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  • General Chemistry  (77,716)
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  • 1
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 17-31 
    ISSN: 1434-193X
    Keywords: Carbene complexes ; Chromium ; Tungsten ; Hexatrienes ; Azahexatrienes ; Transmetalation ; Alkynes ; Polyenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The term 1-metalla-1,3,5-hexatriene applies to a huge number of transition metal carbene complexes which contain a 1,3-butadien-1-yl side chain attached to the carbene carbon atom. Compounds of this type were found to play a crucial role as key intermediates and have also been utilized as stoichiometric reagents in a number of high-yielding transformations, which may successfully be applied to organic syntheses. Typical reaction pathways, structural features, and preparations of such compounds (with focus on M = Cr and W) have been summarized.
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  • 2
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 61-71 
    ISSN: 1434-193X
    Keywords: Binaphthyls ; Chirality ; Circular dichroism ; Dendrimers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The absorption and CD spectra of a series of Fréchet (compounds 7-10) and backfolding (compounds 11-12) dendrimers derived from enantiopure (S)-2,2′-dihydroxy-1,1′-binaphthalene have been recorded (THF) in the range 200-350 nm. All the compounds examined show a positive couplet between 200 and 240 nm (1B transition of the 2-naphthol chromophore), the intensity of which (Δεmax of the low-energy branch) ranges between 100 and 40. By means of the DeVoe polarizability model the intensity of the 1B couplet has been calculated vs. the dihedral angle θ. This analysis provides θ angles of 95-110° for the Fréchet dendrimers and 100-110° for the backfolding compounds. These values clearly indicate that the torsional angle θ, defined by the two naphthalene planes, never exceeds the critical value of 110°. This investigation confirms the wide utility of CD spectroscopy to provide geometrical information that cannot be obtained by other types of structural analysis.
    Additional Material: 5 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 51-60 
    ISSN: 1434-193X
    Keywords: Dithioles ; Anthracenes ; Lithiation ; Electron donors ; Crystal structures ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The synthesis of new derivatives of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene has been achieved by two different routes. Deprotonation of 8 using LDA in THF at -78 °C, followed by in situ quenching of the lithiated intermediate 9 with N,N-dimethylformamide, N-methyl isothiocyanate and methyl chloroformate gave aldehyde, thioamide and methyl ester derivatives 10-12, respectively. Sulfur insertion into the lithiated species 9 followed by reaction of the transient thiolate anion with benzoyl chloride gave the thioester derivative 13 which served as a convenient shelf-stable precursor of other mono-functionalised derivatives of 8. Debenzoylation of 13 and trapping of the transient thiolate anion with iodomethane and 6-bromohexan-1-ol yielded 14 and 15, respectively. Reaction of cation salt 17 with the anion of anthrone 18 gave compound 20, the thiolate anion of which reacted with 6-bromohexan-1-ol to afford the alcohol derivative 21. Subsequent reactions gave alcohol derivative 25 of the title system. The unexpected product 29 was obtained from reaction of 28 with triethyl phosphite. The X-ray crystal structures of compounds 12, 14, 28, and 29 are reported. The molecules adopt a saddle-like conformation; the bis(1,3-dithiole)benzoquinone system is U-shaped through an ‘accumulating bend’ comprising the boat conformation of the central (quinonoid) ring, folding of both 1,3-dithiole rings along S···S vectors, and out-of-plane tilting of the exocyclic C=C bonds, all in the same (inward) direction.
    Additional Material: 3 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 99-104 
    ISSN: 1434-193X
    Keywords: Allyl alcohols ; Organocerium reagents ; Alkenes ; Addition reactions ; Cerium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Alkylcerium reagents add to the multiple bonds of allyl and propargyl alcohols in good yields and under mild conditions. The double bond can be reduced with lithium aluminum hydride in the presence of cerium trichloride. The regiochemistry of the attack depends on electronic factors.
    Additional Material: 1 Tab.
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  • 5
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1-15 
    ISSN: 1434-193X
    Keywords: Amino acids ; Peptides ; Stereoselectivity ; Chirality ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---While geminally disubstituted α-amino acids are helix-inducing residues in α-peptides, gem-disubstituted β-amino acids are predicted not to fit into any of the three major secondary structures of β-peptides recognized to date [the 314 helix, the 12/10/12 helix, and the pleated sheet (Figure 1)]. In order to be able to synthesize and structurally identify β-peptides containing such building blocks, or consisting entirely of them, and in order to establish the chirality of secondary structures they may form, achiral and chiral gem-disubstituted β-amino acids must be readily available. The methods of preparation of 3-amino carboxylic acids with two carbon substituents at the 2- or 3-position (β2,2-/β3,3-amino acids, Figure 2) are reviewed. While there are numerous essentially classical routes to achiral and rac-β-amino acids of this type (Schemes 1-4), their EPC synthesis is currently the subject of investigations. These include the nucleophilic addition to (R)- or (S)-N-sulfinimines (Schemes 6-10) and other Mannich-type transformations (Schemes 19-22), stereoselective alkylations of various chiral hydropyrimidines (Schemes 11, 12, 18), of esters or amides of 2-cyano-alkanoic acids (Schemes 13, 14, 16), and of Li2 derivatives of non-racemic N-protected 3-amino-alkanoates (Scheme 17), as well as sequences of reactions involving enantiopure gem-disubstituted succinic acid derivatives and a Curtius degradation (Schemes 23-26). Oligomers of the achiral gem-disubstituted compounds 1-(aminomethyl)-cyclopropane and -cyclohexane carboxylic acid have already been shown to form 8- and 10-membered hydrogen-bonded rings, respectively (Figure 5), which provide novel motifs for the possible construction of turns, links, or steps in β-peptidic chains.
    Additional Material: 5 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 125-132 
    ISSN: 1434-193X
    Keywords: Aldol condensations ; Carbohydrates ; C-C coupling ; Isomerases ; Controlled ozonolysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A short enzymatic route for the synthesis of L-fucose analogs modified at the nonpolar terminus is reported. In particular, fucose derivatives bearing extended linear (1b) and branched (1e) saturated, or various unsaturated (1c, 1d) aliphatic chains have been prepared, in order to increase hydrophobic contacts. The rather general approach involves a sequential application of the recombinant enzymes L-fuculose 1-phosphate aldolase (FucA) and L-fucose ketol isomerase (FucI) from E.coli. Enantiomerically pure L-fucose analogs have been prepared in up to 30% overall yield starting from the appropriate hydroxyaldehyde precursors and dihydroxyacetone phosphate as readily available components. Unsaturated 2-hydroxyaldehydes have been efficiently prepared by alk(en/yn)yl Grignard addition to cinnamaldehyde followed by controlled ozonolysis of the styrene fragment.
