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  • 1
    Electronic Resource
    Electronic Resource
    Synthesis of Hetaryl Glycosides and Their Glycosyl Donor Properties (1998)
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 1353-1360 
    Details
    ISSN: 1434-193X
    Keywords: Heterocycles ; Substitution ; Carbohydrates ; Anomeric O-hetarylation ; Glycosylation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Anomeric O-hetarylation of tetra-O-benzyl- and tetra-O-acetylglucose (1a, b) can be directly performed with electron-deficient heteroaromatic/heterocyclic systems 2-14, which contain imide halide moieties. The reactions were carried out in the presence of a base and led, through an exchange of the halide by the glucopyranosyloxy moiety, to the products 2a-14a, 7b-14b. Predominantly or exclusively β-products were obtained. Systems bearing more than one imide halide moiety, such as cyanuric fluoride (15) or 5-chloro-2,4,6-trifluoropyrimidine (16), can be employed for successive anomeric O-hetarylations. Investigation of the glycosyl donor properties of O-glucosyl heteroaromatic imidates with 6-O- and 4-O-unprotected glucose derivatives 18 and 19 as acceptors and comparison of the results obtained with data for the corresponding β-trichloroacetimidates 17aβ and 17bβ, reveals that 2,3,5,6-tetrafluoropyridin-4-yl glucopyranosides 14aβ and 14bβ exhibit similar properties. For specific tasks, for instance α-glucopyranoside formation, 14aβ may even be advantageous.
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  • 2
    Electronic Resource
    Electronic Resource
    Structural motifs of the dimeric lewis glycolipids as determined by NMR spectroscopy and molecular dynamics simulations (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 981-988 
    Details
    ISSN: 0947-6539
    Keywords: computer simulations ; conformations ; Lewis glycolipids ; molecular dynamics ; thioglycosides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several monomeric and dimeric Lewis glycolipids have been investigated by NMR spectroscopy, and structural aspects were modelled by computer. From the pseudo-C2-symmetric tetrasaccharide unit that forms the recognition domain of the Lewis Y and Lewis b antigens, a totally C2-symmetric tetrasaccharide was designed that contains the structural element common to all Lewis antigens. Finally, a model for the presentation of dimeric Lewis antigens at membrane surfaces was derived. The overall shapes of the dimeric Lewis oligosaccharides are defined by the connectivity of the sugar residues within rigid tri- and tetrasaccharide building blocks.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020811
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  • 3
    Electronic Resource
    Electronic Resource
    Asymmetric Control in the Pictet-Spengler Reaction by Means of N-Protected Amino Acids as Chiral Auxiliary Groups (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1566-1571 
    Details
    ISSN: 0947-6539
    Keywords: amino acids ; asymmetric syntheses ; carbolines ; chiral auxiliaries ; Pictet-Spengler reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Aromatic and aliphatic Schiff bases of tryptamine react with Fmoc- or phthaloyl-protected amino acid chlorides to form N-acyliminium intermediates, which, in the presence of titanium alkoxides at room temperature, undergo Pictet-Spengler reactions to give tetrahydro-β-carbolines with diastereomeric ratios of up to 99:1. The chiral auxiliary can be removed from the Pictet-Spengler adducts by means of a simple reduction. To rationalize the observed stereoselectivity a transition-state model is proposed in which the titanium atom coordinates both the carbonyl group of the N-acyliminium ion and the amino acid protecting group.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960021215
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  • 4
    Electronic Resource
    Electronic Resource
    Synthesis, Structure and Hydrolysis Studies of Dimethyltris(Trimethylsilyl)Methylmetallanes of Aluminium and GalliumDedicated to Professor Manfred Weidenbruch on the occasion of his 60th birthday (1997)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 3 (1997), S. 1783-1792 
    Details
    ISSN: 0947-6539
