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  • General Chemistry  (643)
  • Cell & Developmental Biology  (273)
  • 1995-1999  (916)
  • 1945-1949
  • 1998  (916)
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  • 1995-1999  (916)
  • 1945-1949
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  • 1
    ISSN: 1434-193X
    Keywords: Pyrrolizidine alkaloids ; Male pheromones ; Butterflies ; Chrysomelidae ; Insect defence ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Pyrrolizidine alkaloids are taken up from plants by various butterflies and moths belonging to the Danainae, Ithomiinae, and Arctiidae, which use them for defence and for the production of male pheromones. A complex distribution scheme of these plant alkaloids by the insect has been established in some species. The transformations involved include conversion into insect alkaloids, metabolism to male pheromones, and transfer of alkaloids from males to females and further into eggs. While these lepidoptera selectively sequester certain compounds from plants in a complex pattern, the leaf beetle Chrysomela lapponica follows a different strategy. Glycosidically bound alcohols present in leaves of their hostplants are liberated and transformed into esters, which are used for defence. Related beetles rely on host-derived single compounds such as salicyl aldehyde or de novo synthesized iridoid monoterpenes. Several aspects of these insect-plant relations, including biosynthetic transformations of plant compounds by the insects, are discussed herein.
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  • 2
    ISSN: 1434-193X
    Keywords: Aquilegiolide ; Glycosides ; Lactones ; Leaf-closing factor ; Menisdaurilide ; Phyllanthus urinaria L. ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Phyllanthurinolactone (1) and its three stereoisomers 18-20 were synthesized, and only 1 was bioactive as the leaf-closing factor of a nyctinastic plant, Phyllanthus urinaria L. X-ray analysis of the tetraacetylglucoside 17 was executed, and the absolute configuration of 1 was determined as 6S,7aR.
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  • 3
    ISSN: 1434-193X
    Keywords: Macrocystidia cucumis ; Cucumins ; Hirsutanes ; Sesquiterpenoids ; Antibiotics ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Arthrosporone (1) and eight new triquinane-type sesquiterpenoids have been isolated from mycelial cultures of the agaric Macrocystidia cucumis. The cucumins A-D (2, 4-6) are highly unsaturated hirsutane derivatives, whereas the cucumins E-G (7-9) represent a new type of linear triquinanes. Cucumin H (10) is a new member of the ceratopicane group. The absolute configuration of cucumin F (8) was assigned by 1H-NMR analysis of the corresponding Mosher esters. Two further metabolites were identified as cyclo(phenylalanylprolyl) (12) and cyclo(leucylprolyl) (13). Cucumin A (2) exhibits antibacterial and cytotoxic activities.
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  • 4
    ISSN: 1434-193X
    Keywords: Carbohydrates ; Glycals ; Arabinogalactans ; 1,2-Anhydrosugars ; Monoclonal antibodies ; CCRC-M7 ; Thioethyl furanosides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The spacer-containing tetrameric arabinogalactans 1-3, suitable for CCRC-M7 epitope characterization, are readily accessible by ZnCl2-assisted stepwise elongation of 11-methoxycarbonylundecanol 9 with the 1,2-anhydrogalactose building unit 6. NIS/cat. TfOH-mediated glycosylation of the newly formed 2-OH, 2′-OH, or 2′′-OH function in the β-(1→6)-galactoside backbone with the 1-thioarabinofuranoside donor 8, followed by deprotection, provided the target tetramers in high overall yield.
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  • 5
    ISSN: 1434-193X
    Keywords: Chromophoric unit ; Crown compounds ; Cyclobis(paraquat-p-phenylene) ; Catenanes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A modified bis-p-phenylene-34-crown-10 ring in which one of the 1,4-dioxybenzene units has been replaced by a 9,10-dioxyanthracene unit has been employed as a template for the formation of cyclobis(paraquat-p-phenylene). The [2]catenane which results has been shown by 1H NMR spectroscopy to exist in solution exclusively in the translationally isomeric form in which (a) only the 1,4-dioxybenzene ring occupies the central cavity of the tetracationic cyclophane and (b) the crown ether ring is prevented from circumrotating through the cyclophane by the large 9,10-dioxyanthracene unit. The absorption spectrum and luminescence properties of this new [2]catenane and of its crown ether component in its free state have been investigated and compared with those of 1,4-dimethoxybenzene and a model anthracene derivative, carrying methylated triethylene glycol chains on the 9 and 10 positions of the anthracene ring. While the absorption spectrum of the crown ether is the sum of the spectra of the two component chromophoric moieties, its emission spectrum shows only the fluorescence band of the 9,10-dioxyanthracene-type unit. The excitation spectrum shows that the disappearance of the 1,4-dioxybenzene type emission in the crown ether is due to a very efficient (kq ≥ 4 × 1010 s-1) energy-transfer process from the 1,4-dioxybenzene to the 9,10-dioxyanthracene type unit. The absorption spectrum of the [2]catenane is noticeably different from the sum of the spectra of its two cyclic components, particularly as far as the presence of a very broad charge-transfer (CT) band in the visible spectral region (λmax = 545 nm, εmax = 615 M-1 cm-1) is concerned.Comparison with the CT band of a model compound shows that the very broad CT band of the [2]catenane is in fact the result of two component bands originating from the interaction of the two different electron-donor units (1,4-dioxybenzene and 9,10-dioxyanthracene type) present in the crown ether with the electron-acceptor bipyridinium-type units of the cyclobis(paraquat-p-phenylene). The emission spectrum of the [2]catenane does not show any band because of the quenching action (rate constant kq ≥ 5 × 1010 s-1) of the low-energy non-luminescent charge-transfer levels on the higher energy, potentially luminescent levels of the crown ether.
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  • 6
    ISSN: 1434-193X
    Keywords: Azides ; Betaines ; Carbenes ; Nitrogen heterocycles ; Nucleophilic aromatic substitution ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nitrogen-rich mesoions have been synthesized by the reaction of the 5-chloro-1,3-diaryltetrazolium salt 1 with various nitrogen nucleophiles. The reactions with aqueous ammonia and hydroxylamine gave tripolar mesoionic amide 2 and mesoionic hydroxylamide 4, respectively. N-Substituted and N,N-disubstituted hydrazines yielded the corresponding hydrazides 5, whereas N,N-diphenylhydrazine gave the rearranged product 6. The reaction with sodium azide gave 5-azidotetrazolium salt 8. The azido group of 8 was reduced to give aminotetrazolium salt 9, deprotonation of which yielded the corresponding conjugate base 3. Hard nucleophiles attacked the tetrazolium carbon atom of 8 to give substitution products, whereas soft nucleophiles added the terminal nitrogen atom of the azido group to give addition products. Azidotetrazolium salt 8 reacted further with sodium azide to give a high yield of the tetrazol derivative 11, together with a small amount of triazene 17. The intermediacy of mesoionic carbene 19 is postulated.