    Additional Material: 1 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 149-153 
    ISSN: 1434-193X
    Keywords: Gabosine ; Secondary metabolite ; Carbohydrates ; Chemical screening ; Biomolecular-chemical screening ; DNA binding ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---In addition to the known gabosines A (4), B (5) and C (6), three new gabosines L (1), N (2) and O (3) were detected by chemical screening as secondary metabolites of Streptomyces (strains GT 041230, GT 051024 and S 1096). The constitutions of 1, 2 and 3 were established by spectroscopic techniques and derivatization reactions. The absolute stereochemistry of 1 and 2 was determined by Helmchen's method and has been verified in the case of gabosine N (2) by X-ray analysis. The DNA-binding properties of the gabosines were investigated and analyzed by binding studies using a recently developed thin-layer chromatography technique (bimolecular-chemical screening).
    Additional Material: 1 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 141-148 
    ISSN: 1434-193X
    Keywords: Fluorine ; Cryptands ; Macrocycles ; Macrocyclic ligands ; Cyclophanes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Novel fluorine-containing macrocyclic molecules have been synthesized in order to clarify the interaction or coordination ability of the C-F unit towards metal ions. The cage compounds 1 and 2 were prepared by direct coupling reactions between the appropriate diamines and dibromides, while bond isomers of the cage compounds were synthesized via fluorinated diaza[3.3]metacyclophanes. Complex formation with alkali metal cations, NH4+, and Ag+ ions has been assessed by picrate extraction experiments. Comparison of the cation affinities of hosts 1, 2, and 4, shows that the spatial arrangement of the fluorine atoms strongly affects the donor ability of the host molecules. The hexafluoro cage compound 1, with six fluorine atoms in an octahedral geometry, exhibits relatively strong coordination ability towards K+, NH4+, and Ag+ ions, while compound 2, with four fluorine atoms in a structure similar to that of 1 shows only poor affinity for these ions. Compound 4, which has six fluorine atoms arranged in a quasi planar fashion, was found to show weak affinity towards NH4+ and Ag+ ions. Thus, octahedrally arranged fluorine atoms evidently provide the best fit for spherical cations. Compound 1 shows characteristic 1H-, 13C-, and 19F-NMR-spectral changes upon complexation. The crystal structure of 1 has been elucidated and compared to that of the K+ complex. The C-F bonds are found to be slightly elongated in the K+ complex, which is clearly indicative of coordination of the fluorine atom to K+.Supporting information for this article is available on the WWW under //http://www.wiley-vch.de/contents/jc_2046/2000/99243_s.pdf or from the author.
    Additional Material: 4 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 603-609 
    ISSN: 1434-193X
    Keywords: Benzopyranones ; (E)-2-Nonenal ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The bright orange-red color of the toadstool Tricholoma aurantium is due to the benzotropolone pigment aurantricholone (1). The compound is at least partially present as the calcium salt. Minor pigments are the yellow 2H-furo[3,2-b]benzopyran-2-one derivatives aurantricholides A (7a) and B (7b), which exhibit strong green fluorescences. Their structures have been established by total syntheses.
    Additional Material: 3 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 269-274 
    ISSN: 1434-193X
    Keywords: Cage compounds ; Calixarenes ; Cavitands ; Carcerands ; Gold ; Interfaces ; Monolayers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The synthesis of sulfide-functionalized (hemi)carceplexes suitable for self-assembly on gold is described. A noncentrosymmetrical carceplex 9 (guest DMF) was synthesized by the combination of calix[4]arene 4 and resorcin[4]arene 3, while a noncentrosymmetrical hemicarceplex 12 (guest NMP) was prepared by the combination of two different resorcin[4]arenes, 3 and 10. These adsorbates form ordered self-assembled monolayers on gold with their quasi C4v axis perpendicular to the surface. The two positions possible for the incarcerated guest lead to diastereotopic surfaces.
    Additional Material: 1 Ill.
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  • 11
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 257-267 
    ISSN: 1434-193X
    Keywords: Allyl azides ; Vinyl azides ; Azirines ; Rearrangements ; Photochemical cis-trans isomerizations ; Rearrangements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A reinvestigation of the reaction between 2,3-diphenyl-2H-azirine (1) and phenyldiazomethane (2) has shown that a literature report has to be corrected since no vinyl azide 4 but rather the allylic compound 3-azido-1,2,3-triphenyl-1-propene (3) is produced. This stable substance, which can also be prepared by substitution reactions of allylic bromide (E)-10 or from alcohol (E)-11, may be separated into its geometrical isomers (E)-3 and (Z)-3, although these equilibrate through rapid [3,3] sigmatropic migration of the azido group. Attempts to synthesize 4 by dehydration of azido alcohols 7 using methanesulfonyl chloride and sulfur dioxide or by elimination of hydrogen chloride from azides 15 led only to 3 and 2-benzyl-2,3-diphenyl-2H-azirine (14). This heterocycle, which can also be prepared by Neber rearrangement, has been found to be the thermal and photochemical decomposition product of the unstable vinyl azides 4. However, dehydrations of 7 using thionyl chloride at low temperature have led to the first isolation of 1-azido-1,2,3-triphenyl-1-propenes (4). Starting with 3 and various other allylic azides, rearrangement reactions involving sigmatropic shift of the azido group or photochemical cis-trans isomerization have been investigated, as have base-catalyzed (prototropic) rearrangements to give vinyl azides.
    Additional Material: 1 Tab.
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  • 12
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 291-295 
    ISSN: 1434-193X
    Keywords: Diarrhetic Shellfish Poisoning ; NMR spectroscopy ; Yessotoxin ; Natural products ; Carboxyyessotoxin ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A new analogue of yessotoxin (YTX), carboxyYTX, 3, was isolated from the digestive glands of mussels cultured on the Italian Adriatic coast. Its structure was determined by MS and NMR spectroscopy.
    Additional Material: 3 Ill.
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  • 13
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 281-289 
    ISSN: 1434-193X
    Keywords: Phosphorus acid amphiphiles ; (α-Aminoalkyl)phosphonic acids ; Carboxyalkyl (α-aminoalkyl)phosphonic acid monoesters ; Spiro compounds ; Pudovik reaction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The addition reaction between the P-H bond of tetraoxyspirophosphoranes 1-2 and long-chain imines 3a-h (decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, and oleyl imines) occurs instantaneously at room temperature. It is diastereoselective, and quantitatively leads to the corresponding (α-aminoalkyl)spirophosphoranes 4a-h and 5e. The influence of the pentacoordinated phosphorus atom on the stereoselectivity of the Pudovik reaction might be attributed to the involvement of the rigid spirophosphoranide (PV) intermediate in the addition reaction. Selective and one-pot hydrolysis of these P-C bond spirophosphoranes readily proceeds either at room temperature in the presence of moist solvents to give the corresponding carboxyalkyl (α-aminoalkyl)phosphonic acid monoesters 6a-h and 7e, or the reaction may be carried out in the presence of 20% aqueous hydrochloric acid under reflux, to afford the free (α-aminoalkyl)phosphonic acid amphiphiles 8a-h in high yields. In contrast to their sodium salts, these single- and double-chained free and monoester phosphonic acid amphiphiles exist as zwitterions and are not soluble in water.
    Additional Material: 2 Tab.
