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  • 1
    ISSN: 0894-3230
    Keywords: intramolecular thermal (4σ + 2π) dyotropy ; kinetic isotope effects ; limiting barrier parameters ; quantum tunnelling ; 2H transfer processes ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Primary deuterium kinetic isotope effects (PDKIE) in parazoline-annelated syn-sesquinorbornenes exhibiting irreversible intramolecular (4σ + 2π) thermal dyotropy reveal unambiguous evidence for a tunnelling contribution to the kinetics in one instance but not for a close analogue. For analogous dyotropy of a cyclohexadiene-annelated syn-sesquinorbornene, the tunnelling components of the kinetic behaviour is small by comparison. The H atom traverse between alternative loci for the pyrazolines, deduced from x-ray and neutron diffraction data, is in agreement with approximate barrier parameters obtained by fitting of the PDKIE data to the Bell equation; barrier penetration is 3.22 kcal below the computed barrier corrected for the tunnelling contribution. The relative kinetic effect of systematic variation of the π-donor/acceptor groups on aryl ring substituents at C and N in the pyrazoline ring is consistent with a pericyclic process for dyotropy of these compounds, but not with rearrangement mediated by biradicals resulting from single H atom transfer in the rate-limiting step. Computer modelling of the transition state for dyotropy of these compounds is also consistent with a thermal, orbital symmetry conserved pericyclic reaction. Copyright © 1998 John Wiley & Sons, Ltd.
    Additional Material: 6 Ill.
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  • 2
    ISSN: 1573-4927
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract The blood catalase of a hypocatalasemic mouse mutant has been compared with that of the wild-type (normal) animal and with that of the heterozygote. Comparison is on the basis of stability to heat and to urea. Electrophoretic evidence is of no value, because all forms tested show the same mobility. Because the heterozygote heat and urea inactivation curves differ from those of the two parental forms, and because the curves are smoothly S-shaped, with no shoulders or other irregularities, it is suggested that this heterozygote produces only a single molecular form of the enzyme.
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  • 3
    ISSN: 1573-4927
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract Antibody to normal mouse catalase will stabilize blood and liver catalase of the acatalasemic mouse against a variety of agents which damage protein tertiary structure (urea, guanidine, trypsin) but not against agents which affect the heme group (azide, hydroxylamine). The antibody will also stabilize catalase against inhibition by 3-amino 1,2,4-triazole (AT), the specific site of action of which is known. The antibody is able also to protect normal mouse catalase from urea denaturation, but it is without effect on AT inhibition of normal catalase. A hypothesis is proposed which explains these results and which helps localize the site of the mutation on the catalase molecule.
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  • 4
    ISSN: 1434-193X
    Keywords: Amino acids ; Spiro compounds ; Michael addition ; Copper reagents ; Curtius degradation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Racemic spiropentylglycine (8) has been synthesized by sodium borohydride reduction of benzyl (E/Z)-2-chloro-2-spiropentylideneacetate (5-Bn), nucleophilic substitution of the chlorine in the product 6 with azide and hydrogenolytic deprotection of the resulting 7 (overall yield 15%). An alternative approach to 8 consisted of the coupling of the higher-order cuprate 10, generated by halogen-metal exchange from bromospiropentane (9), with the electrophilic glycine equivalent 11 followed by deprotection (overall yield 47%). Enantiomerically pure (1′-aminospiropentyl)acetic acid [(R)-16] (overall yield 16% from 5-Me) and 1-aminospiropentanecarboxylic acid [(R)-23] (29% from 5-Me) were obtained from the Michael adduct 14-Me of (4R,5S)-4,5-diphenyloxazolidin-2-one (13) and methyl (E/Z)-2-chloro-2-spiropentylideneacetate (5-Me). Racemic 1-aminospiropentanecarboxylic acid (R/S-23) was prepared by rhodium-catalyzed addition of dimethyl diazomalonate to methylenecyclopropane and subsequent Curtius degradation of the halfester 28 via the azide 29 (overall yield 14%). Upon standing in aqueous solution, 23 underwent complete rearrangement to the new 1-amino-2-methylenecyclobutanecarboxylic acid (24). The interesting derivative of azabicyclo[3.1.0]hexane-1-carboxylate 34 with an annelated spiropentane moiety and a β-amino acid fragment was incidentally obtained in a one-step intermolecular domino reaction starting with the addition of lithium benzylamide to methyl 2-chloro-2-cyclopropylideneacetate (32, 41% yield).