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  • 7
    ISSN: 1434-193X
    Keywords: Azomethine ylides ; Difluorocarbene ; Cycloadditions ; Heterocycles ; Oxazolidines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Iminiodifluoromethanides generated by the reaction of difluorocarbene with benzaldehyde and benzophenone imines undergo regioselective 1,3-dipolar cycloaddition to aldehydes to give oxazolidine derivatives.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 155-161 
    ISSN: 1434-193X
    Keywords: Mechanical bonding ; Interlocked molecules ; Template synthesis ; Calixarenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the amide-based rotaxane 7a bearing calix[4]arene blocking groups is described for the first time. While rotaxane formation fails if a calix[4]arene is functionalized at the upper rim with only an amino or methylamino group lacking any spacer, the prolonged amine 5a works successfully as stopper unit preventing dethreading of the dimeric wheel 1a by its size. Rotaxane formation of 8b was observed only by MALDI-TOF mass spectrometry of the reaction mixture of the amine 5b, the axle 6 and 1a. With the larger trimeric wheel 1b no stable rotaxane could be obtained. It either does not act as a concave template or its opening is too wide, even for the bulky calixarene stoppers.
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  • 9
    ISSN: 1434-193X
    Keywords: Stelladiones ; Photoelectron spectroscopy ; Calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Tricyclo[3.3.0.03,7]octane-2,4-dione (2,4-stelladione, 3), tricyclo[3.3.0.03,7]octane-2,6-dione (2,6-stelladione, 4), and 2-oxotricyclo[3.3.0.03,7]octane-6-thione (5) were synthesized. Key steps in the procedures were a Paterno-Büchi reaction followed by an oxetane splitting with a strong base. Structure analysis on single crystals of 4 and 5 revealed long central C-C bonds (1.59 Å) of the stellane skeleton. The He(I) photoelectron spectra of 3, 5, and 4-methylenetricyclo-[3.3.0.03,7]octan-2-one (9) were recorded and interpreted on the basis of quantum-mechanical calculations (HF-SCF, 6-31G* basis). The large energy difference between the first PE bands of 3 (0.8 eV) is due to a strong interaction between the lone pairs on the oxygen atoms and the σ frame.
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  • 10
    ISSN: 1434-193X
    Keywords: Acetylated glycosides ; 13C NMR ; Black chokeberries (Aronia melanocarpa Elliot) ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Following acetylation of an anthrocyan concentrate from black chokeberries (fruits of Aronia melanocarpa Elliot = Sorbus melanocarpa Heynh., Rosaceae), which is used as a natural food colouring agent, hexa-O-acetyl-sorbitol, penta-O-acetyl-glucopyranose (1), tetra-O-acetyl-β-D-glucopyranosyl benzoate (2), and hepta-O-acetyl-amygdalin (7) were detected by comparison with authentic samples. The structures of the hitherto unknown compounds 4-O-(tetra-O-acetyl-β-D-glucopyranosyl)pentan-2-one (3) and hepta-O-acetyl (2-pentanyl)-β-D-gentiobioside (8) were elucidated from their 1H,1H- and 13C,1H-COSY spectra. The structure of tetra-O-acetyl parasorboside was confirmed by crystal structure analysis.
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  • 11
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 201-212 
    ISSN: 1434-193X
    Keywords: Carbenes ; Carbocations ; Diazo compounds ; Radicals ; Cyclizations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: N-Aziridinylimines are readily obtained from carbonyl compounds and 1-aminoaziridines, of which the 2-phenyl and trans-2,3-diphenylaziridine derivatives are the most popular. Diazo compounds are formed upon thermolysis or photolysis of N-aziridinylimines, with elimination of alkenes. The diazo compounds can be trapped by intramolecular addition to alkenes, but otherwise decompose as they are formed, giving rise to carbenes and products derived therefrom. The transformation R2CO → R2CN2 → R2C is most often achieved with the anions (salts) of arenesulfonylhydrazones. However, the non-ionic and weakly basic N-aziridinylimines have the advantage that they are compatible with nonpolar solvents and with a wide range of substituents. These properties were exploited in the fragmentations of α,β-epoxyketones (→ alkynones) and α,β-epoxyaldehydes (→ β-hydroxyvinylidenes → cyclopentenols) as well as in the syntheses of unsaturated nitriles, silanes, and ethers. Laser flash photolysis of N-aziridinylimines in fluorinated alcohols was used to demonstrate the protonation of carbenes and to measure absolute reaction rates of carbocations. The Shapiro reaction of N-aziridinylimines, performed with catalytic amounts of R2NLi, was found to produce alkenes with excellent regio- and stereoselectivity. As radical acceptors, N-aziridinylimines are superior to alkenes. On this basis, selective (tandem) cyclizations were designed for the synthesis of natural products.
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  • 12
    ISSN: 1434-193X
    Keywords: Configuration determination ; Repellent, mosquito ; Rotundial ; Terpenoids ; Vitex rotundiforia ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The enantiomers of rotundial (1), a mosquito repellent from the leaves of Vitex rotundiforia, were synthesized from the enantiomers of limonene oxide (2). The absolute configuration of natural rotundial was determined to be R.
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  • 13
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 237-241 
    ISSN: 1434-193X
    Keywords: Cycloallenes ; Cycloadditions ; Diradicals ; Strained molecules ; Rearrangements ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The treatment of 3-bromo-2H-chromene (7), dissolved in furan, 2-methylfuran, or 2,5-dimethylfuran, with potassium tert-butoxide, leads to formation of the epoxybenzo[c]chromene derivatives 8-11 in yields of 28-59%. Likewise, in styrene solution, exo-2-phenylcyclobuta[b]chromene 12 is produced (41% yield). With tetrahydrofuran as the solvent, 2-tert-butoxy-2H-chromene (13) is observed as the only product (79% yield). From these results, it is concluded that 7 is converted by β-elimination into the title compound 5. This reactive intermediate is then intercepted by the furans and styrene in cycloaddition reactions to give compounds 8-12, whereas reaction with KOtBu/HOtBu transforms it to the acetal 13. The epoxybenzochromenes 8, 9, and 11 rearrange on heating at 110°C to give the epoxyxanthene derivatives 16-18.
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  • 14
    ISSN: 1434-193X
    Keywords: Electron transfer ; Cytochrome P-450 ; α-Alkylbenzyl alcohols ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The chemical and microsomal oxidations of a number of 4-methoxy substituted α-alkylbenzyl alcohols 4-MeOPhCH(R)OH (Eox = 1.6-1.7 V vs SCE) were investigated. Using potassium 12-tungstocobalt(III)ate, a bona fide one electron oxidant, competition between Cα-H and Cα-Cβ bond cleavage in the intermediate radical cation was observed when the side-chain alkyl group R was Et (2) and iPr (3). With R = Me (1), only C-H bond cleavage took place, whereas with R = tBu (4) C-C bond cleavage was the exclusive fragmentation process. In contrast, the microsomal oxidation of the two substrates 3 and 4 led in both cases to the exclusive formation of the corresponding ketone. Thus, an electron transfer mechanism appears unlikely for the microsomal oxidation of α-alkylbenzyl alcohols, even though the oxidation potential of these species is lower than or comparable to that of the active oxidant in the enzyme. A hydrogen atom transfer mechanism is more in line with these results.