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  • 14
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 617-626 
    ISSN: 1434-193X
    Keywords: Radicals ; Photochemistry ; Aliphatic ethers ; Hydrogen abstraction ; Urazolyl radical ; α-Alkoxy-alkyl radical ; Diones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The photoaddition of 4-methyl-1,2,4-triazoline-3,5-dione (4-MTAD) with a wide variety of acyclic, cyclic and crown aliphatic ethers has been investigated. Monochromatic (λ = 514.5 nm) or polychromatic (λ ≥ 310 nm) irradiations give identical mono-urazolyl ethers as reaction products. Unsymmetrical acyclic ethers afford a mixture of the two α and α′ mono-urazolyl ethers. In the case of 12-crown-4, mono and di-substituted products are obtained. ESR experiments and quantum calculations at the AM1 and 6-31G* levels were performed and a possible reaction mechanism is proposed in which the most probable photochemical process is the H-abstraction leading to a urazolyl radical.Supporting information for this article is available on the WWW under //http://www.wiley-vch.de/contents/jc_2046/2000/099269_s.pdf or from the author.
    Additional Material: 3 Ill.
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  • 15
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 645-655 
    ISSN: 1434-193X
    Keywords: INOC ; Pyrrolidines ; Piperidines ; Stereoselective synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Intramolecular nitrile oxide olefin cycloaddition (INOC) reactions of oximes 1-3 and of 24-27 derived from L-amino acids have been found to proceed stereoselectively, yielding tricyclic fused pyrrolidines and piperidines. Further manipulation led to chiral hydroxymethyl-substituted fused piperidines 33-35 and to 3-amino-4-(1-hydroxypropyl)-2-mercaptomethyl-N-methylpiperidine (36). The structures and stereochemistries of the fused systems, as well as those of the piperidines, have been established by NMR.
    Additional Material: 3 Ill.
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  • 16
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 807-811 
    ISSN: 1434-193X
    Keywords: Proline analogues ; Asymmetric synthesis ; Schöllkopf's method ; Silicon ; Amino acids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The asymmetric synthesis of a new proline surrogate, incorporating the dimethylsilyl group at position 4 of proline using Schöllkopf's bis-lactim ether method, is described.
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  • 17
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 837-840 
    ISSN: 1434-193X
    Keywords: Chiral auxiliaries ; Analytical methods ; Through-space interactions ; Lactic acid ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The enantiomeric purity of a chiral compound is determined by NMR spectroscopic analysis of the diastereomers formed with a chiral derivatizing agent. Fluorinated O-aryllactic acids (FAC) 1 are efficient chiral reporters, whose spectacular remote anisotropic effects allow an easy identification and measurement of diastereomers. The remote effects are attributed to the particular design of FAC esters relative to other CDAs.
    Additional Material: 4 Ill.
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  • 18
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 857-859 
    ISSN: 1434-193X
    Keywords: α,α-Dialkylated amino acids ; Isotopic labeling ; Strecker synthesis ; Solvent effects ; Stereoselective hydrolysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---[15N]-D-isovaline was prepared from DL-[α-15N]-α-aminoisovaleramide by enzymatic resolution with Mycobacterium neoaurum. The 15N-isotope was introduced during the Strecker synthesis of its precursor, e.g. aminoisovaleronitrile. Attempts to prepare the amino nitrile precursor of [15N]-α-aminoisobutyric acid (Aib) led to a poor yield and loss of the label. Significantly, improved results were obtained when a cosolvent is present during formation of aminoisobutyronitrile.
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  • 19
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 861-866 
    ISSN: 1434-193X
    Keywords: DNA ; Dihydroxyacetone monophosphate ; Aldolase ; Carbon-13 ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A synthesis of [3′,4′-13C2]thymidine (1) is described in which [13C2]acetic acid (2) is converted into the nucleoside in twelve steps with 9% overall yield. D-2-Deoxyribose-5-phosphate aldolase (DERA, EC 4.1.2.4) and triosephosphate isomerase (TPI, EC 5.3.1.1) are used for the stereocontrolled formation of D-[3,4-13C2]-2-deoxyribose-5-phosphate (8) from [2,3-13C2]dihydroxyacetone monophosphate (DHAP, 7) and acetaldehyde in 80% yield. The route permits the introduction of isotopically enriched carbon atoms at any position or combination of positions in the furanose ring and the product can be coupled with any of the four naturally occurring base moieties.
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  • 20
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 867-867 
    ISSN: 1434-193X
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---No Abstract.
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  • 21
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 873-877 
    ISSN: 1434-193X
    Keywords: Spiro compounds ; Ring-closing metathesis ; Pauson-Khand reaction ; Carbohydrates ; Enynes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The synthesis of the ketoglycosidic enynes 5, 7 and 8 starting from 2,3,4,6-tetra-O-benzyl-D-glucopyranolactone (2) is described. These enynes are subjected to ruthenium-mediated ring-closing metathesis and Pauson-Khand cyclisation to afford the highly functionalised carbohydrate spiroacetals 9 and 11-14.
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  • 22
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 849-855 
    ISSN: 1434-193X
    Keywords: Marine alkaloids ; Heterocycles ; Natural products ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Ascididemine (9H-quino[4,3,2-de][1,10]phenanthrolin-9-one) (1) and an isomer (9H-quino[4,3,2-de][1,7]phenanthrolin-9-one) (4) have been synthesized starting from 1,4-dimethoxyacridone (7). The acridone was converted into 1,4-dimethoxy-9-ethynylacridine (11) by a triflate coupling. The ethynylacridine was converted in one-pot into 3H-6-methoxypyrido[2,3,4-kl]acridine (15) by reaction with sodium diformylamide; the mechanism of this key transformation is discussed. Conversion into 6H-4-bromopyrido[2,3,4-kl]acridin-6-one (19) and 6H-pyrido[2,3,4-kl]acridin-6-one (17), followed by reaction of each of these under high pressure conditions with acrolein N,N-dimethylhydrazone, gave ascididemine and its isomer, respectively.
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  • 23
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 879-892 
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Addition reactions ; Sulfones ; Alkylations ; Amines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The aza-Michael addition of enantiopure 1-aminopyrrolidines to (E)-alkenyl sulfones in the presence of a catalytic amount of ytterbium trifluoromethanesulfonate [Yb(OTf)3] yields β-hydrazino sulfones in moderate to good yields and with diastereoselectivities of up to ≥ 98%. The latter undergo reductive N-N bond cleavage with BH3 · THF and, after N-protection with Boc2O or benzyl bromide, afford N-protected β-amino sulfones with moderate to high enantiomeric excesses (ee = 42 to ≥96%) without racemization. Subsequent α-alkylation of the N,N-dibenzyl protected β-amino sulfones with various electrophiles yields α-alkyl-β-amino sulfones in excellent yields (88-97%) with high diastereomeric (de ≥96 to ≥98%) and enantiomeric purity (ee = 94 to ≥96%). The absolute configuration of the new stereogenic centre was determined by X-ray structural analysis and confirmed by NMR spectroscopy (NOE experiments). Possible reaction mechanisms for the conjugate addition and α-alkylation are presented.