    Keywords: aluminium ; gallium ; hydrolyses ; Si ligands ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reactions of [(Me3Si)3CLi2thf] with Me2MCl (M = Al, Ga) afford the mixed trialkylmetallanes [(Me3Si)3-CAlMe2·thf] (1) and [(Me3Si)3 CGaMe2·thf] (2) in high yields. The coordinated THF molecule of compound 2 can be removed by sublimation in vacuo to yield the solvent-free product [(Me3Si)3-CGaMe2] (3). Hydrolysis of compound 2 with one equivalent of water at 0°C gives the trimeric hydroxide [{(Me3Si)3CGaMe(μ-OH)}3] (4), while the reaction with two equivalents of water at room temperature yields the unusually stable gallium hydroxide-water complex [{(Me3Si)3CGaMe(OH)(μ-OH)MeGaC(SiMe3)3) H2O·2thf] (6). On heating, compound 6 is converted to the hydroxide [{(Me3Si)3C}4Ga4(μ-O)2(μ -OH)4] (5), which has a heteroadamantane-like core. The hydrolysis of compound 1 with one equivalent of water at - 25°C gives the dimeric hydroxide [{(Me3Si)3CAlMe(μ-OH)}2·2thf] (7), while the reaction with two equivalents of water results in the formation of the novel hydroxide [{(Me3Si)3C}4Al4(μ-O)2 (μ-OH)4] (8), which is isostructural to the gallium compound 6 with the adamantane-like structure. The molecular structures of compounds 1, 2, 4, 5·3THF, 6, 7 and 8·0.5 THF have been determined by X-ray structure analysis. Compound 7 is the first structurally characterised aluminium hydroxide containing methyl groups, and 8 is the smallest structurally characterised galloxane hydroxide described in literature.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19970031109
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  • 5
    Electronic Resource
    Electronic Resource
    C-Disaccharides of Ketoses (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 502-510 
    Details
    ISSN: 0947-6539
    Keywords: alkynes ; C-glycosides ; cobalt complexes ; cyclizations ; enzyme inhibitors ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of gluconolactone 2 with allylmagnesium bromide at low temperatures afforded ketopyranose 3, which could easily be converted into open-chain ketoses (R)-6 and (S)-6. Their reaction with lithioacetylide 9 afforded propargylic alcohol derivatives (R)-10 and (S)-10, which could not be cyclized directly to the desired C-ketosides. They were converted by standard procedures into (R)-14 and (S)-14 and then into dicobalthexacarbonyl complexes (R)-16 and (S)-16. A facile acid-catalyzed ring closure gave the desired C-ketosides (R)-18 α/β and (S)-18α/β, respectively, in different ratios. In order to demonstrate that removal of the protective groups and hydrogenation of the CC triple bond proceed smoothly, (R)-18 α was transformed into the deprotected target molecule (R)-1 α. For the assignment of the new chiral centers at C-2/2′ and at C-8, (S)-18α was transformed into azido derivative (S)-22α, which underwent intramolecular cycloaddition to afford the spiro derivative (S)-25α. Because of the conformational constraints in this molecule, unequivocal configurational assignment was possible with the help of NMR data.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19960020508
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  • 6
    Electronic Resource
    Electronic Resource
    Photoreactions of phenyl-substituted N-(pent-4-enyl-1-oxy)pyridine-2(1 H)-thiones (1996)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 2 (1996), S. 1014-1023 
    Details
    ISSN: 0947-6539
    Keywords: alkoxy radicals ; cyclizations ; pyridinethiones ; radicals ; tetrahydrofurans ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of hitherto unknown N-(pent-4-enyl-1-oxy)pyridine-2(1 H)-thiones (6) were prepared from substituted pent-4-enyl tosylates or benzylic chlorides. On irradiation with incandescent light heterocycles 6 liberated alkoxy radicals 2, which were studied for rearrangement reactions. Surprisingly, all transformations involving the 1-phenylpent-4-enyl-1-oxy radical (2a), for example, to give the substituted thioether 8, 2-bro-momethyl-5-phenyltetrahydrofuran (11), or the tetrahydrofuran 14a, were not stereoselective. On the other hand 2-, 3- mono-, and 1,5-disubstituted pent-4-enyl-1-oxy radicals 2d-e and 2g cyclized in good yields and with good to excellent stereoselectivities to give the corresponding 2,4-cis- and 2,3-trans-phenyltetra-hydrofurfuryl radicals 3d-e, and the trans-2-benzyl-5-methyl substituted intermediate 3g. The major reaction mode of the 4-phenylpent-4-enyl-1-oxy radical (2f) was the 6-endo cyclization, which afforded 3-phenyltetrahydropyran (13f) as the major product (endo:exo = 93:7) after trapping with hydrogen donors. According to the experimental data of the present study, the unusual reactivity of the 1-phenylpent-4-enyl-1-oxy radical (2a) in 5-exo-trig ring closures could be caused by a coplanar arrangement of the benzyloxy moiety in the transition state of the cyclization. This interaction would lock the radical center in 2a in a preferred conformation, which would result in similar steric effects for both cis- and trans- cyclizations.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.1460020816