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  • 15
    ISSN: 1434-193X
    Keywords: Cyclophanes, hydroxy- ; Macrocycles ; Calixarenes ; O-Benzylation ; Template effect ; Conformation ; Hydrogen bonding, intramolecular ; Chirality ; Protecting groups ; Deprotections ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Regioselective O-benzylations of [3.1.3.1]metacyclophanes with intraannular OH groups are described. O-Benzylation of 6,13,22,29-tetra-tert-butyl-9,16,25,32-tetrahydroxyy[3.1.3.1]metacyclophane 1 with 2 equiv. of benzyl bromide in the presence of Na2CO3 leads to exclusive formation of the monosubstituted product 3. In contrast, use of K2CO3 as the base leads to di-O-substitution, resulting in the disubstituted product, distal-4, as the major product, along with some 1,2-proximal-4, in spite of using a large excess of benzyl bromide. Under the same reaction conditions in the presence of Cs2CO3, a mixture of two conformers of the tetra-O-benzylated product 2 in a ratio of 80:20 (cone-2-1,4-alternate-2) is obtained in 83% yield. Thus, the alkali metal cation plays not only an important role with regard to the regioselectivity, but also in determining the number of O-benzylations that occur, as a consequence of the template effect. Regioselective syntheses of dimethoxy- and trimethoxy[3.1.3.1]metacyclophanes are accomplished by a protection-deprotection strategy, using the benzyl residues as protecting groups. The 1H-NMR spectra of these macrocyclic metacyclophanes are also discussed.
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  • 16
    ISSN: 1434-193X
    Keywords: Resorcinols ; Cross-coupling ; Demethylation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Suzuki and Stille cross-coupling reactions were utilized in the synthesis of 5-substituted 1,3-dimethoxybenzene and 5-substituted resorcinol derivatives. The substituted resorcinol derivatives were obtained in only three steps from inexpensive reagents. 1,3-Dimethoxybenzoic acid and 1-chloro-3,5-dimethoxybenzene were transformed into 1-iodo-, 1-bromo-, 1-trimethyltin-3,5-dimethoxybenzene and 3,5-dimethoxyphenyl boronic acid. 5-Allyl-1,3-dimethoxybenzene and 3,5-dimethoxybiphenyl derivatives were obtained via cross coupling reactions under mild conditions. HI, BBr3 and AlI3 were used to demethylate these dimethoxybenzenes into their resorcinol derivatives.
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  • 17
    ISSN: 1434-193X
    Keywords: Conformation analysis ; Dynamic NMR spectroscopy ; Crown compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformations of (±)-trans-2,3-Bis(1-naphthyl)-15-crown-5 (1) and -18-crown-6 (2) are investigated by MM2 and dynamic 1H-NMR spectroscopy. The most stable conformation is the diequatorial one in both the free ligands and the alkali metal complexes 1 · NaClO4 and 2 · KClO4. Three rotamers, pseudo-aa, pseudo-ee, and pseudo-ae, with respect to the spatial arrangement of the naphthyl groups, were found as calculated minima by MM2 and experimentally by low-temperature NMR, with fair agreement of the rotational barriers. In the cation-reinforced crowns the rotation rates are significantly diminished. The distribution of the rotamers depends on the size of the crown and its state, free or complexed.
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  • 18
    ISSN: 1434-193X
    Keywords: Aziridines ; Cleavage reactions ; Diradicals ; EPR spectroscopy ; Photolysis ; Rearrangements ; Ring contractions ; Small ring systems ; Strained molecules ; Tetrazoles ; Valence isomerization ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Deprotonation of the annulated tetrazolium salts 4, 6, 8, 10, and 12 with sodium or potassium hydride yields the alkylidenedihydrotetrazoles 5, 7, 9, 11, and 13, respectively. While 5a and b are unstable, even in solution at low temperatures, 7, 9, 11, and 13 form yellow oils that are distilled under high vacuum. - Irradiation of solutions of 7, 9, and 11 in [D8]toluene at -60°C yields, besides molecular nitrogen, annulated iminoaziridines that have an exocyclic CN double bond, i.e. 14, 16, and 18, respectively. In addition, an equal amount of the isomer 19 with the endocyclic CN double bond is formed from 11. On thermolysis, 14, 16, and 18 undergo [2 + 1] cycloreversion into methyl isocyanide and the cyclic imines 15, 17, and 20, respectively. By contrast, 19 rearranges thermally to yield 18. While the doubly bridged alkylidenedihydrotetrazole 13a affords only unidentified decomposition products on photolysis, its methyl homologue 13b is converted into the hexahydronaphthyridine 22 which is also formed on thermolysis. - Irradiation of 13b in a 2-methyltetrahydrofuran or butyronitrile matrix at 77 K yields a triplet diradical showing a four-line EPR spectrum centred at 3362 G and a half-field transition (at 1669 G) with a hyperfine structure. The zero-field splitting parameters |D-hc| = 0.031 cm-1 and |E-hc| = 0.0014 cm-1 are obtained by simulation of the EPR spectrum. The signal-carrier is assigned the diazatrimethylenemethane structure 23 on the basis of the close similarity between its EPR spectrum and those of trimethylenemethane (28) and tris(N-methylimino)methane (29). - Structural features are discussed that are responsible for the observed differences between the photochemical pathways.
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  • 19
    ISSN: 1434-193X
    Keywords: Dioxirane ; Oxidation ; Allylic alcohols ; Regioselectivity ; Diastereoselectivity ; Chemoselectivity ; Epoxides ; Enones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The solvent-dependent shift in the regioselectivity of the geraniol epoxidation by methyl(trifluoromethyl)dioxirane (TFD) reveals that as for the less reactive dimethyldioxirane (DMD), hydrogen bonding stabilizes the transition state of the epoxidation. In protic media, the hydrogen bonding is exerted intermolecularly by the solvent, whereas in unpolar, non-hydrogen-bonding solvents intramolecular assistance through the adjacent hydroxy functionality comes into the play and the attack on the allylic alcohol moiety is favored. For chiral allylic alcohols, additional steric interactions control the π-facial selectivity in the conformationally fixed transition state. Analogous to DMD, the preferred dihedral angle in the hydrogen-bonded transition state of the TFD epoxidation constitutes approximately 130°, but contrary to DMD and for synthetic purposes important, the allylic alcohols and derivatives 1 and 3-5 investigated here are chemoselectively epoxidized by TFD without formation of the corresponding enones.
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  • 20
    ISSN: 1434-193X
    Keywords: Amino acids ; Bicyclopropylidene ; Methylenecyclopropane ; Palladium catalysis ; Zinc reagents ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Palladium-catalyzed cross-coupling reactions of bromo(methylenecyclopropanes) 1c, 2c with the sodium enolate of dimethyl malonate 4a and the chlorozinc enolates of the glycine equivalent (diphenylmethyleneamino)acetate 4c and diethyl malonate 4d, respectively, have been found to proceed with opening of the three-membered ring in each case, to give the corresponding dienyl-substituted CH-acidic compounds 5-7 in moderate to good yields. On the other hand, coupling of bicyclopropylidenylzinc chloride (2d) with diethyl bromomalonate (4e) and the electrophilic glycine equivalent ethyl 2-acetoxy-2-(diphenylmethyleneamino)acetate (4f) gave 7 and 6 in 27 and 29% yield, respectively.