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  • 24
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1007-1012 
    ISSN: 1434-193X
    Keywords: β-Blockers ; Protecting groups ; Deprotection ; Carbamates ; Oxazolidinone ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The iodinatied carbamates 3 (E + Z), with potential β-blocking properties, were synthesized. The first route chosen, from 4-aminophenol and the chloroformate β7, had to be abandoned because of the formation of the oxazolidinone 10 during the epoxidation step. The aminoalcohol 17 prepared from the practolol 1 finally gave the target compounds by condensation with the iodoallylic chloroformates 8 (E + Z). The secondary Boc-protected amine function was regenerated without removing the carbamate function situated in the p-postion, by using mild reaction conditions (1 N HCl).
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  • 25
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1061-1063 
    ISSN: 1434-193X
    Keywords: Photochemistry ; Matrix isolation ; Density functional calculations ; Isomerizations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Ultraviolet irradiation of matrix-isolated formamide (1) in solid argon with light of wavelength 248 nm leads to the formation of formimidic acid (2). Comparison of the experimental IR spectrum of the photoproduct with the calculated IR spectrum of 2 shows clearly that two rotamers 2a and 2b are formed.Supporting information for this article is available on the WWW under //http://www.wiley-vch.de/contents/jc_2046/2000/99501_s.pdf or from the author.
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  • 26
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1051-1059 
    ISSN: 1434-193X
    Keywords: Chirality ; Dendrimers ; Fullerenes ; Lipofullerenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The facile synthesis of enantiomerically pure C3-symmetrical fullerene dendrimers all-R-fA-7, all-S-fC-8, all-R-fC-9, and all-S-fA-10, as well as lipofullerenes all-S-fA-11 and all-S-fC-12 involving an octahedral [3:3] addition pattern is described. The C3 symmetrical tris[bis(4-phenyl-2-oxazoline)methano]fullerenes 1, 2, 3, and 4 with known absolute configuration were used as precursors. The completion of the octahedral addition pattern was achieved by a threefold cyclopropanation with the dendritic 3,5-dihydroxybenzylic bromomalonate 5 and the lipophilic dioctadecyl bromomalonate 6. The absolute configurations of the precursor trisadducts were retained in the inherently chiral C3-symmetrical hexakisadducts 7-12. The structure and the isomeric purity of all hexakisadducts 7-12 were unambiguously proven by NMR spectroscopy and other techniques. Preliminary experiments with the dendritic hexakisadducts all-S-fC 8 and all-S-fA 10 revealed that these macromolecular ligands posess catalytic properties with respect to stereoselective cyclopropanations of styrene with ethyl diazoacetate.
    Additional Material: 6 Ill.
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  • 27
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1065-1076 
    ISSN: 1434-193X
    Keywords: Cyclodextrins ; Inclusion compounds ; Carcinine ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A new modified β-cyclodextrin (β-CD) derivative 1 that was functionalized in position 6 with Boc-Carcinine was synthesised and its crystal structure was determined. The structure reveals a “sleeping swan”-like shape, the covalently bonded Boc-Carcinine moiety forming a folded structure with the Boc group inserted within the hydrophobic cavity of the β-cyclodextrin. The conformation of the Carcinine moiety is determined by the inclusion of the Boc group and is further stabilised by three intramolecular hydrogen bonds, two between the amide N1-H group, the carbonyl C′1=O1 group and a primary hydroxylic group of the glucose unit 5, one between the carbonyl C′0=O0 group and the primary hydroxylic group of the glucose unit 2. The β-CD macrocycle differs only slightly from unmodified β-CDs, maintaining an approximate sevenfold symmetry. The solution structure of the new β-CD derivative was investigated by NMR spectroscopy and circular dichroism (c.d.) spectroscopy. In addition to a complete (1H and 13C) assignment of the pendant Boc-Carcinine group, the NMR study allowed the assignment of all the proton resonances associated with the β-CD macrocycle. Furthermore, NMR and c.d. results indicated that the self-inclusion of the Boc group within the β-CD cavity is retained in aqueous solution. In order to estimate the strength of this self-inclusion complex a series of competition experiments with the external guest 1-adamantanol was carried out using c.d. spectroscopy.
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  • 28
    Electronic Resource
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1091-1106 
    ISSN: 1434-193X
    Keywords: Polycyclic anions ; Aromaticity ; Annulene anions ; Corannulene ; Fullerene anions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Stable π-conjugated anions are formed in the reaction of π-conjugated systems with alkali metals. Reactivity, aromatic properties and aggregation of the anions are described. Bowl-shaped and fullerene anions, as well as helicene and planar polycyclic anions, are discussed vis-à-vis their aromaticity and charge delocalization pattern.
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  • 29
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    Liebigs Annalen 2000 (2000), S. 1081-1090 
    ISSN: 1434-193X
    Keywords: Atropisomerism ; Conformation analysis ; Liquid chromatography ; Chiral resolution ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A series of bis(oxo/thioxothiazolinyl) aromatic atropisomers with two chiral C(aryl)-N(heterocycle) axes was synthesized. The separation of antiparallel, parallel diastereomers was achieved by chromatography on silica. Conformational assignment for the bis-thione atropisomers 1a,b-4a,b and bis-one atropisomers 9a,b-12a,b was based on polarimetric results in analytical chiral chromatography on microcrystalline cellulose triacetate. The relationship conformation-1H NMR chemical shifts, found for the bis-thiones and bis-ones was successfully applied for the conformational assignment of thione-one atropisomers not assigned by chiral liquid chromatography.
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  • 30
    Electronic Resource
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1077-1080 
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Phosphazene base ; Pyridyl epoxide ; Furyl epoxide ; Pyridyl alcohol ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---trans-1-(2-Pyridyl)-, trans-1-(3-pyridyl)-, trans-1(2-furyl)-, and trans-1(3-furyl)-2-phenyl epoxides with enantiomeric purities ranging from 96.8 to 99.8% [in favor of the (+, EtOH)-isomer] are obtained in two steps from pure (R,R,R)-oxathiane which is recovered (85-90%) and reused. The chiral bidentate ligand 2-phenyl-(S)-1-(2-pyridyl)ethanol with 99.6% ee was obtained in three steps and 67% overall isolated yield.
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  • 31
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1107-1112 
    ISSN: 1434-193X
    Keywords: Photochemistry ; Radicals ; EPR spectroscopy ; Quantum mechanical calculations ; Silicon compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Trisilane 7 has been synthesized as a potential photochemical precursor of tetra-tert-butylsilatetrahedrane (5) and/or tetra-tert-butylsilacyclobutadiene (6). Astonishingly, only one Si,Si bond is broken upon irradiation of 7 and the silacyclobutenyl radical 9 can be identified as the reaction product. The structure of radical 9, which in the absence of oxygen is persistent even at room temperature, has been elucidated by comparison of its experimental and calculated ESR spectra.