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  • 7
    Electronic Resource
    Electronic Resource
    The Aluminum-Nitrogen Bond in Monomeric Bis(amino)alanes: A Systematic Experimental Study of Bis(tetramethylpiperidino)alanes and Quantum Mechanical Calculations on the Model System (H2N)2AlY (1998)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 1095-1114 
    Details
    ISSN: 1434-1948
    Keywords: Aminoalanes ; Alanes ; Ab initio calculations ; 27Al-NMR spectroscopy ; Structure elucidation ; Substituent effects ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reactions of metallated nucleophiles MIY [Y = OR, SR, NR2, PR2, AsR2, CR3, Si(SiMe3)3, R = organyl, H] with bis(2,2,6,6-tetramethylpiperidino)aluminum halides [tmp2AlX, (X = Cl, Br, I)] offer facile access to a variety of bis(amino)alanes of the type tmp2AlY. As indicated by 27Al-NMR spectroscopy, mass spectrometry and X-ray crystal structure determina-tions, all of these compounds are monomeric in the solid state, in solution, and in the gas phase. Even Al-E single bonds (E = Si, P, As etc.) that are not commonly encountered are stabilized by the supporting tmp fragments. The results of a systematic analysis of the bonding parameters determined for the tmp2AlY compounds, combined with a quantum mechanical study on model compounds (H2N)2AlY, not only reveal the presence of a highly polar Al-N bond, but at the same time rule out AlN-pp(π) bonding, in contrast to the situation in the analogous tmp2BY compounds. It is shown that the Al-N bond length depends on the acidic character of the protic species HY: the shorter d(Al-N), the less basic is Y.
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  • 8
    Electronic Resource
    Electronic Resource
    η5-Pentamethylcyclopentadienyliridium(III) and -rhodium(III) Labeling of Amino Acids with Aromatic Side Chains - The Importance of Relativistic Effects for the Stability of Cp*Irııı Sandwich Complexes (1998)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 1991-1998 
    Details
    ISSN: 1434-1948
    Keywords: Iridium ; Rhodium ; Amino acids ; Sandwich complexes ; Bioorganometallic chemistry ; Relativistic effects ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: η5-Pentamethylcyclopentadienyliridium(III) and -rhodium(III) sandwich complexes of the type [(η5-Cp*)M(η6-aa)](CF3SO3)2 (M = Ir, Rh; 3-14) containing L-tyrosine, L-tryptophan and L-phenylalanine derivatives (aa) can be prepared by treatment of [(η5-Cp*)ML3] (CF3SO3)2 [L = thf, (CH3)2CO, CH3CN] with the appropriate bioligand in thf for N-protected compounds and in CF3COOH for α-amino acids with unprotected amino groups. Coordination to the Cp*MIII fragments stabilizes the ketonic form of the tyrosine aromatic side chains, leading to a marked enhancement in the acidity of the p-hydroxy function. The crystal structure of [Cp*Ir(ActyrOMe)] (CF3SO3)2 (3b, ActyrOMe = N-acetyltyrosine methyl ester) confirms a marked distortion towards an η5-oxohexadienyl coordination mode as may be gauged from the tilting of the p-OH plane C13/C14/C15 by no less than θ = 12.9° from that of the remaining ring atoms. Facial isomers are present in an effective 1:1 ratio for all tryptophan derivatives. Whereas the Cp*IrIII sandwich complexes of aromatic α-amino acids are stable in polar solvents, rapid decay is observed for analogous Cp*RhIII complexes of N-unprotected derivatives in polar solvents. Comparative nonrelativistic and relativistic all-electron density functional calculations on the cationic sandwich complexes [Cp*M(η6-C6H5Me)]n+ (n = 2, M = Ir, Rh; n = 1, M = Ru) confirm that all three metals bind more tightly to Cp* than to toluene as gauged by the respective force constants (k1 〉 k2). A much larger relativistic enhancement of k2 for M = Ir (279 vs 207 Nm-1) could be responsible for the greater stability of Cp*IrIII complexes in solution.