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  • 21
    ISSN: 1434-193X
    Keywords: Thione S-imides ; Thiaziridines ; Thione S-sulfides (thiosulfines) ; Sulfur heterocycles ; Reaction mechanism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: N-(1-Adamantyl)hexafluorothioacetone S-imide (1) reacts readily with aromatic thioketones 4a-e to afford 1,4,2-dithiazolidines 5a-e as products of [3 + 2] dipolar cycloadditions. Unexpectedly, cycloadducts 5d and 5e, obtained from thioxanthione (4d) and 4,4′-(dimethoxy)thiobenzophenone (4e), respectively, are found to decompose at room temperature and could not be isolated as pure compounds. Unlike aromatic thiones, adamantanethione (4f) did not react with 1 at ambient temperature. However, reaction did occur upon heating in a sealed tube, and the new 1,4,2-dithiazolidine 9, bearing two adamantyl moieties, was isolated as the major product. The structure of 9 has been determined by X-ray diffraction analysis. The connectivity of the heterocyclic ring in this product indicates that the mechanism of its formation must proceed by a different route involving another in situ generated sulfur-centered 1,3-dipole. Retrocycloaddition of the primary adamantanethione cycloadduct 13 liberates hexafluorothioacetone, which is subsequently captured by S-imide 1 to give tetrakis(trifluoromethyl)-1,4,2-dithiazolidine 8 as a crystalline product. The structure of 8 has also been confirmed by X-ray diffraction analysis.
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  • 22
    ISSN: 1434-193X
    Keywords: Purpurosaminides C ; Glycosyl donors ; Synthesis ; Biocatalysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Both enantiomers of purpurosamine C-type glycosyl donors [(ent)-9, (ent)-10, (ent)-11] and of a 2-azido epimer [(ent)-14] with a modified pattern of protecting groups have been prepared from racemic 3,4-dihydro-2H-pyran-2-carbaldehyde (acrolein dimer, rac-1, “indirect aziridination”, “azidonitration”). In two cases (rac-23β: methyl 6-O-acetyl-2,3,4-trideoxy-2α-trifluoroacetylamino-β-D/L-hexopyranoside, rac-32: methyl 6-O-acetyl-2β-azido-2,3,4-trideoxy-β-D/L-hexopyranoside), efficient resolution has been achieved biocatalytically.
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  • 23
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 509-514 
    ISSN: 1434-193X
    Keywords: Azasugars ; Pyrrolo-oxazolidinones ; Amidoalkylation ; Sakurai reaction ; Asymmetric synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hydroxy-substituted pyrrolidines are important structural elements in azasugars. Effective precursors for such compounds are pyrrolo-oxazolidinones of type 1. Short asymmetric syntheses starting from easily available low-priced precursors are desirable. Two efficient pathways to these structures starting from the 4-methoxylated oxazolidinones 2 and 3, easily accessible from the chiral pool, have been successfully developed. These oxazolidinones can be used as amidoalkylation reagents. Via Sakurai reactions and subsequent intramolecular cyclization, the synthesis of bicyclic pyrrolidines is possible in a stereocontrolled way.
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  • 24
    ISSN: 1434-193X
    Keywords: Ene reactions ; Chiral tiglic acid 2,2-dimethyloxazolidines ; Triazolinedione ; Singlet oxygen ; Evans′ oxazolidinones ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The diastereoselectivity in the ene reactions of triazolinedinone and singlet oxygen with chiral oxazolidine or oxazolidinone derivatives of tiglic acid depends both on the chiral auxiliary used and the size of the attacking enophile. While chiral 2,2-dimethyloxazolidine amides of tiglic acid give with triazolinediones TAD only one single ene adduct, the corresponding chiral oxazolidinone derivatives (Evans′ auxiliaries) display a very low diastereoselectivity. No selectivity is observed in the singlet-oxygen ene reaction with dimethyloxazolidine amides of tiglic acid. These stereochemical results are rationalized in terms of the differences in the steric demand of the singlet oxygen and TAD enophiles rather than electronic factors.
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  • 25
    ISSN: 1434-193X
    Keywords: Tetrasulfido[20]annulenes ; Thiophenophanes ; Porphycenes ; Alkynes ; McMurry coupling ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The syntheses of tetraalkynylated tetrasulfido[20]annulenes 2a-d are described. Treatment of the corresponding brominated tetrasulfido[20]annulene 5a (easily available by McMurry coupling of the brominated 2,2′-bithiophene-5,5′-dicarbaldehydes, 3a) with various acetylenes in the presence of Pd(II) and Cu(I) in NH(iPr)2 yields acetylenic tetrasulfido[20]annulenes 2a-d.
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  • 26
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    Liebigs Annalen 1998 (1998), S. 553-553 
    ISSN: 1434-193X
    Keywords: Correction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The compound names in the title of the original paper indicate a wrong ring size; the correct title should read as above (in the rest of the article those names are correct).
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  • 27
    ISSN: 1434-193X
    Keywords: Bond-dissociation energies ; Arenes ; Transition metals ; Ionization energies ; Gas-phase chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bond-dissociation energies (BDEs) of more than twenty arene-M+ complexes (M = Cr, Fe, Co) have been studied by mass spectrometry, using the kinetic method. With minor exceptions, the same relative order of BDEs is found for the three metals and the series of substituted arenes: electron-donating substituents, like alkyl or amino groups, increase the arene-metal BDE whereas electron-withdrawing substituents, e.g. halogens, lower the BDE compared to the unsubstituted benzene complexes. Interestingly, inverse linear correlations of the arene-M+ BDEs and the arenes′ ionization energies (IEs) exist for particular classes of substituents. The present results may serve as a guide to predict BDEs of similar organometallic complexes based on the IEs of substituted arenes and vice versa.
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  • 28
    ISSN: 1434-193X
    Keywords: Enthalpy of formation ; Enthalpy of vaporization ; Enthalpy of combustion ; Cycloalkanes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The standard enthalpies of formation ΔHf°(g) of mono- and gem-di(alkoxycarbonyl)-substituted cyclopropanes 1, cyclobutanes 2 and cyclopentanes 3 have been calculated from the standard enthalpies of combustion ΔHc°, which were measured calorimetrically, in combination with the standard enthalpies of vaporization ΔHvap°. The latter were obtained for 1a-c, 2b-c and 3b-c from the temperature dependence of the vapor pressures, which were measured in a flow system. Contrary to suggestions in the literature, only weak stabilization (≤ 8 kJ · mol-1) of small rings by gem-alkoxycarbonyl substituents was observed. In this context, we give a revised value for the enthalpy of combustion of methyl cyclobutanecarboxylate. It is concluded that the known high rates of ring closure to gem-dialkoxycarbonyl cyclopropanes are not attributable to a ‘stabilizing effect’ resulting from conjugation between the alkoxycarbonyl substituents and the cyclopropane ring, as has been suggested in the literature. The operation of a Thorpe-Ingold- or gem-dimethyl-type effect would seem to offer a more satisfactory interpretation.
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  • 29
    ISSN: 1434-193X
    Keywords: Cycloaddition ; High pressure chemistry ; Quinones ; Cyclohexanones ; Retro Diels-Alder process ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The selectivities of typical transformations of the p-benzoquinone Diels-Alder adduct 2 and its dihydro derivative 3 are shown to be highly dependent on the mechanistic path followed. To avoid ambiguities and to make sure of clearly defined regioselectivity, the monoketal 13 was examined and proven not only to be an excellent dienophile but, of course, also to lead to reliable regioselectivity in subsequent transformations. This led to the correction of an earlier provisional assignment of the alkylation products 11 and 12.