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  • 32
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    Liebigs Annalen 2000 (2000), S. 1113-1120 
    ISSN: 1434-193X
    Keywords: Peptidomimetics ; Reverse turn mimetics ; Glycomimetics ; Heterocycles ; Lactams ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -D-Glucurono-3,6-lactone and L-cysteine combine in a highly stereoselective manner to give the 7,5-bicyclic thiazolidinlactam 2. The α-hydroxy group of the D-glucurono-3,6-lactone was exchanged for an amino function (to give 13) and, after condensation with L-cysteine methyl ester, the polyol dipeptide 7 was obtained. Peptide couplings proceed without the need to protect the three secondary hydroxy groups of the seven-membered ring. The amino group of 7 was deprotected and selectively elongated to the pseudo-tripeptide 16. The depsipeptide 17 was obtained by condensation of Boc-Ala-OH with the polyol 2. Elongation at the carboxy terminus yielded 19 and 20. The bicyclic scaffold populates a well-defined solution conformation; the hydroxy groups mimic the side chains of hydrophilic amino acids and can be further functionalized.
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  • 33
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1121-1130 
    ISSN: 1434-193X
    Keywords: Catenanes ; Cyclophanes ; Electrochemistry ; Electronic spectroscopy ; Template-directed synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -Catenanes composed of two, three, five, or seven interlocked macrocycles have been synthesized in yields ranging from 1 to 30%. Their template-directed syntheses rely on a series of cooperative noncovalent bonding interactions between π-electron rich 1,5-dioxynaphthalene ring systems and π-electron deficient bipyridinium units which are incorporated within the macrocyclic components. The interlocked structure associated with one of the [3]catenanes was demonstrated unequivocally by single crystal X-ray analysis which also revealed the formation of polar stacks stabilized by intermolecular [π···π] interactions. The number of interlocked components of each catenane was determined by liquid secondary ion, matrix-assisted laser desorption ionization/time-of-flight, and/or electrospray mass spectrometries. The absorption spectra, emission spectra, and electrochemical properties of the macrocyclic components and of the catenanes have been investigated. Two kinds of charge-transfer absorption bands (intramolecular in the cyclophanes containing electron-donor and electron-acceptor units, intercomponent in the catenanes) have been found. Such charge-transfer excited states are responsible for the quenching of the potentially fluorescence units of the cyclophanes, and of the crown ethers in the catenanes. Charge-transfer electronic interactions are also evidenced by the electrochemical behavior. Correlations among the redox potentials of the various compounds are reported and discussed.
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  • 34
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    Liebigs Annalen 2000 (2000), S. 995-1005 
    ISSN: 1434-193X
    Keywords: Asymmetric hydroxylations ; Allyl ethers ; Carbohydrates ; Molecular modeling ; Molecular dynamics ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The catalytic asymmetric dihydroxylation of several allyl 2-O-benzyl-α-D-xylosides with AD-mix β and PYR(DHQD)2 shows almost no diastereofacial selectivity if the 3- and 4-OH groups are unprotected or acetylated. Acetal, benzyl ethers and benzoyl esters enhance the diastereoselectivity, in the opposite sense to that predicted by the “AD mnemonic”, which is completely lost using AD-mix α. In an attempt to understand this behaviour, computational studies of the asymmetric dihydroxylation (AD) of olefins using Sharpless' and Corey's catalysts have been carried out using molecular dynamics. A three-step algorithm was developed taking advantage of the enzyme-like behaviour of catalyst-olefin systems and applied using an ESFF force field. To validate our approach, the first sampling step procedure was then refined and performed using a modified CVFF force field. This led to a U-shaped model in good agreement with that proposed by Corey for the AD of allyl 4-methoxybenzoates, which brings to the fore a role for the methoxy group. This model also accounts for the observed enantioselectivity of styrene dihydroxylation. When applied to the AD of allyl xylosides using AD-mix β, our model accounts well for the observed diastereoselectivity. Both synthetic and modelling results confirmed that aromatic groups on the olefin could be involved in π-π stacking interactions with the aromatic rings of the catalyst and should be important, if not a prerequisite, to achieve high enantio- and diastereoselectivity.Supporting information for this article is available on the WWW under //http://www.wiley-vch.de/contents/jc_2046/2000/99372_s.pdf or from the author.
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  • 35
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    Liebigs Annalen 2000 (2000), S. 1735-1739 
    ISSN: 1434-193X
    Keywords: Sulfoxides ; β-Amino sulfones ; Mannich type reaction ; Lithium perchlorate ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The LiClO4-mediated one-pot reaction of aldehydes with (trimethylsilyl)dialkyl amines and the lithium salt of sulfoxides or sulfones, affords the corresponding β-(dialkylamino) sulfoxides and β-(dialkylamino) sulfones in high yields. The aminosulfoxidation reaction of aliphatic or aromatic aldehydes lacks diastereoselectivity, but the diastereomeric sulfoxides can be separated by HPLC or column chromatography for further use.
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  • 36
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    Liebigs Annalen 2000 (2000), S. 1719-1726 
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Nitrogen heterocycles ; Heterocycles ; Pyrrolidines ; Piperidines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Chiral, nonracemic 2-substituted pyrrolidines and piperidines were prepared in high ee and moderate to good chemical yields in three steps from (R)-phenylglycinol and γ- or δ-chloroketones. The key step of the synthesis was the stereoselective reductive ring-opening of chiral bicyclic 1,3-oxazolidines prepared by condensation of (R)-phenylglycinol and the corresponding ketones.
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  • 37
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    Liebigs Annalen 2000 (2000), S. 1727-1734 
    ISSN: 1434-193X
    Keywords: Self-assembly ; Vapor-pressure osmometry ; Resorcin[4]arenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Resorcin[4]arene tetracarboxylic acids 5,6 (A) and resorcin[4]arene tetrapyridines 2,3 (P) self-assemble in chloroform solution to form stable heterotopic AP dimers. Data from NMR titration and dilution experiments, as well as from vapor-pressure osmometry (VPO), indicate that the AP dimer is formed with an association constant greater than 107 M-1. Solid-solution extraction experiments are indicative of the formation of a 2:1 trimer (A2P), while self-associated homotopic species (A2 and A3) can be detected by NMR and VPO. Analysis of the heterotopic noncovalent assembly process over a range of compositions shows that these other species are much less stable than the AP heterodimer, which is the exclusive species at an A/P concentration ratio of 1:1 (〉 99.7% of the total at 10 mM).
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  • 38
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    Liebigs Annalen 2000 (2000), S. 1741-1744 
    ISSN: 1434-193X
    Keywords: Bridged pyranose derivatives ; Ring enlargement ; Carbenes ; Glycosides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Addition of dichlorocarbene to the glycal (±±)-2 followed by cyclopropyl-allyl rearrangement leads to the chloro-2H-pyran (±±)-4. Oxidation of (±±)-4 and reduction of the obtained hydroxypyranone (±±)-5 gave the methyl pyranoside (±±)-6. The relative configuration of (±±)-6 was established by X-ray structural analysis of the corresponding acetate (±±)-7. The synthesis of the optically active starting materials (+)-2 and (-)-2 is also reported.