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  • 5
    ISSN: 1434-1948
    Keywords: Niobium ; Tin ; Hydrides ; Group 5 ; Group 14 ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of niobocene trihydride [Cp2NbH3] with one equivalent of an alkyl- or aryltin halide XnSnR4-n (n = 1, 2) leads to products of the displacement of hydride by tin, i.e. [Cp2NbH2(SnX3-nRn)] [R = Me, n = 3 (1); R = Me, × = Cl, n = 2 (3); R = Ph, n = 3 (4); R = Ph, × = Cl, n = 2 (5); R = Et, × = Br, n = 2 (6)]. Use of an excess of the tin reagent results in a halodealkylation exchange reaction at the tin atom bound to the metal center. Reaction of the mono(silyl) complexes [Cp2NbH2(SiXMe2)] (X = Cl, Ph) with organotin halides XnSnR4-n gives products of the displacement of silicon by tin, whereas the bis(silyl) complex [Cp2NbH(SiPhMe2)2] does not undergo this reaction. The bis(tin)-substituted niobocene [Cp2NbH(SnMe3)2] has been prepared by the reaction of [Cp2NbH2(SnMe3)] with the tin amide Me3SnNMe2 under thermal conditions, as well as by the literature method of thermolysis of [Cp2NbH(η2-C2H3Ph)] in the presence of Me3SnH. Reaction of [Cp2NbH(SnMe3)2] with two equivalents of Cl2SnMe2 affords the bis(tin)-substituted complex [Cp2NbH(SnClMe2)2] (9). The possible presence of non-classical hypervalent H···Sn-Cl interactions in this compound has been assessed by means of X-ray structure analysis. The structural data for 9 are in accord with an essentially classical description as a tin-substituted dihydride complex without significant Sn···H interactions.
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  • 6
    ISSN: 1434-193X
    Keywords: Amino acids ; Bicyclopropylidene ; Methylenespiropentane ; Spiro compounds ; Organolithium derivatives ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Racemic bicyclopropylidenyl- (rac-11) and methylenespiropentyl- (rac-17) substituted alanines have been synthesized by iodination of bicyclopropylidenyl- and methylenespiropentylmethanols 7, 13, nucleophilic substitution of the iodine in 8, 14 with the enolate of ethyl α-(diphenylmethyleneamino)acetate (O'Donnell's glycine equivalent) and deprotection of 9, 15 in 24 and 18% overall yield, respectively. N-Methylbicyclopropylidenylalanine rac-22, was obtained from the Michael adduct of (bicyclopropylidenyl)magnesium bromide 18 to enamine 19 and deprotection of the carbamate 20 (23% overall yield). Racemic (1-amino-2-methylenespiropentane)- (37), (1-amino-2-methylenecyclopropane)- (3), and (1-aminobicyclopropylidene)carboxylic acid (39) were prepared as hydrochlorides by tert-butoxycarbonylation of the lithiated methylenespiropentane (6), methylenecyclopropane (4), or bicyclopropylidene (5) intermediates with di-tert-butyl pyrocarbonate (Boc2O), repeated lithiation of the tert-butyl esters 29, 30, and 33 with LDA and subsequent carboxylation, Curtius degradation of the half esters 31, 32, and 34 followed by deprotection in 11, 45, and 4% overall yields, respectively. Compound 37 was also prepared from bicyclopropylidene (5) following the same procedure, but with rearrangement in the last but one step, in 19% overall yield. An attempted Hofmann degradation of the bicyclopropylidenecarboxamido ester 40 with NBS failed and gave only bromohydrin 44 (27%), but with bis(acetoxy)iodobenzene provided carbamate 46a, b in 76 amd 79% yield, respectively. Along this route with subsequent deprotection of 46b, the amino acid 39 could be prepared in up to 10% overall yield from bicyclopropylidene.
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  • 7
    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.
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  • 8
    ISSN: 1434-193X
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: -The synthesis of 2,6-dialkoxy-9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene derivatives 15 and 16 is described. Solution electrochemistry shows that 15 and 16 display three redox waves, representing the sequential formation of the dication, radical trication and tetracation species in an EqEqEq process. The X-ray crystal structures of neutral compounds 15 and 16 and the charge transfer complex (15)2+(TCNQ-•)2· 2MeCN are reported. The neutral 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 and folding of both 1,3-dithiole rings. In the complex (15)2+(TCNQ-•)2· 2MeCN the anthracene system is planar and aromatic; the dithiolium cations form a dihedral angle of 78° with the anthracene plane. The TCNQ anion radicals form a stack of dimers with interplanar separations of 3.15 Å within a dimer and 3.50 Å between the dimers. The structure contains unusually short intermolecular S···N contacts [2.865(3) Å].
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  • 9
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
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  • 10
    ISSN: 0570-0833
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
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