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  • 30
    ISSN: 1434-193X
    Keywords: Indolizino[3,4-b]quinoline ; Receptors ; Molecular tweezers ; Tentacle molecules ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A convenient diastereoselective synthetic route to the molecular tweezer bis(indolizino[3,4-b]quinolyl)methane 9 and the rigid indolizino[7′,8′:2,3]quinolino[8,7-h]indolizino[8,7-b]quinolines 14, 15 as potential receptor molecules has been developed, involving double imine condensation followed by Lewis acid catalyzed biscyclization of prolinal-derived bis(imines) 8 and 13, respectively. Whereas the use of SnCl4 leads to the formation of the planar polycycle 15, the corresponding concave product 14 is formed in the presence of EtAlCl2. Both compounds 14, 15, as well as tweezer 9 have been characterized by X-ray crystal-structure analysis. Although tris(imines) 20, 24 derived from 1,3,5-triaminobenzene (18) and tris(4-aminophenyl)amine (23) could be obtained similarly by molecular sieve-catalyzed condensation, the corresponding triscyclization could not be achieved. However, by attaching preformed indolizino[3,4-b]quinoline subunits 25 and 31 to an aromatic core, the bidentate receptors 30, 33 and the tentacle molecule 28 were accessible.
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  • 31
    ISSN: 1434-193X
    Keywords: Cross-coupling, Heck ; Palladium catalysis ; Indanes ; Arenes, dialkenyl-N ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: o-Bromostyrenes 2-Br react with various alkenes in the presence of palladium catalysts to give either substituted indene 6 or o-diethenylbenzene derivatives 3, depending on the reaction conditions. Under oxidative conditions the latter can be cyclized to indene derivatives as well.
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  • 32
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    Liebigs Annalen 1998 (1998), S. 689-696 
    ISSN: 1434-193X
    Keywords: Ferrocene ; Asymmetric synthesis ; Nucleophilic 1,2-addition ; Hydrazones ; Cleavage reactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The aldehydes 1 and 5 were converted to the corresponding SAMP hydrazones 2 and 6, respectively. Subsequent nucleophilic 1,2-addition of organolithium reagents to the C-N double bonds and cleavage of the N-N hydrazine bond using an excess of BH3·THF afforded (1-ferrocenylalkyl)amines (N-protection → 4) and 1,1′-bis(1-aminoalkyl)ferrocenes (N-protection → 8) in good overall yields (32-88%), with very high enantiomeric excesses (ee = 90-98%) and dl/meso ratios of up to 95:5.
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  • 33
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    Liebigs Annalen 1998 (1998), S. 719-724 
    ISSN: 1434-193X
    Keywords: Steroids ; Cyclocholates ; Supramolecular chemistry ; Sodium borohydride reduction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Syntheses of bile acid cyclooligomers with 12- and 7,12-oxo groups (6a-d, 7a-c, 8a-b) by the Yamaguchi method are described. Cyclotrimerization is the principal reaction route for these cholic acid systems. Conversion of 7- and 12-hydroxy groups in cholic acid (1a-b) to oxo groups (4a-c, 5a-c), followed by macrocyclization (6a-d, 7a-c, 8a-b) and selective reduction of the oxo groups back to hydroxy ones without cleaving the 24-carboxylic ester linkages (11) constitutes a new strategy in the synthesis of cyclocholates having unprotected hydroxy groups.
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  • 34
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    Liebigs Annalen 1998 (1998), S. 735-743 
    ISSN: 1434-193X
    Keywords: Obituary ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 35
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    Liebigs Annalen 1998 (1998), S. 759-761 
    ISSN: 1434-193X
    Keywords: Catalysis ; Carbonyl compounds ; Iron compounds ; Michael reactions ; Tautomerism ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A double acceptor-activated cycloalkene 1 reacts in an FeCl3 · 6 H2O catalysed Michael reaction surprisingly as a donor. The constitution of the product 2 results from a reaction of 1 in the γ-position, thus the Michael reaction is vinylogous with respect to the donor. A tautomerism between the enone 1 and the dienol 4 is found to be a precondition for this reactivity.
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  • 36
    ISSN: 1434-193X
    Keywords: Antibiotics ; Dipeptides ; Heterocycles ; β-Amino acid ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: TAN-1057 (1a, b) - a new natural dipeptide antibiotic active against methicillin resistant strains of Staphylococcus aureus - was synthesized starting from Nα, Nδ, Nω-tri-Z-L-arginine 20b via the corresponding diazoketone 21b. This upon photolysis rearranged to the ketene which was trapped by (±)-2,4,5,6-tetrahydro-5-methylamino-2-ureidopyrimidin-4-one (3) to yield the fully protected dipeptide 23 (30%). The latter was deprotected by hydrogenolysis to give the final compound as a mixture of two epimers - TAN-1057A, B - isolated previously from a strain of Flexibacter sp. PK-74. The intermediate 3 was prepared from 3-amino-2-(N-Z-N-methylamino)propionic acid methyl ester hydrochloride (16) and 2-methyl-2-thiopseudobiuret hydroiodide (18) in one step in 35% yield.
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  • 37
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    Liebigs Annalen 1998 (1998), S. 847-851 
    ISSN: 1434-193X
    Keywords: Fluorescent dyes ; Perylenes ; Heterocycles ; Carboxylic acids ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and properties of perylene-bisimides 4 with one carboxylic acid anchor group are described. The properties of 4 are strongly influenced by the spacer between the carboxylic acid group and the imide moeity. Dyes with aliphatic (4d-4g) and aromatic (4a-4c) spacers have been prepared.
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  • 38
    ISSN: 1434-193X
    Keywords: Lithiation ; Diazomethane ; Solid-state structure ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Reaction of n-butyllithium with Me3SiCHN2 in THF affords the first example of a lithiated diazomethane with C-Li and N-Li bonds.
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  • 39
    ISSN: 1434-193X
    Keywords: N-Alkylation ; Diketopiperazine building block ; Peptidomimetics ; RGD analogues ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: As a new scaffold for peptidomimetic synthesis, a highly constrained bifunctional diketopiperazine, 4, has been prepared by smooth N-alkylation with tert-butyl bromoacetate. As a first application, we describe herein the synthesis of new peptidomimetics of the Arg-Gly-Asp (RGD) sequence. The product 30, which shows a selective platelet-aggregation inhibiting activity, can be used as a lead for the preparation of more potent products.
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  • 40
    ISSN: 1434-193X
    Keywords: AZT ; cycloSal-pro-nucleotide ; Prodrugs ; Nucleotide delivery ; HIV chemotherapy ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The application of our cycloSaligenyl- (cycloSal) pronucleotide concept to the approved anti-HIV dideoxynucleoside 3′-azido-3′-deoxythymidine AZT (1) is reported. This pro-nucleotide concept has been designed to deliver the corresponding 3′-azido-3′-deoxythymidine monophosphate AZTMP (2) by selective chemical hydrolysis from the lipophilic precursors cycloSal-AZTMP 4a-h. All derivatives 4a-h were synthesized using differently substituted salicyl alcohols 7a-h as starting materials. In hydrolysis studies, compounds 4 decomposed selectively releasing AZTMP (2) and the salicyl alcohols 7 following the designed tandem reaction. Furthermore, due to the electronic properties introduced by substituents, the half-lives of the triesters 4 could be ajusted over a wide range. Phosphotriesters 4 exhibited considerable biological activity in HIV-1 and HIV-2 infected wild-type human T-lymphocyte (CEM/O) cells, whereas, contrary to our expectations, nearly all activity was lost in HIV-2 infected thymidine-kinase-deficient CEM cells.