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  • 39
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    Liebigs Annalen 2000 (2000), S. 1745-1758 
    ISSN: 1434-193X
    Keywords: CMP-Neu5Ac analogues ; Enzyme inhibitors ; Substrate analogues ; Transition state analogues ; Transferases ; Carbohydrates ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Quinic acid was transformed into phosphitamides 16, 25, and 36, which could be readily linked to 5′-O-unprotected cytidine derivative 17. Ensuing oxidation of the obtained phosphite triesters with tBuO2H and hydrogenolytic de-O-benzylation furnished the corresponding phosphate diesters 18, 26, and 38. Base catalyzed removal of acetyl protecting groups, and methyl ester hydrolysis furnished CMP-Neu5Ac analogues 1d, 1e, and 2. Quinic acid was also transformed into 1,2-unsaturated diallyl α-hydroxymethyl-phosphate derivatives (R)- and (S)-46, which on reaction with cytidine phosphitamide 47 afforded the phosphite triesters. Subsequent oxidation with tBuO2H and then treatment with NEt3 gave phosphate diester derivatives (R)- and (S)-48. Deallylation, acetyl group removal, and methyl ester hydrolysis furnished (R)- and (S)-3, respectively. Treatment of (R)- and (S)-48 with DBU as a base led to acetic acid elimination, thus yielding, after de-O-allylation, acetyl group cleavage, and ester hydrolysis, diene derivative (E)-4. Donor substrate analogues 1d and 1e exhibited good α(2-6)-sialyltransferase inhibition (Ki: 2.0·10-4 and 2.0·10-5 M). However, transition state analogues (R)-, and particularly (S)-3 showed excellent inhibition properties (Ki: 1.6·10-6 and 2.7·10-7 M).
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  • 40
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    Liebigs Annalen 2000 (2000), S. 1759-1765 
    ISSN: 1434-193X
    Keywords: Cyclitols ; Aminocyclitols ; Glycosidase inhibitors ; α-Mannosidase inhibitors ; Deoxygenation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Three deoxy derivatives 2-4 of the α-mannosidase inhibitor mannostatin A (1) were synthesized, and their inhibition of Jack bean α-mannosidase was evaluated in order to elucidate the roles of each of the three hydroxyl groups of the inhibitor. The 1- and 2-deoxy derivatives 2 and 3 retained some inhibitory activity, although reduced by a factor of about 100 relative to the parent, whereas it was completely lost with the 3-deoxy derivative 4. Structure and activity relationships are discussed in the light of these findings.
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  • 41
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    Liebigs Annalen 2000 (2000), S. 1535-1537 
    ISSN: 1434-193X
    Keywords: Catalytic hydrogenation ; Ruthenium complexes ; Sulfoxides ; Sulfoxides ; Asymmetric induction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The diastereoselective catalytic hydrogenation of β-keto sulfoxides in the presence of ruthenium complexes was studied. Optically pure β-keto sulfoxides were hydrogenated in the presence of achiral catalysts leading to moderate yields and stereoselectivities. Using chiral ruthenium catalysts such as [(R)-MeO-BIPHEPRuBr2] and [(S)-MeO-BIPHEPRuBr2], the hydrogenation proceeded in good yields with very high diastereoselectivity. Chirality at the secondary centre of the β-hydroxy sulfoxides produced was controlled by the catalysts.
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  • 42
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    Liebigs Annalen 2000 (2000), S. 1773-1783 
    ISSN: 1434-193X
    Keywords: Cycloadditions ; Carbenes ; Nitrones ; Nitrilimines ; Pyrazolines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The reaction of tert-butylalkynyl chromium Fischer carbene complex 1 with nitrones 2 affords β-enamino-ketoaldehydes 4 by the light-promoted rearrangement of the corresponding [3+2] cycloadduct carbene complexes 3. On the other hand, [3+2] cycloaddition of chiral nonracemic Fischer alkenyl carbene complexes 19 with nitrilimines 10 yields enantiomerically pure Δ2-pyrazolines with high regio- and diastereoselectivity.
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  • 43
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    Liebigs Annalen 2000 (2000), S. 1547-1562 
    ISSN: 1434-193X
    Keywords: Inositolphosphoglycans ; Oligosaccharide synthesis ; Insulin mediators ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A block synthesis approach to the inositol-containing pseudohexasaccharide 1 is presented. The myo-inositol building block 6 has been prepared using a key regioselective acylation through a boron-tin exchange reaction and the 2-azido-2-deoxy glycosyl donors 15 and 17 have been synthesized from D-glucosamine using a diazo transfer reaction. The anomeric position of the mono- and disaccharide building blocks has been temporarily protected as phenyl thioglycoside and this function was then converted into the different leaving groups to perform the glycosylation reactions. Both trichloroacetimidates and fluorides have been used as glycosyl donors for the construction of the different glycosidic linkages. The protected pseudohexasaccharides 44, 48-50, which are precursors of pseudohexasaccharide 1, have been efficiently prepared and fully characterized. Pseudohexasaccharide 1 contains the fundamental structural features which have been proposed for type A inositolphosphoglycans, which may be involved in the insulin-signaling process.
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  • 44
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    Liebigs Annalen 2000 (2000), S. 1563-1567 
    ISSN: 1434-193X
    Keywords: Triplet recombination ; Electron transfer ; Radical ions ; Photochemistry ; Terpenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The triphenypyrylium tetrafluoroborate (TPT)-sensitized reactions of several terpene donor molecules, including sabinene (1), α-phellandrene (4), α-terpinene (5) and γ-terpinene (6) give rise to significantly different products than reactions induced by other electron-transfer sensitizers, such as 1,4-dicyanobenzene (DCB). The divergent reactions require decidedly different key intermediates; the products obtained with TPT can be explained by dissociative recombination of the intermediate radical-radical cation pair in the triplet state, generating donor-derived biradicals.
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  • 45
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1569-1576 
    ISSN: 1434-193X
    Keywords: Aziridines ; Oxazolines ; Ring expansion reactions ; Amino acids ; Amino acids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A new method for the preparation of 2,2,3-trisubstituted methyl 1-benzoylaziridine-2-carboxylates is reported. These compounds have been obtained starting from α-alkyl β-amino acids by formation of the lithium dianion and reaction with iodine. The aziridines undergo ring expansion or ring opening, depending on the substituents of the aziridine ring and on the reaction conditions. Following these methods, both α-substituted α-hydroxy β-amino acids and α-substituted β-hydroxy α-amino acids have been synthesised.
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  • 46
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 1577-1587 
    ISSN: 1434-193X
    Keywords: Azides ; Cleavage reactions ; Cycloadditions ; Nitrogen heterocycles ; Polycycles ; Ring expansion ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---2-Alkyl-1-methylquinazolinium hexafluorophosphates 9 are deprotonated by sodium or potassium hydride to afford solutions of 2-alkylidenedihydroquinazolines 10, which were investigated by NMR spectroscopy. Trapping with methanesulfonyl azide (5a) of 10 in situ or subsequent treatment with trifluoromethanesulfonyl azide (5b) gives mixtures of colourless (15) and intensely yellow N-sulfonylimino-1,4-benzodiazepines 16 along with products due to cleavage of the exocyclic double bond of 10, viz. 11 and 13. The ethylidene compound 10b yields the bicyclic products 18 and 19, apparently by complex sequences of reactions that are triggered by removal of the acidic proton at C-2 of 16b and 16f. The structures of the products are based on spectroscopic evidence and X-ray diffraction analyses performed on 15b, 16d, 16e, and 19.