    Additional Material: 7 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Reactions of Dimethyl[tris(trimethylsilyl)methyl]metalanes of Aluminum and Gallium with H2S and Elemental Chalcogens - Crystal Structures of [RAl(μ-S)]2·2 THF, [RGa(μ3-S)]4, [{RAl(μ3-S)}3MeAl(μ3-S)], [RAlMe(μ-SeMe)]2, and [RGaMe(μ-TeMe)]2 [R = C(SiMe3)3] (1998)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 2033-2039 
    Details
    ISSN: 1434-1948
    Keywords: Aluminum ; Chalcogens ; Cubanes ; Gallium ; Main group elements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of the trialkylalane (Me3Si)3CAlMe2 × THF (1) with H2S in toluene at room temperature results in the formation of the dimeric sulfido compound [(Me3Si)3CAl(μ-S)]2 × 2 THF (3), while the reaction of the homologous gallane (Me3Si)3CGaMe2 (2) with H2S under similar conditions affords the heterocubane [(Me3Si)3CGa(μ3-S)]4 (4). Heating of compound 3 in vacuo gives a mixture of the symmetrically substituted [(Me3Si)3CAl(μ3-S)]4 (5) and the unsymmetrically substituted heterocubane [{(Me3Si)3CAl(μ3-S)}3MeAl(μ3-S)] (6). Reaction of the trialkylmetalanes 1 and 2 with elemental selenium in refluxing toluene results in the formation of the corresponding selenolates [(Me3Si)3CMMe(μ-SeMe)]2 (7, M = Al; 8, M = Ga). The compounds exhibit a high thermal stability and can not be converted to the corresponding selenides [(Me3Si)3CM(μ3-Se)]n (M = Al, Ga) upon heating. In contrast, the trialkylalane 1 does not react with tellurium metal in refluxing toluene at all, while the trialkylgallane 2 affords the tellurolate [(Me3Si)3CGaMe(μ-TeMe)]2 (9) under similar conditions. Compounds 3-9 have been characterized by 1H- and 29Si-NMR, mass, and IR spectroscopy. Furthermore, the molecular structures of compounds 3· C6H6, 4· 1.5 C6H6, 6, 7· C6H6 and 9 have been determined by X-ray crystallography.
    Additional Material: 5 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    Diastereoselektive sequentielle Umsetzung von Benzothiopyrylium-Salzen zu anellierten BenzothiopyranonenDiese Arbeit wurde vom Fonds der Chemischen Industrie und von der Universität Göttingen gefördert. H. G. dankt dem Fonds der Chemischen Industrie für ein Promotionsstipendium. Wir danken Professor Tietze für seine großzügige Unterstützung und sein stetes Interesse.Professor Wolfgang Lüttke zum 75. Geburtstag gewidmet (1995)
    Weinheim : Wiley-Blackwell
    Zeitschrift für die chemische Industrie 107 (1995), S. 705-707 
    Details
    ISSN: 0044-8249
    Keywords: Anellierungen ; Kaskadenreaktionen ; Schwefelverbindungen ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/ange.19951070608
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