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  • 41
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    Liebigs Annalen 1998 (1998), S. 865-870 
    ISSN: 1434-193X
    Keywords: Alkaloids ; Enaminones ; Indolizidines ; Sulfide contraction ; Total synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the racemic title alkaloid 1 has been accomplished in eight steps and 7.2% overall yield from pyrrolidine-2-thione (5) and ethyl hex-2-enoate (6). Key steps include a ring closure that takes advantage of the nucleophilicity of a vinylogous urethane 8, and stereoselective reduction of the C=C double bond of a bicyclic vinylogous amide 12.
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  • 42
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    Liebigs Annalen 1998 (1998), S. 889-895 
    ISSN: 1434-193X
    Keywords: Sponge alkaloids ; Protecting groups ; Macrocycles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general method was developed for the synthesis of a variety of symmetric and asymmetric macrocyclic bispyridines. 3-Hydroxyalkylpyridines 18-20 were prepared, converted to the corresponding iodides and protected on the pyridine nitrogen with a p-methoxybenzyl group to give the crystalline precursors 21, 22 and 23. Reaction with a second 3-hydroxyalkylpyridine gave the corresponding bis (3-alkylpyridinium) salts, which could easily be deprotected by refluxing in pyridine (24-29). Cyclization of the iodides in refluxing acetonitrile under high dilution conditions (1-2 mM) gave the desired macrocyles in overall yields between 65 and 74%. This efficient procedure resulted in the synthesis of the muscarinic receptor antagonists cyclostellettamines A-F (1-6) and some unnatural cyclophanes (12 and 31).
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  • 43
    ISSN: 1434-193X
    Keywords: Radical reactions ; Electron transfer ; Nitration ; Radical ions ; Nitric oxides ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: During the irradiation of phenol (1) with 15N-enriched tetranitromethane in acetonitrile, and during the reaction of 1 with 15N-enriched nitric acid and nitrous acid, emission due to the nitration products o- and p-nitrophenol (2a, 2b) is observed in the 15N-NMR spectra. The CIDNP effects are built up by radical pairs formed by the encounters of the radicals NO2· and 1+· or PhO·. During the reaction of 1 with nitrous acid, 2b is formed, in part due to a non-radical reaction, via oxidation of p-nitrosophenol (3).
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  • 44
    ISSN: 1434-193X
    Keywords: Nitric acid derivatives ; Nitric oxide ; Angina pectoris ; Thiol derivatives ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: As part of our research into new organic nitrates for the treatment of angina pectoris, we have investigated a series of hexitol and aminodeoxyhexitol mononitrate derivatives containing a sulfur group. Since the depletion of tissue stores of sulfhydryl groups appears to play an important role in the development of this phenomenon, the addition of a sulfur group to a nitrate derivative could prevent the development of nitrate tolerance during a long-term treatment. Before studying the duration of action, and the possible influence on tolerance phenomenon, it was important to check the vasorelaxing effects of our compounds compared to those of commercial organic nitrates of similar structures such as isosorbide mononitrate or isosorbide dinitrate. All the compounds were tested on isolated rat aortas; some of these products exhibited an interesting activity.
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  • 45
    ISSN: 1434-193X
    Keywords: Crystal engineering ; Host-guest chemistry ; Nanotubes ; Sandwich complexes ; Self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An analysis is presented of the different contributions that give rise to the packing observed in the crystal structures of a wide range of bipyridinium-based molecular assemblies and supramolecular arrays. It is demonstrated how the various interactions - electrostatic, van der Waals, and π-π interactions - that contribute to the solid-state arrangement of these molecules and supermolecules can be utilized in order to design a series of tetracationic cyclophanes that can potentially self-organize in a highly ordered way in the solid state by virtue of the fact that they contain π-electron donors as well as π-electron acceptors. The syntheses of these cyclophanes is outlined and the tunability of the self-assembly methodology in their construction is demonstrated. One of these tetracationic cyclophanes - comprising π-electron-rich hydroquinone rings and π-electron-deficient bipyridinium units - has been shown to pack as highly ordered two-dimensional, mosaic-like sheets in the solid state. Its dicationic precursor also forms extended π-π-stacked layers in the solid state. An analogous cyclophane - containing two π-electron-rich resorcinol rings in place of the two hydroquinone rings - forms, in the solid state, one-dimensional arrays wherein the component resorcinol rings interact through their parallel π-π stacking. It has also been established that the first of the aforementioned tetracationic cyclophanes forms a 1:1 adduct with ferrocene in both the solution and solid states. X-ray crystallography, performed on the 1:1 adduct, reveals that not only is the ferrocene molecule complexed in a π-π stacking sense within the tetracationic cyclophane, but the 1:1 adduct also packs in a manner that is remarkably similar to the supramolecular organization of the free cyclophane in the crystalline state.
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  • 46
    ISSN: 1434-193X
    Keywords: Cyclopropanation ; Chromium ; Catalysis ; Carbene complex ; Diazo compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The [2 + 1] cycloaddition reaction of electron-rich alkenes such as enol ethers with 9-diazo-9H-fluorene (1) is efficiently catalyzed by pentacarbonyl(η2-cis-cyclooctene)chromium(0) (2). This cyclopropanation reaction shows a pronounced preference for electron-rich C=C bonds, as demonstrated by the regioselective reactions of allyl vinyl ether and 2-vinyloxyethyl acrylate; the [2 + 1] cycloaddition proceeds via the carbene complex intermediate 13, which has been detected by 13C-NMR spectroscopy in the course of the reaction. (Z)-Propenyl benzyl ether yields spirocyclopropane 21 with retention of the configuration of the former olefinic double bond. Whereas diazo compounds 22 and 23 react with ethyl vinyl ether to give low yields of cyclopropanes, the dibenzocycloheptenylidene and diarylcarbene precursors 24 and 25 afford moderate yields of olefin metathesis products 29-31. The competition between cyclopropanation and olefin metathesis reflects the propensity of the carbene complex intermediates to undergo decarbonylation.
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  • 47
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    Liebigs Annalen 1998 (1998), S. 1045-1053 
    ISSN: 1434-193X
    Keywords: C, C couplings ; Cycloaromatization ; Dienediynes ; McMurry reaction ; Neocarzinostatin ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 6-ring/10-ring dienediyne model 11 of the antitumor agent neocarzinostatin chromophore 1 and its 6-ring/11-ring homolog 12 have been obtained in 41 and 18% yields, respectively, by McMurry cyclizations of ketoaldehydes 8 and 9 using TiCl3·2DME and Zn/Cu couple. Compounds 8 and 9 were obtained by multistep syntheses starting from the readily available acetylenic aldehydes HC≡CC(CH3)2-(CH2)nCH=O (18, n = 1; 21, n = 2). Dienediyne 11 was converted into the dienediyne ketone 33 which cycloaromatized at room temperature giving the octahydrophenanthrone 35 in 16% yield.
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  • 48
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    Liebigs Annalen 1998 (1998), S. 1085-1087 
    ISSN: 1434-193X
    Keywords: Wittig reaction ; Dynamic NMR ; Oxaphosphetanes ; Metal ion complexation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A stable betaine lithium salt adduct is observed during the course of a Wittig reaction. The stabilization of this adduct has been achieved by complexation with lithium ions and by using the chelating effect of pyridyl ligands. Dynamic NMR spectra are observed when the corresponding oxaphosphetanes complex with sodium or lithium ions.