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  • 47
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    Liebigs Annalen 2000 (2000), S. 1589-1593 
    ISSN: 1434-193X
    Keywords: Azaallenium ions ; Azaallylium ions ; Iminium ion ; Kinetics ; Linear Free Energy Relationships ; Ab initio calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The rate constants for the reactions of the 2-azaallenium ion 1b+, the 2-azaallylium ion 2a+ and the iminium ion 3+ with different nucleophiles were determined by 1H NMR spectroscopy. By correlation with the Linear Free Enthalpy Relationship (LFER) lg k20°C = s (E + N), developed by Mayr and Patz, the electrophilicity parameters E(1b+) = -3.7, E(2a+) ≍ -16 and E(3+) = -10.43 were obtained. They show that the relative reactivities of these ions are approximately 1012:1:106. Quantum chemical calculations (ab initio, DFT) of the methyl anion affinities for the ions 1b+,2a+ and3+ are in agreement with the experimental E values. The X-ray structure of 3+·CF3SO3- is reported for the first time; it shows no strong interaction between the cation and the anion.
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  • 48
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    Liebigs Annalen 2000 (2000), S. 1595-1601 
    ISSN: 1434-193X
    Keywords: Oxazoline N-oxide ; Cycloadditions ; Cycloadditions ; Lactams ; Thienamycin ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---[3+2] Cycloaddition between a camphor-derived oxazoline N-oxide 9 and the γ,δ-unsaturated enamino ester 11 afforded the single adduct 6. A stereoselective reduction of the enamino ester side chain allowed the control of the absolute configuration of the two additional asymmetric centres. Nitrogen protection and oxidative hydrolysis of the resulting product 13, followed by further functional group manipulations, led to the β-lactam derivative 1, a known precursor of the β-methylthienamycin derivative2a.
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  • 49
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    Liebigs Annalen 2000 (2000), S. 357-364 
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Sulfoxides ; Asymmetric hydroxylations ; Lactones ; Sulfoxides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A general synthetic strategy towards the two bis(lactones) (-)-colletol (1) and (+)-colletodiol (2) is described. A common intermediate in this synthesis is the 6-membered hydroxy lactone (+)-(3R,5R)-3-hydroxy-5-hexanolide (6), readily prepared by stereoselective reduction of (+)-(SR)-methyl 3,5-dioxo-6-(p-toluenesulfinyl)hexanoate (7). Stereoselective hydroxylation of this hydroxy lactone has allowed efficient access to (+)-colletodiol (2).
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  • 50
    Electronic Resource
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 381-385 
    ISSN: 1434-193X
    Keywords: Enynes ; Macrocycles ; Coordination chemistry ; Crown compounds ; Radicals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The complex molecular structure and interesting activation mechanisms of naturally occurring enediynes have inspired the synthesis of a variety of simpler model systems to mimic their properties. While in most model compounds nucleophilic attack, isomerization or bioreduction are used to activate the unsaturated system for diradical cyclization, some attempts have been made to employ metal-ion coordination for this purpose. Significant enhancement of the thermal reactivity has been achieved by metal-ion induced conformational and electronic changes of suitably substituted enediynes, such as 1, 5, 6, 17 and 18. Enediyne activation by stoichiometric or catalytic formation of vinylidene complexes, such as 22, from terminal alkynes has also been investigated. This paper summarizes recent results pursuing the activation of enediyne diradical cyclization by metal ions.
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  • 51
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    Liebigs Annalen 2000 (2000), S. 1617-1622 
    ISSN: 1434-193X
    Keywords: Tetrakis(dimethylamino)naphthalenes ; Basicity ; Hydrogen bonds ; Cyclicvoltammetry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---For comparison to the recently described 2,3,6,7-tetrakis(dimethylamino)naphthalene (1) the three isomers 2,3, and 4 were synthesized. The basicities of this group of isomers are strongly dependent upon the different mutual orientations of the pairs of dimethylamino substituents: only the isomers 3 and, partially, 4, both with dimethylamino groups in adjacent peri-positions of the naphthalene, are strong “proton sponges”. For the isomers 1 and 2 with the same number and kind of twofold dimethylamino substituents in neighbouring ortho-positions, however, no significant basicity increase is observed. To explain this difference between the two groups of isomers it is suggested that in the ortho-pairs of 1 and 2 the C-N bonds diverge considerably, leading to an increased N···N distance and consequently to less stable [N···H···N]+ hydrogen bonds in contrast to the parallel C-N bonds in the peri-substituted isomers 3 and 4. X-ray crystal structure analyses of the bases and of some of the salts derived therefrom were solved and are discussed. Cyclic voltammetry indicates that 1 to 4 are strong electron donors, reacting easily to radical cations or dications which with suitable acids have been obtained as salts.
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  • 52
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 419-423 
    ISSN: 1434-193X
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The first enantioselective synthesis of enantiomerically pure (-)-wine lactone, (-)-1a, a fragrance constituent of various white wines, and its epimer (+)-1b, was carried out. The key steps are allylic substitution of (±)-2-cyclohexen-1-yl acetate (2) with dimethylmalonate using palladium complexes of phosphanyldihydrooxazol L1 or of the phosphanylcarboxylic acid L2 as catalyst, subsequent decarboxylation, iodolactonization and elimination, furnishing enantiomerically pure bicyclic lactone (+)-7 in 47% overall yield. The diastereoselective introduction of methyl groups by SN2′-type substitution with an organocopper compound and by enolate alkylation gave lactone (-)-1a in 43% overall yield from (+)-7.
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  • 53
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 391-399 
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Amino alcohol ; Bispidine ; Enantioselective catalysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Two generally applicable routes for the synthesis of chiral amino alcohols embodying the bispidine framework have been developed. In linear route A the bispidine framework is built up successively from chiral primary amines via intermediate formation of a piperidinone and a bispidinone. In convergent route B an achiral bispidine is formed first and then the N-substituents are introduced by reaction of the nitrogen bases with chiral electrophiles. In order to determine if the bispidine core and its N-substituents can influence the steric course of enantioselective transformations, bispidine amino alcohols built up by these two routes were investigated as chiral ligands in the enantioselectively catalyzed addition of diethylzinc to aldehydes and chalcone. In general, tridentate ligands containing one chiral amino alcohol fragment and a second amino substituent without a stereogenic center were more efficient than tetradentate ligands with two amino alcohol structural units. With the best ligands the enantioselective addition of diethylzinc to aromatic and aliphatic aldehydes proceeded with 83-98% ee and the nickel-catalyzed addition of diethylzinc to chalcone was achieved with up to 85% ee.
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  • 54
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 2000 (2000), S. 425-437 
    ISSN: 1434-193X
    Keywords: Cyclic voltammetry ; Fluorescence spectroscopy ; Mixed oligomers ; Structure-property relationships ; Organic light-emitting diodes ; Oligothiophenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Novel mixed oligoheterocycles 1-5, containing thiophene/thiazole, thiophene/1,3,4-thiadiazole, thiophene/oxazole, or thiophene/1,3,4-oxadiazole moieties, were synthesized. The introduction of electronegative heteroatoms such as oxygen and nitrogen into the conjugated π-system leads to a more pronounced acceptor character than that found in the analogous oligothiophenes. Characterization of the redox properties reveals that the reduction of the mixed oligomers is facilitated while oxidation is shifted to higher potentials. For this series, clear structure-property relationships could be found by comparing optical properties, in particular absorptions, emissions and fluorescence quantum yields in solution. The X-ray structural determination of mixed thiophene/1,3,4-oxadiazole heptamer 5 indicates that the replacement of thiophene units by 1,3,4-oxadiazoles has a strong influence on the molecular arrangement and intermolecular interactions in the solid state.
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  • 55
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    Liebigs Annalen 2000 (2000), S. 439-448 
    ISSN: 1434-193X
    Keywords: Aplykurodins ; Marine steroids ; C-C coupling ; Cyclic peroxides ; Lactonization ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---An approach to the synthesis of aplykurodins, ichthyotoxic marine lactones, is presented. The carbon framework was derived from vitamin D3 by conversion of the readily accessible allyl alcohol 13 to the protected Grundmann's hydroxy ketone 22 and subsequent introduction of the C2 side chain through a Pd0-promoted coupling. Highly stereoselective hetero Diels-Alder reaction with O21 produced the key intermediate peroxide 25. Functional group transformations, coupled with a series of chemo- and stereoselective reactions, finally resulted in the synthesis of the unnatural analogue 17,18-dihydro-3,9-di-epi-aplykurodinone B (6).
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  • 56
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    Liebigs Annalen 2000 (2000), S. 449-453 
    ISSN: 1434-193X
    Keywords: Cyclodextrins ; NMR spectroscopy ; NOE ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The conformational features of alkylated, benzoylated, and benzylated cyclodextrins in solution were analysed by NOE and proton-selective relaxation methods and were compared to those of native compounds.
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  • 57
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    Liebigs Annalen 2000 (2000), S. 455-465 
    ISSN: 1434-193X
    Keywords: Fullerenes ; Cycloadditions ; Donor-acceptor systems ; Cyclic voltammetry ; Photoinduced electron transfer ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---The new C60 and C70 adducts 1b, 1d-1k, 1m, 6d, 7d, and 8d have been synthesized by [2+3] cycloadditions of the appropriate nitrile oxides. Variations in the distance and geometry of the donor and acceptor substituents are seen to have an influence on the redox behavior of the fullerene adducts in cyclic voltammetry experiments. The isoxazolo-fullerenes 1c, 1d, and 1i show shifts of about 30 mV or 40 mV to more negative values compared with the reference compound 1a. On the other hand, strong acceptor properties are detected in the case of compound 1e, which shows a positive shift of 30 mV relative to 1a. Moreover, time-resolved fluorescence spectroscopy has shown that upon excitation of the fullerene moiety in the polar solvent benzonitrile, an electron is transferred from the donor substituent to the first excited singlet state of the fullerene, thereby reducing the excited-state lifetime. Our data demonstrate that the electron-transfer rate in donor-substituted fullerenes can be controlled by the electron-donating property of the substituent as well as the electronic structure and/or length of the spacer used. The C70 regioisomers 6d, 7d, and 8d exhibit differences in their spectroscopic characteristics.
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  • 58
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    Liebigs Annalen 2000 (2000), S. 467-472 
    ISSN: 1434-193X
    Keywords: Zeolites ; Radical ions ; Ring opening ; Electron transfer ; Oxidations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Incorporation of trans-1,2-diphenylcyclopropane (1) and its 3,3-D2-isotopomer into the channels of a redox-active pentasil zeolite (Na-ZSM-5) resulted in the formation of exo,exo-1,3-diphenylallyl radical (2•) and its 2-D1 isotopomer, respectively. This conversion requires oxidation, ring opening, and deprotonation, in an unspecified sequence. The allyl radical 2• is also formed upon incorporation of trans-1,3-diphenylpropene (3). A comparison with the solution photochemistry, in the presence or absence of added base, shows the conversion of 1 into 2• to be a zeolite-specific reaction. Incorporation of arylcyclopropanes 9 (R = H, OCH3) into ZSM-5 generated trans-propenylbenzene radical cations 10•+ (R = H, OCH3); the 2,2-D2 isotopomer of 9 (R = OCH3) gave rise to three different isotopomers of 10•+ (R = OCH3).
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  • 59
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    Liebigs Annalen 2000 (2000), S. 479-484 
    ISSN: 1434-193X
    Keywords: Conformation analysis ; NMR spectroscopy ; MM calculations ; NMR spectroscopy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---A dynamic process has been detected in the solution 13C-NMR spectra oftBu-Si(iPr)3 (4) in the temperature range -145 °C to -163 °C, the corresponding free energy of activation being 5.5 kcal mol-1. Molecular mechanics calculations have identified three types of conformations that are markedly more stable than all the others, but the one with the highest symmetry cannot be populated, as is evident from the appearance of the spectrum at low temperature. This spectrum can be interpreted in terms of restricted rotation of the tert-butyl group and of the isopropyl groups, leading to a structure centered around the (a,g,-g) conformation. In the case of the analogous (iPr)3SiCl derivative (5), this dynamic process was not observed in solution, but an appropriate spectrum for the same (a,g,-g) conformation was obtained by the solid-state CP-MAS NMR spectroscopy at -100 °C.
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  • 60
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    Liebigs Annalen 2000 (2000), S. 473-477 
    ISSN: 1434-193X
    Keywords: Zeolites ; Electron transfer ; Proton transfer ; Radical ions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: ---Incorporation of N,N′-diphenylhydrazine into HZSM-5 led to the formation of azobenzene and aniline (70-80 mol-% mass balance). The formation of azobenzene can be followed by the appearance in diffuse reflectance UV/Vis spectroscopy of a characteristic band (λmax = 420 nm). IR spectra of the extracted solids indicate that some aniline is retained in the zeolite, probably due to acid-base interaction with active sites. The products are rationalized by two competing mechanisms: the formation of azobenzene by electron transfer to an oxidizing site, the generation of aniline by proton transfer from a Brønsted acid site. A long-wavelength diffuse-reflectance band (λmax = 800 nm) in the initial stages of the reaction supports the intermediacy of a radical cation. These results show that acid zeolites can exhibit a dual behavior against basic substrates with low oxidation potential, serving simultaneously as electron acceptor and Brønsted acid.
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  • 61
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    Liebigs Annalen 2000 (2000), S. 485-490