracil derivatives 6b, c and 13b, c, respectively, show up, carrying at C(5) α-oxobutanenitrile side chains. Cleavage of the acyl cyanide functions in 6a-c and 13a-c with nucleophilic agents produces various acids, esters, or amides, i.e. derivatives 8a-c and 15-c, respectively. The methyl esters 8a (X = MeO, R = H) and 15a (X = MeO, R = H) are also formed directly from the adducts 3 and 10, respectively, with acid or base catalysis in presence of MeOH. The cycloadducts 17a and 17c, resulting from the reaction of 1a and 1c with 16, respectively, have a Me group at the ring junction C(4a) and are stable. The structure of 17c proves that this hetero-Diels-Alder addition of inverse electron demand follows the endo-mode.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19930760510
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  • 2
    Electronic Resource
    Electronic Resource
    Hetero-Diels-Alder Additions of α,β-Unsaturated-Acyl Cyanides. Part 3Part 1: [7], part 2: [8].. Syntheses of 3-bromo-2-ethoxy-3,4-dihydro-2H-pyran-6-carbonitriles, and about their transformation to 2-ethoxy-2H-pyrans (1995)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 78 (1995), S. 151-164 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cycloadditions of the α,β-unsaturated-acyl cyanides 1-3 with (Z)-or (E)-1-bromo-2-ethoxyethene (4) may be performed at moderate temperatures and provide in good yields the 3-bromo-2-ethoxy-3,4-dihydro-2H-pyran-6-carbonitriles 5-7, respectively (Scheme 1). Diastereoisomeric pairs of products result at room temperature merely from the ‘endo’- and ‘exo’-transition states; more complex mixtures appear above 60° as a consequence of (Z)/(E)-isomerization of 4. The relative stability of the anomers of 5 and 6 is explored by treatment with BF3·Et2O. Acid alcoholysis (MeOH or EtOH) of 5 leads to acetals 9a, b of 4-bromo-5-oxopentanoate. Alkyl (2Z,4E)-5-ethoxypenta-2,4-dienoates 12, 17, and 20, are formed in alcoholic alkoxide solutions from 5, 6, and 7, respectively, which is compatible with the intermediacy of 2-alkoxy-2H-pyrans and their valence tautomers, α,β-unsaturatedacyl cyanides. Methoxide addition to the CN group competes with dehydrobromination in case of 5; it leads to 3-bromo-3,4-dihydro-2H-pyran-6-carboximidate 13 (ca. 50% at -20°) which can be hydrolyzed to the methyl carboxylate 14. DBU (1,8-diazabicyclo[5,4,0]undec-7-ene) in benzene converts 5 to 6-ethoxy-2-oxohexa-3,5-dienenitrile (11), the ring-opening product of an obviously unstable 2-ethoxy-2H-pyran; the same reagent dehydrobrominates 6 to 2-ethoxy-4-methyl-2H-pyran-6-carbonitrile (15). HBr Elimination from 7 takes place with great ease in presence of pyridine, or even during chromatography on alumina, and leads to the stable ethyl 6-cyano-2-ethoxy-2H-pyran-4-carboxylate (18); this dimerizes at room temperature to give a 1:3 mixture of tricyclic adducts ‘endo’-21 and ‘exo’-21. The structure of the latter is established by an X-ray crystallographic analysis.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19950780115
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  • 3
    Electronic Resource
    Electronic Resource
    NMR of Terminal Oxygen. Part 14Part 13: [1].. Kinetically stabilized simple enols containing methylated uracil groups: Application of a 17O-NMR test of H-bonding (1994)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 77 (1994), S. 317-322 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the crystalline N,N′-dimethylated uracil derivatives 2a, b, the kinetically stabilized enol group forms an H-bond with O—C(4), as demonstrated by increased shielding of specifically labelled 2a and 2b in the 17O-NMR spectra (Δδ(17O)(C(4) - O) = -30 ppm); absence of dilution and solvent effects show that the H-bridge is intra-molecular, forming an eight-membered chelate ring. The (apparent) shielding effect Δδ(17O) in 2a, b is larger than that in salicylamide. The strong H-bond explains why the enols 2, in spite of the absence of steric hindrance, are kinetically stabilized.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19940770131
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  • 4
    Electronic Resource
    Electronic Resource
    NMR of Enaminones. Part 7Part 6: [l]. The general name enaminone is used for compounds containing the O=C—C=C—N moiety. 1H-, 13C-, and 17O-NMR and X-ray structure determinations of 1,2-disubstituted conjugated 3-[(tert-Butyl)amino]enones (1997)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 80 (1997), S. 2137-2147 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1H-, 13C-, and 17O-NMR spectra for the 2-substituted enaminones MeC(O)C(Me)=CHNH(t-Bu) (1), EtC(O)C(Me)=CHNH(t-Bu) (2), PhC(O)C(Me)=CHNH(t-Bu) (3), and MeC(O)C(Me)=CHNH(t-Bu) (4) are reported. These data show that 3 exists mainly in the (E)-form, 4 in (Z)-form, and 1 and 2 as mixtures of both forms. Polar solvents favour the (E)-form. The (Z)- and (E)-forms exist in the 1,2-syn,3,N-anti and 1,2-anti,1,N-anti conformations A and B, respectively. The structures of the (E)- and (Z)-form are confirmed by X-ray crystal-structure determinations of 3 and 4. The shielding of the carbonyl O-atom in the 17O-NMR spectrum by intramolecular H-bonding (ΔλHB) ranging from -28 to -41 ppm, depends on the substituents at C(l) and C(2). Crystals of 3 at 90 K are monoclinic. with a = 9.618(2) Å, b = 15.792(3) Å, c = 16.705(3) Å, and β = 94.44(3)°, and the space group is P21/c with Z = 8 (refinement to R = 0.0701 on 3387 independent reflections). Crystals of 4 at 101 K are monoclinic, with a = 16.625(8) Å, b = 8.637(6) Å, c = 11.024(7) Å, and β = 101.60(5)°, and the space group is Cc with Z = 4 (refinement to R = 0.0595 on 2106 independent reflections).
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19970800713
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