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  • 49
    ISSN: 1434-193X
    Keywords: Calixarenes ; Chromophores ; Nonlinear optics ; Pre-organization ; Supramolecular chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A systematic investigation of the conceptofpre-organization of nonlinear optical (NLO) active chromophoric groups in calix[4]arene derivatives and the influence on the absolute second-order nonlinear optical coefficients is reported. Several calix[4]arenes were synthesized by modification of the electron-withdrawing groups at the upper rim of the aromatic and extension of the conjugated π system of the pre-organized chromophoric groups. Electrical field induced second harmonic generation (EFISH) experiments showed high μβ(0) values up to 1165·10-48 esu. Compared with the corresponding reference compounds, enhancements of the μβ(0) values varying up to 2.5 times per chromophore were observed which proves the benefit of pre-organization of NLO-active units in a multi-chromophoric system. Another important advantage is that the increase in NLO activity observed for these systems is not accompanied with a shift of the absorption band to longer wavelengths exceeding 20 nm. This makes these calix[4]arene derivatives promising building blocks for the development of stable, NLO-active materials that are suitable for frequency doubling.
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  • 50
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    Liebigs Annalen 1998 (1998), S. 1123-1134 
    ISSN: 1434-193X
    Keywords: Asymmetric synthesis ; Catalysis ; Hydroformylations ; Rhodium compounds ; Catalyst-directing groups ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Introducing ortho-diphenylphosphanyl benzoyl as a substrate bound catalyst directing group (CDG) allows an efficient substrate-directed diastereoselective hydroformylation of acyclic homomethallylic alcohols 5, making use of 1,3-asymmetric induction. The corresponding anti-aldehydes 10 were obtained as the major diastereomer in all cases, with diastereomer ratios of ca. 91:9 (anti:syn). Supporting evidence could be obtained for the ability of the o-DPPB group to act as a catalyst-directing group (CDG) via a reversible catalyst coordination. Finally, a model has been devised that rationalizes the origin of the 1,3-asymmetric induction. This model is based on a conformational analysis (NMR studies, MACROMODEL/MM3 calculations) of the homomethallylic substrates and indicates a relationship between the preferred substrate conformation and the experimentally determined stereoselectivities. In agreement with this model was the predicted significant improvement in stereoselectivity upon hydroformylation of the anti-homomethallylic alcohol derivative 15 (→ 21).
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  • 51
    ISSN: 1434-193X
    Keywords: Copper(II) ions ; Amino esters hydrolysis ; Metal catalysis ; Cooperativity ; Metallopolymers ; Solvent effect ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Polymeric (2) and oligomeric (4, 5) materials made of repeating units of 2,6-diaminomethylpyridine and 4,4′-diphenylmethane have been synthesized with the number of monomeric units (n) ranging from 2 to 29. In 1:1 DMSO/water solutions, these materials are fully soluble and strongly bind CuII ions. The complexes catalyze to different extents the hydrolysis of the p-nitrophenyl esters of α-, β-, and γ-amino acids. Only CuII complexes of polymeric 2 (n ≥ 10) are more effective catalysts than free CuII ions in the cleavage of β-amino esters. Such enhanced reactivity, which in the case of β-alanine p-nitrophenyl ester (β-AlaPNP) amounts to almost two orders of magnitude when the comparison is made with the CuII complex of monomeric ligand (N, N′-benzyl)-2,6-aminomethylpyridine (3), is observed in 1:1 (v/v) DMSO/H2O only when a certain degree of polymerization is reached (6 〈 n 〈 10). In 1:1 (v/v) CH3CH2OH/H2O the kinetic benefits of the complexes of polymer 2 (n = 10) diminishes and vanishes in 9:1 (v/v) CH3CH2OH/H2O. Analysis of rate data suggests that two neighboring CuII ions bound to the polymeric ligands cooperate for the occurrence of the hydrolytic process: one of them coordinates the amino group of the substrate so that the carbonyl of the carboxylate faces the second metal ion which delivers a bound hydroxyl acting as the nucleophilic species. The selectivity toward β-amino ester is likely associated with a rather rigid conformation of these metallopolymers which places two metal centers at the appropriate distance one from the other. It is suggested that the onset of the metal ion cooperativity is connected to a conformational change of the metallopolymer from an extended to a globular structure, likely triggered by hydrophobic forces.
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  • 52
    ISSN: 1434-193X
    Keywords: Diazo compounds ; Carbenes ; Heterocycles ; Enaminones ; Indoles ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Copper-catalyzed decomposition of alkyl diazoacetates in the presence of semicyclic enaminones 1a-o leads primarily to enamino esters 2 which constitute formal products of C-C insertion of the carbene unit. In the case of N-methyl (1a-e) and N-benzyl (1f-i) enaminones, 2a-i are accompanied by 2,3,5,6-tetrahydroindoles 3a-i, in which two carbene moieties are incorporated. At 250 °C, compounds 2b-e, which could not be isolated in pure form, undergo cyclocondensation to form 1,2,3,5-tetrahydrocyclopenta-[b]pyrroles 4b-e. In contrast, 2f-i can be isolated as Z,E mixtures and are transformed thermally into 4f-i only in the presence of silica gel. Carbene transfer to N-phenyl enaminones 1j-m and N-methyl enamino esters 1n,o leads only to the 1:1 adducts, enamino esters 2j-o, which do not undergo the cyclocondensation reaction under the previous conditions. Dehydrogenation of tetrahydroindoles 3c-e with tetrachloro-p-benzoquinone can be controlled to give either 1,2-dihydroindole-6,7-dicarboxylates 5 or indole-6,7-dicarboxylates 6.
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  • 53
    Electronic Resource
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    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 1189-1191 
    ISSN: 1434-193X
    Keywords: Carbohydrates ; Fucose ; Amino sugar ; 2′-(Acetamido)fucobioside ; Azidonitratisation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 2-azidofucosyl donor 4 was synthesised via a azidonitratisation and stereoselectively α-linked to the benzyl fucoside acceptor 5 by methyl triflate catalysis. The resulting disaccharide 6 could be transformed to the corresponding acetamido derivative and deprotected to 2′-(acetamido)fucobioside 10
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  • 54
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 1213-1218 
    ISSN: 1434-193X
    Keywords: Cycloadditions ; Diazatetraasteranes ; Crystal structure ; Solid-state chemistry ; Photochemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Upon irradiation, crystalline 4-methoxyphenyl-1,4-dihy-dropyridines 1 undergo nearly quantitative [2+2]cyclo-addition to form, via the ring-open intermediates 2, the centrosymmetric, polyfunctionalized 3,9-diazatetraasteranes 3. The centrosymmetric character of 3 was confirmed by 1H NMR spectral data as well as by X-ray crystal structure analysis. The dimerization reactions prove to be topochemically controlled by the nearest distance between potentially reacting double bonds, as well as by the conformationally determined packing restraints. This latter factor is shown by the X-ray crystal structure analysis of one dimerizing and one photostable derivative of the monomers 1.
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  • 55
    ISSN: 1434-193X
    Keywords: Glycosides ; Glycosylations ; Glycosyl phosphites ; Metal perchlorates ; LiClO4-solvent mixtures ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Glycosyl phosphites 1-3 derived from glucose are activated under neutral conditions and without the addition of any further promoter in 1 M solutions of LiClO4, Mg(ClO4)2 or Ba(ClO4)2 in ether, CH2Cl2, or CH3CN and react under these conditions with the alcohols5-9 to give the glycosides 10-14. The best results are obtained in the presence of Ba(ClO4)2. In CH2Cl2 and ether the α-anomers predominate, in CH3CN the β-isomers are formed in excess. Whereas the methyl phosphite 1 and the benzyl phosphite 3 deliver the desired glycosides in yields of 35-66%, the ethyl phosphite 2 is a more reactive glycosyl donor, giving the glycosides 10, 11 and 13 in 62-95% yield.
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  • 56
    ISSN: 1434-193X
    Keywords: Correction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Correction
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  • 57
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 1275-1283 
    ISSN: 1434-193X
    Keywords: Dendrimers ; Amino-functionalized dendrons ; Building blocks ; Protecting groups, orthogonal ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The multi-gram scale synthesis of first- and second-generation dendrons with Boc- and Cbz-protected amino groups in the periphery and methyl/ethyl esters at the focal points is described. Saponification of the esters and deprotection of the amines are shown to be strictly orthogonal processes which makes these dendrons valuable building blocks for future constructions.
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  • 58
    ISSN: 1434-193X
    Keywords: Matrix isolation ; Ab initio calculations ; Photochemistry ; Silylenes ; Silaethenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Flash pyrolyses of four suitable precursors - namely 1,1,1-trimethyl-2-propargyldisilane (32), its allenyl (34) and propynyl isomer (35), as well as 2-ethynyl-1,1,1,2-tetramethyldisilane (31) - lead to the formation of three C3H4Si species (7, 11, and 15). By examination of their photochemistry five additional compounds of the same composition (12, 13, 14, 17, and 18) were identified. Except ethynylsilaethene (18), all compounds observed are silylenes. Their identification is based on the comparison of the experimental and calculated (BLYP-6-31G*) IR spectra. Silacyclobutadiene (4) or silatetrahedrane (25) could not be detected.
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  • 59
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1998 (1998), S. 1353-1360 
    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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  • 60
    ISSN: 1434-193X
    Keywords: Amino acid synthesis ; Chiral glycine building block ; Dihydroimidazole ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The title compound BDI is prepared on multigram scale, either by resolution of the precursor 2-tert-butylimidazolidin-4-one (from glycine amide and pivalaldehyde) through diastereomeric salts (Scheme 2) or by preparative chromatographic enantiomer separation on a chiral column. Lithiated BDI derivatives are highly nucleophilic species, combining the structural elements of a Li enaminate, of an enolether and of an N-Boc-enaminate (E, G). They react with complete diastereoselectivity (NMR analysis) from the face trans to the tert-butyl group. The electrophiles employed are primary and secondary alkyl, allyl, benzyl, and propargyl halides (Schemes 3 and 5), enoates (in Michael additions, Scheme 7), as well as aliphatic and aromatic aldehydes (in aldol additions, Scheme 8). When a third, exocyclic, stereocenter is formed in these reactions, there is a high degree of enantiomer differentiation (with rac.sec. halides, products 10-12) and of enantiotopic face differentiation (with enoates and aldehydes, products 40-50). The reactions are so clean that highly efficient in-situ double alkylations are feasible, in which the sequence of addition of the two different electrophiles determines the configuration at the newly formed stereogenic center (Scheme 5). In contrast to derivatives of previously reported chiral glycine reagents, the products from BDI are converted to methyl esters of amino acids under mild conditions and without concomitant formation (… and the need for recovery or removal) of a chiral auxiliary; the method is compatible with acid-sensitive side chains in the α-amino acids and α-branched α-amino acids to be synthesized (Schemes 4 and 6). The addition of Li-BDI to aldehydes furnishes, after hydrolysis, α-amino-β-hydroxy acids of erythro configuration (allo-threonine analogs, Scheme 8); a model for the stereochemical course of this reaction (rel. topicity unlike) is proposed, and compared with the corresponding conversions of analogous oxazolidinone and imidazolidinone Li enolates which occur with rel. topicity like.
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  • 61
    ISSN: 1434-193X
    Keywords: Homoenolate equivalents, chiral ; Alkylation ; 1-Aminoallyl anions, chiral ; β-Methyl ketones, chiral ; Asymmetric synthesis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 3-Substituted aliphatic ketones 10 have been obtained in excellent optical yields by alkylation of the aminoallyllithium compound endo-8, a homoenolate equivalent of hexane-3-one, using prolinol ether (SMP) as the chiral auxiliary. The intermediate endo-8 was generated by tin-lithium exchange of the 3-stannylated enamine 7a with butyllithium. An improved hydrolysis procedure for the resulting enamines is described. Some mechanistic implications with respect to the formation as well as the alkylation of endo-8 are discussed.
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  • 62
    ISSN: 1434-193X
    Keywords: Calorimetry ; Molecular modelling ; Crown compounds ; Cryptands ; Macrocyclic ligands ; Conformational analysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stability constants and enthalpy changes determined by calorimetric titrations and supported by selected NMR titrations are reported for the complexation of sodium and potassium cations with 18 different crown ethers containing nitrogen atoms with different number, location and substitution pattern. The data, measured in methanol mostly with potassium salts, are compared to literature data; they show striking differences between all-oxygen analogs and the macrocycles with NH groups. In contrast, affinities with aza crown ethers bearing alkyl groups at the nitrogen as well as with the cryptand [2.2.2] come closer to the complexation free energies predicted from the number and electron donating capacity of the ligand heteroatoms. This is rationalised on the basis of molecular mechanics calculations, showing that a NH-containing crown predominates in conformations with axial N lone pairs, due to their repulsive electrostatic interactions with the ring oxygen atoms. Replacement of the hydrogen by alkyl groups forces the lone pairs to an equatorial position, thus enabling better complex formation, as borne out by experiment. In line with these arguments the lgK differences are with some exceptions more due to ΔH than to TΔS differences. The calorimetric data show linear isoequilibrium correlations between TΔS and ΔH, with slopes between those observed with other crown ether and cryptand complexes. Preliminary investigations of some synthetic macrocyclic amide precursors yield appreciable complexation only, if the two carbonyl oxygens can come in close contact with the guest cation. Computer aided molecular modelling shows that this is possible in a small 15C5-derivative, in which the polyethylenglycol cycle only serves as ring template without binding contributions from the ether oxygen atoms.
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  • 63
    ISSN: 1434-193X
    Keywords: Phosphoris ylides ; Iminophosphoranes ; Phosphorus heterocycles ; Rearrangements ; Diazaphosphaazulene ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 1,2-Dihydro-1,3,2-diazaphosphinine 2 reacts with acetylene-dicarboxylic acid esters to give chemoselectively the adducts 5 or 6a, b, depending on the reaction conditions. Compound 6a is alternatively synthesized by treatment of equimolecular amounts of the previously described monoadduct 3(3′) and dimethyl acetylenedicarboxylate. A reaction pathway is proposed based primarily on i) the isolation and characterization of intermediate 8(8′) and ii) the reactivity of 3(3′) towards electrophiles, in particular with tetracyanoethylene (compound 10).
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  • 64
    ISSN: 1434-193X
    Keywords: Cyclophanes ; Dilution principle ; Heterocycles ; Medium membered rings ; Ring strain ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology