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  • 101
    Electronic Resource
    Electronic Resource
    Total Synthesis of (+)-(8S,13R)-CyclocelabenzineDedicated to Professor Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1295-1304 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An asymmetric synthesis of the spermidine alkaloid (+)-cyclocelabenzine (1a) and its (-)-(13S)-epimer 1b is described using optically active (+)-(3S)-3-amino-3-phenylpropionic acid as the chiral building block. The isoquinolin-1-one fragment 15 was synthesized by a modified Bischler-Napieralski reaction. The relative configuration of the (-)-isomer was determined by an X-ray crystal-structure analysis, which enabled us to determine the absolute configuration of natural (+)-1a as (8S,13R).
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790506
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  • 102
    Electronic Resource
    Electronic Resource
    A Reinvestigation of the Oxidative Rearrangement of Yohimbane-Type Alkaloids. Part BPart A: [1].. Formation of Oxindol (= 1,3-Dihydro-2H-indol-2-one) Derivatives (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1361-1378 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The methanolysis of the epimeric 7-chloro-7H-yohimbine derivatives 2 and 3 was reinvestigated. In case of the 7α-epimer 2, the reaction was uneventful and conformed with earlier observations, i.e., under sufficiently mild conditions, only the imino ether 4 (= imino ether A) was produced. Under the same conditions, the less reactive β-isomer 3 furnished a mixture of both imino ethers 4 and 5, accompanied by the elimination product 11, and by equal amounts of yohimbine (1) and 3,4,5,6-tetradehydroyohimbine (12), which are believed to arise through a disproportionation process of the putative intermediate 5,6-didehydroyhimbine (23). The nature of this divergent reactivity and of the ready equilibration of 4 and 5 was investigated by means of extensive force-field and semi-empirical calculations (AM1 and PM3) of various conformers of the compounds 2-5 and of some possible reaction intermediates.
    Additional Material: 7 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790510
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  • 103
    Electronic Resource
    Electronic Resource
    2-Alkynylcyclopent-2-enols from 2-Alkynylcyclohex-2-enones via 1-Alkynyl-7-oxabicyclo[4.1.0]heptan-2-ones (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1428-1434 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2-Alkynylcyclohex-2-enones 1a-c and 2a-c react with H2O2/NaOH in MeOH to afford 1-alkynyl-7-oxabicyclo[4.1.0]heptan-2-ones 3a-c and 4a-c, respectively. The 3-unsubstituted bicyclic epoxy ketones 3a, 3b, and 4a, 4b react further with H2O2/NaOH, undergoing ring contraction and (formal) decarbonylation to give 2-alkynylcy-clopent-2-enols 5a, 5b, and 6a, 6b, respectively. Epoxy ketones 3 are also obtained under neutral conditions on irradiation (λ = 350 nm) of cyclohexenones 1 in air-saturated benzene solution. Similarly, under neutral conditions oxo-cycloalkenecarbonitriles 8 react (thermally) with H2O2 in MeCN to give the oxabicyclic carbonitriles 9.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790515
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  • 104
    Electronic Resource
    Electronic Resource
    Xylose-DNA Containing the Four Natural Bases (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1451-1461 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Oligonucleotides containing (2′-deoxy-β-D-xylofuranosyl)guanine have been prepared. For this purpose 2-aminoadenosine (5) was synthesized and converted to 2′-deoxy-β-D-xyloguanosine (1). The related 2′-deoxy-β-D-xyloisoguanosine (3) and 2′-deoxy-β-D-xyloxanthosine (4) were also synthesized. Compound 1 was converted to the phosphonate and phosphoramidite building blocks 10 and 11, respectively. The oligodeoxynucleotide (5′-3′)d(xG-xT-xA-xG-xA-xA-xT-xT-xC-xT-xA-xC-T) (18) formed a duplex with the same Tm as the parent (5′-3′)-(G-T-A-G-A-A-T-T-C-T-A-C) (19), but with an inverted CD spectrum.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790518
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  • 105
    Electronic Resource
    Electronic Resource
    Masthead (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790601
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  • 106
    Electronic Resource
    Electronic Resource
    Biosynthesis of Dodecano-4-lactone in Ripening Fruits: Crucial Role of an Epoxide-Hydrolase in Enantioselective Generation of Aroma Components of the Nectarine (Prunus persica var. nucipersica) and the Strawberry (Fragaria ananassa) (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1488-1496 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Early steps in the biosynthesis of dedecano-4-lactone in ripening nectarines (Prunus persica var. nucipersica) and strawberries (Fragaria ananassa) were studied using cis-9,10-([18O]epoxy)heptadecanoic acid (5), instead of the natural C18-precursor epoxystearic acid 1. In nectarines, an epoxide hydrolase catalyzed, in a highly regio- and enantioselective reaction, the attack of H2O at C(9) of epoxy acid 5 yielding preferentially (9R,10R)-dihydroxyheptadecanoic acid 6a as an early intermediate and, after further metabolization, undecano-4-lactone 7a. In strawberries, the same transformation exhibited only a moderate regio- and enantioselectivity. Besides the previously established function of phytogenic epoxide hydrolases involved in generating oxygenated fatty-acid derivatives, which are used in cutin monomer production and plant defence, a new rǒle for these enzymes now emerges in the enantioselective production of aroma components of ripening fruits. The cis-9,10-([18O]epoxy)heptadecanoic acid (≥ 98% 18O; 5) was obtained in good overall yield (68%) from isopropyl (9Z)-heptadec-9-enoate using 18O2 gas in a free-radical chain reaction in the presence of 2-methylbutanal.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790521
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  • 107
    Electronic Resource
    Electronic Resource
    Synthese von Trifluoromethyl-substituierten Schwefel-Heterocyclen unter Verwendung von 3,3,3-Trifluorobrenztraubensäure-DerivatenVorgetragen von H. H. am ‘Symposium on Fluoroorganic Chemistry’, Warschau, 17. Mai 1996. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1537-1548 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of Trifluoromethyl-Substituted Sulfur Heterocycles Using 3,3,3-Trifluoropyruvic-Acid DerivativesThe reaction of methyl 3,3,3-trifluoropyruvate (1) with 2,5-dihydro-1,3,4-thiadiazoles 4a, b in benzene at 45° yielded the corresponding methyl 5-(trifluoromethyl)-1,3-oxathiolane-5-carboxylates 5a, b (Scheme 1) via a regioselective 1,3-dipolar cycloaddition of an intermediate ‘thiocarbonyl ylide’ of type 3. With methyl pyruvate, 4a reacted similarly to give 6 in good yield. Methyl 2-diazo-3,3,3-trifluoropropanoate (2) and thiobenzophenone (7a) in toluene underwent a reaction at 50°; the only product detected in the reaction mixture was thiirane 8a (Scheme 2). With the less reactive thiocarbonyl compounds 9H-xanthene-9-thione (7b) and 9H-thioxanthene-9-thione (7c) as well as with 1,3-thiazole-5(4H)-thione 12, diazo compound 2 reacted only in the presence of catalytic amounts of Rh2(OAc)4. In the cases of 7a and 7b, thiiranes 8b and 8c, respectively, were the sole products (Scheme 3). The crystal struture of 8c has been established by X-ray crystallography (Fig.). In the reaction with 12, desulfurization of the primarily formed thiirane 14 gave the methyl 3,3,3-trifluoro-2-(4,5-dihydro-1,3-thiazol-5-ylidene)propanoates (E)-and (Z)-15 (Scheme 4). A mechanism of the Rh-catalyzed reaction via a carbene addition to the thiocarbonyl S-atom is proposed in Scheme 5.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790604
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  • 108
    Electronic Resource
    Electronic Resource
    Duplex Stability of 7-Deazapurine DNA: Oligonucleotides containing 7-bromo- or 7-iodo-7-deazaguanine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1549-1558 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The oligonucleotide building blocks, the phosphonates 1a, b and the phosphoramidites 2a, b derived from 7-iodo- and 7-bromo-7-deaza-2′-deoxyguanosines 3a, b were prepared. They were employed in solid-phase oligonucleotide synthesis of the alternating octamers d(Br7c7G-C)4 (8) and d(I7c7G-C)4 (9) as well as the homo-oligonucleotides d[(Br7c7G)5-G] (11) and d[(I7c7G)5-G] (12). The melting profiles and CD spectra of oligonucleotide duplexes were measured. The Tm values as well as the thermodynamic data were determined and correlated to the major-groove modification of this DNA. The self-complementary octamers 8 and 9 form more stable duplexes compared to the parent oligomer d(G-C)4. The heteroduplex of d[(I7c7G)5-G] (12) with d(C6) is slightly destabilized (ΔTm = -12°) over that of d[(c7G)5-G] with d(C6). However, the complex of 12 with poly(C) is more stable than that of d[(c7G5-G)] with poly(C).
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790605
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  • 109
    Electronic Resource
    Electronic Resource
    The Crystal and Molecular Structure of [Co(tmen)3](C7H7SO3)3·2 H2O and [Co(tmen)3](CH3SO3)2·EtOH·1/2 H2O (tmen = 2,3-dimethylbutane-2,3-diamine) (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1607-1610 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The molecular structure of the two complex ions [Co(tmen)3]3+ and [Co(tmen)3]2+ (tmen = 2,3-dimethylbutane-2,3-diamine) have been determined at 100 K. They show a strong trigonal distortion of the octahedral CoN6 core. The twist angle is 26.2(2.8)° for CoII and 43.9(9)° for CoIII. Average Co—N distances are 2.193(13) Å (CoII) and 1.997(2) Å (CoIII), 0.02 Å longer than for the main body of CoIII amines.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790610
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  • 110
    Electronic Resource
    Electronic Resource
    Hydrolytical Cleavage of TAR-RNA, the trans-Activation Responsive Region of HIV-1, by a Bis(guanidinium) Catalyst Attached to Arginine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1967-1979 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Guanidinium compounds imitating the bis(arginine) structural motif of staphylococcal nuclease (e.g. 3) are known to be powerful catalysts for phosphoryl transfer reactions in dipolar aprotic solvents. Compound 3 also accelerates the hydrolysis of RNA (H2O, pH 7). However, due to diminished substrate affinity in H2O, the rate effects are less pronounced in aqueous than in aprotic solution. To test if a synthetic ribonuclease may be derived from the bis(guanidinium) moiety of 3 by the addition of RNA-binding substructures, the TAR sequence of HIV-1 was chosen as a target. The arginine residue of compound 4 serves as an extremely simplified mimic of tat, a protein responsible for boosting the viral transcription by complex formation with TAR. Here, we present the synthesis of 4 and its ability to bind and to cleave efficiently the truncated TAR sequence 1. In addition, the synthesis of an acridine arginine conjugate, 19, is reported in preliminary form. Compound 19 associates with 1 and completely blocks the cleavage induced by 4.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790719
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  • 111
    Electronic Resource
    Electronic Resource
    Lipid Masking and Reactivation of Angiotensin Analogues (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2023-2034 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Studies on post-angioplastic restenosis have shown the implication of angiotensin II (Ang) as a myoproliferative mediator. The antiproliferative efficacy of non-peptide Ang antagonists on the rat carotid model is of 50%, whereas a continuously infused peptide antagonist at low doses totally blocks neointimal growth. To explore the feasibility of depot forms of Ang that may be introduced during angioplasty and thus prevent restenois, lipid-masked Ang analogues of the following general structure were prepared: [Xxx°, Yyy1]Ang with Xxx = decanoyl or palmitoyl and Yyy = Ser, Cys, Asp, β-lactoyl, 3-mercaptopropanoyl or succinyl. All fatty acylated peptides [Xxx°, Yyy1]Ang were practically inactive, and O- or S-esterified Ser and Cys peptides underwent intramolecular transcylation giving inactive Nx-acylated peptides. O-Acylated [β-mercaptopropanoyl1]Ang were easily hydrolyzed into their biologically active[Yyy1]Ang forms, either by mild saponification or by lipase activity.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790723
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  • 112
    Electronic Resource
    Electronic Resource
    From Inactive Nortopsentin D, a Novel Bis(indole) Alkaloid Isolated from the Axinellid Sponge Dragmacidon sp. from Deep Waters South of New Caledonia, to a Strongly Cytotoxic Derivative (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2075-2082 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nortopsentin D (5), a bis(indole) alkaloid unique for bearing a 2-amino-methylimidazole appendage at the central 1H-imidazol-5(4H)-one nucleus, was isolated in abundance, besides the putative biogenetic precursor 6 of its appendage, from the deep-water axinellid sponge Dragmacidon sp. Structural elucidation of 5 by NMR and MS methods heavily relied on its N-methyl derivatives 8-11. Unusually for topsentin-type structures, natural 5 and semisynthetic methyl derivatives 8 and 10 proved inactive on KB tumoural cells, while introduction of the last three methyl groups, amazingly led to highly cytotoxic 11.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790804
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  • 113
    Electronic Resource
    Electronic Resource
    Model Studies for the Coenzyme-B12-Catalyzed Methylmalonyl→Succinyl Rearrangement. The Importance of Hydrophobic Peripheral Associations (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2100-2113 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interaction between a vitamin B12 derivative containing a peripheral C18 alkyl chain (see 1a) and a (methyl)thiomalonate substrate bearing alkyl chains of various length at the thioester group (see 5) was investigated. A catalytic cycle was established for the methylmalonyl→succinyl rearrangement by using electrochemistry and photolysis (see Scheme 3). Increased yields of the succinate relative to the reduction product were obtained (2:3 ratio), when the reaction was run in MeOH/H2O, and when both the substrate and the catalyst had an octadecyl substituent capable of hydrophobic interactions.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790807
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  • 114
    Electronic Resource
    Electronic Resource
    Electrospray-Ionization Mass Spectrometry. Part III.Part II:[1] Acid-Catalyzed Isomerization of N,N′-Bis[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines by the Zip ReactionPresented by L.B. at the ‘29. Diskussionstagung der Arbeitsgemeinschaft Massenspektrometrie der Deutschen Physikalischen Gesellschaft’, on May 28-31, 1996, University of Bremen, Germany. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2152-2163 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electrospray tandem mass spectra (ESI-MS/MS) of the three N,N′-bis[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines 1-3 displayed the same fragment-ion signals. These isomers could not be differentiated by ESI-MS/MS, since their fragmentation patterns are similar. (E,E)-N-(3-[15N]Aminopropyl)-3,3′-bis(4- hydroxyphenyl)-N,N′-(butane-1,4-diyl)bis[prop-2-enamide] ([15N(1)])-(1) was synthesized in order to get further information about the fragmentation mechanisms. The comparison of the ESI-MS/MS of 1 and [15N(1)]-1 revealed a transamidation, the Zip reaction, under mass-spectral conditions of the [1 + H]+ ions. Because of this reaction, the three isomers 1-3 could not be distinguished.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790810
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  • 115
    Electronic Resource
    Electronic Resource
    Total Synthesis of Cyclothialidine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2219-2234 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A total synthesis of cyclothialidine (1), a new DNA gyrase inhibitor isolated from Streptomyces filipinensis, is described. The synthetic concept was tested by preparing the lactone 13 (Scheme 2) which contains the characteristic bicyclic core entity of 1. Key features of the synthesis of 1 are: preparation of 3,5-dihydroxy-2,6-dimethylbenzoic acid (23) from 3,5-dihydroxybenzoic acid (19) by two consecutive Mannich aminomethylation/hydrogenation sequences; benzylic N-bromosuccinimide bromination of an ester derivative 25 thereof and its subsequent coupling with Boc-Ser-Cys-OMe (11); cyclization of the ω-hydroxy acid 29 29 to the 12-membered lactone 30 using preferably Mitsunobu conditions; completion of the peptidic side chains of 1 using Boc strategy (Scheme 4). Optically pure cis-N-(tert-butoxycarbonyl)-3-hydroxy-L-proline ((-)-14) was obtained by resolution of the racemate via an efficient reaction sequence containing a lipase-catalyzed enantiospecific acetate hydrolysis (Scheme 3). The structure of natural 1 was confirmed by comparison with the synthetic material. The synthetic route described provides also easy access to analogues of 1, e.g., via the intermediate 30.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790816
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  • 116
    Electronic Resource
    Electronic Resource
    Donor/Acceptor-Substituted Tetraethynylethenes: Systematic Assembly of Molecules for Use as Advanced Materials (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2249-2281 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A comprehensive series of tetraethynylethenes (= 3,4-diethynylhex-3-ene-1,5-diynes, TEEs) bearing electrondonating (p-methoxyphenyl or p-aminophenyl) and/or electron-accepting (p-nitrophenyl) groups was prepared via [Pd]-catalyzed Sonogashira cross-coupling reactions. The electronic and photonic properties of these molecules were investigated with a special emphasis on the effects caused by degree and pattern of donor/acceptor substitution around the central TEE core. This analysis showed that intramolecular donor-acceptor interactions, as evidenced by a long-wavelength charge-transfer band, are considerably more effective in TEEs 44 and 46, with trans and cis, linearly-conjugated electronic pathways between donor and acceptor, than in 11, with a geminal, cross-conjugated electronic pathway. UV/VIS Spectroscopy revealed a steady bathochromic shift of the longest-wavelength absorption band (λmax) as the number of donor-acceptor conjugation paths increased upon changing from bis-arylated (11, 44, and 46) to tetrakis-arylated (14, 31, and 35) TEEs. The position of the longest-wavelength absorption was also found to be strongly dependent on the nature of the N-substituents in the R2NC6H4-donor groups. Electronic emission spectroscopic investigations demonstrated a considerable solvent dependency of the fluorescence of donor-acceptor-substituted TEEs such as 11 or 44, in agreement with the presence of highly polarized excited states in these molecules. Correspondingly, fluorescence spectra of TEEs bearing only donor or acceptor substituents showed little solvent dependency. The large majority of the donor/acceptor-substituted TEEs are thermally and environmentally stable molecules. They can be stored for months as solids in the air at room temperature, and many decompose only upon heating to temperatures above 200°. X-Ray analysis showed the conjugated C-atom scaffolds of 44, 46, and 67 to be essentially planar, whereas the aryl substituents in 28 and 30 are rotated out of the plane of the TEE core by varying degrees.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790818
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  • 117
    Electronic Resource
    Electronic Resource
    Mitteilungen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 913-913 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790330
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  • 118
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    Electronic Resource
    β-Peptides: Synthesis by Arndt-Eistert homologation with concomitant peptide coupling. Structure determination by NMR and CD spectroscopy and by X-ray crystallography. Helical secondary structure of a β-hexapeptide in solution and its stability towards pepsin (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 913-941 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The β-hexapeptide (H-β-HVal-β-HAla-β-HLeu)2-OH (2) was prepared from the component L-β-amino acids by conventional peptide synthesis, including fragment coupling. A cyclo-β-tri- and a cyclo-β-hexapeptide were also prepared. The β-amino acids were obtained from α-amino acids by Arndt-Eistert homologation. All reactions leading to the β-peptides occur smoothly and in high yields. The β-peptides were characterized by their CD and NMR spectra (COSY, ROESY, TOCSY, and NOE-restricted modelling), and by an X-ray crystal-structure analysis. β-Sheet-type structures (in the solid state) and a compact, left-handed or (M) 31 helix of 5-Å pitch (in solution) were discovered. Comparison with the analogous secondary structures of α-peptides shows fundamental differences, the most surprising one at this point being the greater stability of β-peptide helices. There are structural relationships of β-peptides with oligomers of β-hydroxyalkanoic acids, and dissimilarities between the two classes of compounds are a demonstration of the power of H-bonding. The β-hexapeptide 2 is stable to cleavage by pepsin at pH 2 in H2O for at least 60 h at 37°, while the corresponding α-peptide H-(Val-Ala-Leu)2-OH is cleaved instantaneously under these conditions. The implication of the described results are discussed.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790402
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    Novel Cholic-Acid-Type Sterones of Deltocyathus magnificus, a deep-water scleractinian coral from the Loyalty Islands, SW Pacific (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 982-988 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cholic-acid-type 3-keto steroids 1-8, all isolated from Deltocyathus magnificus (2-8 after CH2N2 treatment), are the first secondary metabolites obtained from a deep-water scleractinian. Steroids 1-3, 5, and 7 are new, their main structural oddities being loss of C(24) (see 5) or hydroxylation in the side chain (see 3 and 7). Steroids 4, 6, and 8, from the same coral, were previously known from other marine organisms.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790406
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  • 120
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    Acid-Catalysed Backbone Rearrangement of cholesta-6,8(14)-dienes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 989-998 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rearrangement of 5α- and 5β-cholesta-6,8(14)-dienes (13a and 13b, resp.) in the presence of anhydrous toluene-4-sulfonic acid in acetic acid leads to 5α- and 5β-12(13 → 14)-abeo-cholesta-8,13(17)-dienes (15a and 15b, resp.) via 5α- and 5β-cholesta-8,14-dienes (14a and 14b, resp.), respectively. Epimerization at C(20) of the spirosteradienes 15a and 15b occurs with increasing reaction time. Molecular-mechanics calculation of the relative stabilities of these compounds and of congeners thereof is in agreement with the observed reaction pathway.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790407
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  • 121
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    Asymmetric Synthesis of the Alkaloids mayfoline and N(1)-acetyl-N(1)-deoxymayfoline (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1085-1094 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The total syntheses of the spermidine alkaloids (-)-mayfoline (11) and (+)-N(1)-acetyl-N(1)-deoxymayfoline (12) are described. These macrocyclic lactams belong to the most interesting conjugates of the polyamine derivatives very commonly found in nature. The enantioselective syntheses were achieved through resolution of the methyl 3-amino-3-phenylpropanoate (2) by recrystallization of its (+)-L-tartrate salt. Construction of the 13-membered ring ensued through condensation, reductive ring expansion (internal bond cleavage), and finally a transamidation reaction involving a second ring expansion.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790417
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  • 122
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    Electrochemical Behaviour and Antioxidant Activity of Some Natural Polyphenols (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1147-1158 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of natural polyphenols (chlorogenic acid (9), cordigol (11), cordigone (12), danthrone (1), 1,5-dihydroxy-3-methoxyxanthone (2), eriosematin (7), flemichin D (8), frutinone A (6), mangiferin (4), quercetin (5), 1,3,6,7-tetrahydroxyxanthone (3) and verbascoside (10)) were investigated for their redox properties using cyclic voltammetry. The antioxidant properties of these compounds were also examined in two models, namely lipid peroxidation in rat synaptosomes and AAPH-mediated oxidation of serum albumin. Compounds with a catechol group (9, 4, 5, 3 and 10) were oxidized below 0.4 V and inhibited lipid peroxidation with IC50 values between 2 and 8 μM. Compounds having one or more isolated phenolic groups and showing an oxidation potential between 0.45 and 0.8 V (11, 12 and 8) inhibited lipid peroxidation with IC50 between 7 and 9 μM, except 2 (0.45 V), danthrone (0.96 V) and eriosematin which showed no or modest antioxidant activity. Some of the investigated compounds also protected albumin from oxidation, but no structure-activity relationship was apparent, suggesting that other factors beside redox potential influence this activity.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790422
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  • 123
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    Glycosylidene Carbenes. Part 23. Regioselective glycosidation of deoxy- and fluorodeoxy-myo-inositol derivativesDedicated to Edgar Heilbronner on the occasion of his 75th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1169-1191 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: To demonstrate the relevance of the kinetic acidity of individual OH groups for the regioselectivity of glycosylation by glycosylidene carbenes, we compared the glycosylation by 1 of the known triol 2 with the glycosylation of the diol D-3 and the fluorodiol L-4. Deoxygenation with Bu3SnH of the phenoxythiocarbonyl derivative of 5 (Scheme 1) or the carbonothioate 6 gave the racemic alcohol (±)-7. The enantiomers were separated via the allophanates 9a and 9b, and desilylated to the deoxydiols D- and L-3, respectively. The assignment of their absolute configuration is based upon the CD spectra of the bis(4-bromobenzoates) D- and L-10. The (+)-(R)-1-phenylethylcarbamates 13a and 13b (Scheme 2) were prepared from the fluoroinositol (±)-11 via (±)-4 and the silyl ether (±)-12 and separated by chromatography. The absolute configuration of 13a was established by X-ray analysis. Decarbamoylation of 13a ( → L-12) and desilylation afforded the fluorodiol L-4. The H-bonds of D-3 and L-4 in chlorinated solvents and in dioxane were studied by IR and 1H-NMR spectroscopy (Fig. 2). In both diols, HO—C(2) forms an intramolecular, bifurcated H-bond. There is an intramolecular H-bond between HO—C(6) and F in solutions of L-4 in CH2Cl2, but not in 1,4-dioxane; the solubility of L-4 in CH2Cl2 is too low to permit a meaningful glycosidation in this solvent. Glycosidation of D-3 in dioxane by the carbene derived from 1 (Scheme 3) followed by acetylation gave predominantly the pseudodisaccharides 18/19 (38%), derived from glycosidation of the axial OH group besides the pseudodisaccharides 16/17 (13%) and the epoxides 20/21 (7%), derived from protonation of the carbene by the equatorial OH group. Similarly, the reaction of L-4 with 1 (Scheme 4) led to the pseudodisaccharides 28/29 (46%) and 26/27 (14%), derived from deprotonation of the axial and equatorial OH groups, respectively. Formation of the epoxides involved deprotonation of the intramolecularly H-bonded tautomer, followed by intramolecular alkylation, elimination, and substitution (Scheme 4). The regio- and diastereoselectivities of the glycosidation correlate with the H-bonds in the starting diols.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790424
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  • 124
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    A Novel Asymmetric Benzoin Reaction Catalyzed by a Chiral Triazolium Salt. Preliminary communication (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1217-1221 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using the chiral triazolium salt 1 as catalyst, a novel asymmetric variant of the benzoin reaction is reported. For the first time, the scope of the method is extended to a broader range of aromatic aldehydes 2, affording the acyloins 3a-h in yields of 22-72% and enantiomeric excesses up to 86%.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790427
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  • 125
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    Erratum (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1239-1239 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790429
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  • 126
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    Variation and SelectionDedicated to Professor Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1260-1278 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ninety years ago, chemical methods were used in solving biological problems. In our days, methods of evolutionary biology are going to be used in solving problems of synthetic chemistry.
    Additional Material: 8 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790504
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  • 127
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    Oligosaccharide Analogues of Polysaccharides. Part 8. Orthogonally Protected Cellobiose-Derived Dialkynes. A Convenient Method for the Regioselective Bromo- and Protodegermylation of Trimethylgermyl- and Trimethylsilyl-protected DialkynesDedicated to Vlado Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1279-1294 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cellobiose-derived dialkynes 14 and 15 were prepared by glycosidation of the acceptor 9 with the thioglycosides 12(82%) and 13(85%), respectively. The acceptor 9 was prepared from the known alcohol 2 via the lactone 7 in five steps (48% overall), and the donors 12 and 13 were prepared from the alkynylated anhydroglucose derivative 10 (60% overall). Acetolytic debenzylation of 14 and 15 (→16 and 17, resp.) followed by deacylation of 16 yielded 60% of the cellobiose-derived dialkyne 18. Deacylation of 14 (→19), methoxymethylation (→20) and trimethylgermylation led to the orthogonally protected dialkyne 21 (69% overall). Protodesilylation of 21 with K2CO3/MeOH gave 22 (90%), while the Me3Ge group was selectively removed with CuBr (19 mol-%) in THF/MeOH to give 20 (95%). Treatment of 21 with aqueous HCl solution led to 19 (80%). Bromodegermylation of 21 (NBS/AgOOCCF3) led to a mixture of 23 (85%) and 24 (11%). Similar conditions using CuBr instead of AgOOCCF3 gave exclusively the bromoalkyne 23 (93%). The temperature dependence of the δ values of the OH resonances of 18 in (D6)DMSO evidence a strong intramolecular H-bond between C(5′)—O…HO—C(5).
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790505
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  • 128
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    Analogs of Cinchona Alkaloids Incorporating a 9,9′-Spirobifluorene MoietyDedicated to Professor Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1338-1360 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Cinchona alkaloid analogs (+)- and (-)-5 with a quinuclidine-2-methanol residue attached to C(2) of a 9,9′-spirobifluorene moiety were prepared as a racemic mixture by reacting lithiated 2-bromo-9,9′-spirobifluorene 7 with (2-ethoxycarbonyl)quinuclidine (±)-6 to give ketone (±)-8, followed by diastereoselective reduction with diisobutylaluminum hydride (DIBAL-H). The absolute configuration at C(9) and C(8), i.e., at the methanol bridge and the adjacent quinuclidine C-atom, in the two enantiomers of 5 is identical to the configuration at the corresponding centers in (-)-quinine (1) and (+)-quinidine (2), respectively. For the optical resolution of (±)-5, a chiral stationary phase for HPLC was prepared by covalently bonding quinine via a thiol spacer to a silica-gel surface. The enantiomer separation was accomplished at an α value of 1.61 with (±)-5 being eluted last, in agreement with 1H-NMR studies in CDCl3 which showed that (+)-5 underwent a more stable host-guest association with quinine than (-)-5. 1H{1H} Nuclear Overhauser effect (NOE) difference spectroscopical analysis of the host-guest associations with quinine in CDCl3, combined with computer-model examinations, allowed the assignment of the absolute configurations as (+)-(8R,9S)-5 and (-)-(8S,9R)-5. A detailed conformational analysis displayed excellent agreement between the results of computational methods (Monte Carlo multiple minimum simulations, analyses of the total energy as a function of the flexible dihedral angles in the molecule) and 1H{1H}-NOE difference spectroscopical data. It was found that (-)-5 and (+)-5 differ significantly in their conformational preference from their natural counterparts quinine (1) and quinidine (2). Whereas the natural alkaloids prefer the ‘open’ conformation, with the quinuclidine N-atom pointing away from the quinoline ring, analog (±)-5 adopts preferentially (by ca. 4 kcal mol-1) a ‘closed’ conformation, in which the quinuclidine N-atom points into the cleft of the 9,9′-spirobifluorene moiety. Since the basic quinuclidine N-atom in the ‘closed’ conformation is sterically shielded from forming strong H-bonds, the new Cinchona alkaloid analogs form less stable host-guest associations via H-bonding than quinine or quinidine.
    Additional Material: 10 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790509
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  • 129
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    A New Approach to the Synthesis of Long-Chain Polypropionates Based on the Diels-Alder Monoadditions of 2,2′-Ethylidenebis[3,5-dimethylfuran]Part of the Ph.D. thesis of J.A., University of Lausanne, 1995. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1393-1414 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acidic condensation of 2,4-dimethylfuran with acetaldehyde provided 2,2′-ethylidenebis[3,5-dimethylfuran] (7) which added 1 equiv. of methyl bromopropynoate to give a major adduct 8. Regio- and stereoselective hydroboration of the latter 7-oxanorbornadiene derivative followed by alcohol protection and methanolysis of its β-bromoacrylate moiety gave (1RS,2RS,4RS,5SR,6SR,1′RS)-methyl 4-[1′-(3″,5″-dimethylfuran-2″-yl)ethyl]-3,3-dimethoxy-6-exo-[(2-methoxy)ethoxy]-1,5-endo-dimethyl-7-oxabicyclo[2.2.1]heptane-2-endo-carboxylate (24) (Schemes 2 and 3). Reduction of 24 with LiAlH4, followed by H2O and MeOH elimination gave the 3-methyl-idene-7-oxanorbornan-2-one derivative 26 which underwent 7-oxa ring opening through a SN2′ type of reaction with Me2CuLi (Scheme 4). Stereoselective hydrogenation and ketone reduction provided (1RS, 2SR,3RS,4RS,5RS,6RS,1′SR)-1- [1″-(3 ″,5″-dimethylfuran-2″-yl)]-c-3-ethyl-c-5-[(2-methoxyethoxy)m e-c-ethyl-c-c-5-(2-methoxyethoxy)methoxy]-t-4,t-6-dimethyl-cyclohexane-r-1,c-2-diol (32), the oxidative cleavage of which with Pb(OAc)4 generated a 6-oxo-aldehyde 33 (Schemes 4 and 5). Chemoselective protection of 33 and chemo- and stereoselective reductions generated (2RS,3RS,4SR,5SR,6SR,7RS)-7-(3′,5″-dimethylfuran-2′-yl)-2-ethyl-6-hydroxy-4-[(2-methoxyethoxy)methoxy]-3,5-dimethyloct-1-yl pivaloate (36) and its 4-hydroxy 6-epimer 40 (12 and 13 steps, resp., from adduct 8; Scheme 5). Oxidation of the furan ring of 36 led to a (2RS,3SR,4RS,5SR,6RS,7RS)-7-ethyl-3,5,8-trihydroxy-2,4,6-trimethyl-octanoic acid derivative 44, a polypropionate fragment with six contiguous stereogenic centres (Scheme 6).
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  • 130
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    Isomerisierung von (all-E)-Cycloviolaxanthin Herstellung und Charakterisierung von (9Z)-und (13Z)-Cycloviolaxanthin (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1444-1450 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Isomerization of (all-E)-Cycloviolaxanthin. Preparation and Characterization of (9Z)- and (13Z)-CycloviolaxanthinFrom (all-E)-cycloviolaxanthin (ex Capsicum annuum), (9Z)- and (13Z)-cycloviolaxanthin were prepared in a highly pure state. The configuration of both isomers was determined by means of UV/VIS, NMR, and CD spectroscopy.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790517
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    Mitteilungen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1518-1518 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790523
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  • 132
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    Intramolecular Vinyl Radical Cyclization Reactions of cyclohexadienes derived from sequential additions to (arene)(tricarbonyl)chromium complexes. Preliminary communicationDedicated to the memory of our late colleague Wolfgang Oppolzer (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1533-1536 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cyclohexa-1,3-dienes 7-10 with a 1,5,6-substitution pattern were prepared in a one-pot reaction sequence by sequential addition of MeLi and propargyl bromides to the tricarbonylchromium complex 1a. These products were subjected to radical cyclization procedures. Vinyl radical generation by Bu3Sn addition to the propargyl group in 7-10 was followed by regio- and diastereoselective intramolecular ring closure. Two different 5-exo-trig cyclizations are possible via different vinyl radical intermediates and cyclization to one or the other of the termini of the cyclohexadiene moiety. Internal alkynes reacted to yield exclusively the cis-fused hexahydroindene products 12-14, whereas the terminal alkyne yielded bicyclo[3.2.1]octenes 11 as sole products.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790603
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    The First Asymmetric Intramolecular Stetter Reaction. Preliminary Communication (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1899-1902 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The first asymmetric intramolecular Stetter reaction is reported, using the chiral triazolium salt 1 as catalyst. Starting from the easily accessible 4-(2-formylphenoxy)but-2-enoates 2, this protocol opens up an enantioselective pathway to the benzo-annulated pyran-4-ones (chroman-4-ones) 3a-h with good yields and enantiomeric excesses of up to 74%.
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    URL: http://dx.doi.org/10.1002/hlca.19960790712
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    Reactions of 3-(Prop-2-enylidene)azetidin-2-ones with 4,5-Dihydro-3H-1,2,4-triazole-3,5-dionesDedicated to Prof. Gunther Seitz. Marburg. on the occasion of his 60th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1925-1929 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 3-(2-propenylidene)-β-lactams 1a-c react as semicyclic dienes with the dihydrotriazole-diones 2 in a stereoselective fashion leading to the cycloadducts 3. The 3′,3′-disubstituted derivative 4 gives no [4 + 2]-cycloaddition products but forms the adducts 5 and 6. Sterochemistry and reaction pathways are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790715
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  • 135
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    Mechanistic Details of the Fe+-Mediated C—C and C—H Bond Activations in Propane: A Theoretical Investigation (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1939-1956 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Quantum-chemical calculations employing a density-functional theory/Hartree-Fock hybrid method (B3LYP) have been used to explore the mechanistic details of the C—C and C—H bond-activation processes in propane mediated by a bare Fe+ ion. While the theoretically predicted results are in complete accord with all available experimental data, they give rise to a different mechanistic picture than envisaged previously. In contrast to earlier speculation, the activation barriers for the initial insertion steps of Fe+ into a C—H or C—C bond are found to be significantly below the Fe+ + C3H8 channel. The rate-determining steps for both, the C—C and the C—H bond activation branches of the [FeC3H8]+ potential-energy surface rather occur late on the respective reaction coordinates and are connected with saddle points of concerted rearrangement processes. The C—C bond activation, which leads to the exothermic reductive elimination of methane, occurs via the C—C inserted species and not as a side channel originating from a C—H inserted ion, as assumed hitherto. For the C—H bond-activation processes, which finally results in the exothermic expulsion of molecular hydrogen, two energetically similar reaction channels for an [1,2]-elimination exist. The results clearly show, that an [1,3]-H2-elimination mechanism cannot compete with the [1,2]-elimination paths, in line with the experimental findings. Overall, a lower energy demand for the reductive elimination of methane compared to the loss of H2 is obtained, straightforwardly explaining the preference of the former process observed experimentally.
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790801
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  • 137
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    Synthesis of 1,2,5-Thiadiazepine Derivatives by Ring Enlargement of 1,2-Thiazetidin-3-one 1,1-Dioxides with 3-Amino-2H-azirines (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2067-2074 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: At 0° in MeCN, 2,2-disubstituted 3-amino-2H-azirines 1 and 4,4-disubstituted 1,2-thiazetidin-3-one 1,1-dioxides 7 react smoothly to give 1,2,5-thiadiazepin-6-one 1,1-dioxides of type 8 (Scheme 2). The reaction mechanism of this regiospecific ring enlargement to seven-membered heterocycles follows previously described pathways. The structures of 7a and 8b were established by X-ray crystallography (see Figs. 1 and 2).
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790803
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  • 138
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    Biosynthesis of δ-Jasmin Lactone ( = (Z)-Dec-7-eno-5-lactone) and (Z,Z)-Dodeca-6,9-dieno-4-lactone in the Yeast Sporobolomyces odorus (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2088-2099 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (all-Z)-(9,10,12,13,15,16-2H6)Octadeca-9,12,15-trienoic acid ( = α-linolenic acid; D6-4) was synthesized to investigate the biochemical formation of linolenic-acid-derived aroma compounds in cultures of the yeast Sporobolomyces odorus, using an established gas chromatographic/mass spectrometric (GC/MS) method. Three compounds were identified as labeled: (Z)-dec-7-eno-5-lactone (δ-jasmin lactone), (Z,Z)-dodeca-6,9-dieno-4-lactone, and (2E,4Z)-hepta-2,4-dienoic acid. Both lactones were biosynthesized mostly under conservation of the initial configuration from their corresponding oxygenated linolenic-acid intermediates. The application of (13S,9Z,11E,15Z)-13-hydroxy(9,10,12,13,15, 16-2H6)octadeca-9,11,15-trienoic acid (D6-7) as a OH-functionalized precursor of δ-jasmin lactone allowed to gain insight into the stereochemical course of the biosynthesis to both enantiomers of this lactone. In this experiment, 88.3% of the metabolized labeled precursor was transformed under retention of the original configuration of the (R)-enantiomer. This investigation is also a contribution to a better understanding of the C=C bond isomerization steps which took place during the β-oxidative degradation of the substrate.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790806
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  • 139
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    5,5′-Bi-5H-cyclopenta[2,1-b;3,4-b′]dipridinylidene, a New Bridging Ligand for Metal Complexes. Preliminary Communication (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2176-2179 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 5,5′-Bi-5H-cyclopenta[2,1-b;3,4-b′]dipyridinylidene(1) was synthesized in three steps from 9,10-phenanthroline and characterized by UV/VIS and NMR spectroscopy, mass spectrometry, and cyclic voltammetry. Its ability to act as a bridging ligand is demonstrated by the synthesis of the complexes [Ru(bpy)2(1)](PF6)2 (6) and [{Ru(bpy)2}2(1)](PF6)4 (7) (bpy = 2,2′-bipyridine).
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790812
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  • 140
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    Synthesis and Structure of Functionalized Cyclododecadiynes and -dienes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2164-2175 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cyclododecadiynes 3b-d, 8b-d, and 10b-c with functionalities in two propargylic positions, as well as the monofunctionalized diyne 13b have been prepared from simple open-chain building blocks. In the DMPU ( = N,N'-dimethylpropyleneurea)-assisted ring-closing alkylation of 1,7-diynes, the twelve-membered ring compounds have been prepared in yields of 16-55%. The preparation of the diene-diyne 15 and the cyclododeca-5,11-diyne-1,4-dione 18 are described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790811
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  • 141
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    Synthesis of Enantiomerically Pure Carbohydrate-Derived Annulated Cyclopentadienes and Ferrocenes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2201-2218 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cyclopentadienes 3a/b/c, 8b/c, 12a/b/d, and 16a/b/d were prepared as mixtures of regioisomers from the D-mannitol-derivatives 1,6,10, and 14 and transformed into the ferrocenes 17, 18, and 19 (73%; 38:17:45), 23, 24, and 25 (70%; 6:42 :52), 26 (31%), and 27 (27%), respectively. Deprotection of 17-19 with HCl/MeOH gave the H2O-soluble ferrocenes 20-22; chloromercuration and iodination of 17 via 29 led to the C2-symmetric diiodo-ferrocene 30. The mono(chloromercurio) derivative 28, obtained as a by-product, was also transformed into 29. The structure of the ferrocenes 18 and 19, and of the bis(chloromercurio)ferrocene 29 has been established by X-ray analysis. The starting cyclopentadienes 3 were obtained in 50% yield from 1 by dialkylation of CpNa, followed by thermolysis of the spiro-annulated 2. Similarly, dimesylate 6 (from 4) gave the spiro-annulated diene 7 and the annulated dienes 8 in about equal amounts; thermolysis transformed 7 into 8 (62%). The dienes 12 were prepared in 15% yield from the ditriflate 10 via 11, the dimesylate 9 proving insufficiently reactive, and the dienes 16 (49%) from 14 via 15.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790815
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  • 142
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    Synthesis of Fused Triazoles as Probes for the Active Site of Retaining β-Glycosidases: From Which Direction Is the Glycoside Protonated? (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2190-2200 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structure of the β-glycosidase inhibitors 1-7 and 10-13 suggests that protonation of O-C(1) (the glycosidic O-center) of the substrate by a carboxy group of the retaining β-glycosidases does not occur in the plane perpendicular to the ring of the glycon (β-side; ‘from the top’), but in the plane of the ring (‘from the side’). The triazoles 17 and 18 have been prepared in six steps from the L-xylofuranose 21. They possess a CH group instead of the N-center of the related tetrazoles 4 and 5, corresponding to the glycosidic O-atom, and a very similar structure, both in solution and in the solid state. Unlike the tetrazoles, however, which are good-to-medium inhibitors of retaining β-glycosidases, the triazoles do not inhibit the β-glycosidases from sweet almonds, snail, and bovine liver, and only slightly inhibit the β-glucosidase from Caldocellum saccharolyticum. This is in keeping with the proposed direction of protonation in the plane of the saccharide ring and with modelling studies, docking 4 into the active site of the white clover cyanogenic β-glucosidase and 6 into the E. coli β-galactosidase and the Lactococcus lactis 6-phospho-β-galactosidase.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790814
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  • 143
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    Double-Bond Shifts in [4n]Annulenes as a New Principle for Molecular Switches: First Results with Dimethyl Heptalene-1,2- and -4,5-dicarboxylatesDedicated to Vlado Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2282-2315 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new concept for molecular switches, based on thermal or photochemical double-bond shifts (DBS) in [4n]annulenes such as heptalenes or cyclooctatetraenes, is introduced (cf. Scheme 2). Several heptalene-1,2- and -4,5-dicarboxylates (cf. Scheme 4) with (E)-styryl and Ph groups at C(5) and C(1), or C(4) and C(2), respectively, have been investigated. Several X-ray crystal-structure analyses (cf. Figs. 1-5) showed that the (E)-styryl group occupies in the crystals an almost perfect s-trans-conformation with respect to the C=C bond of the (E)-styryl moiety and the adjacent C=C bond of the heptalene core. Supplementary 1H-NOE measurements showed that the s-trans-conformations are also adopted in solution (cf. Schemes 6 and 9). Therefore, the DBS process in heptalenes (cf. Schemes 5 and 8) is always accompanied by a 180° torsion of the (E)-styryl group with respect to its adjacent C=C bond of the heptalene core. The UV/VIS spectra of the heptalene-1,2- and -4,5-dicarboxylates illustrated that it can indeed be differentiated between an ‘off-state’, which possesses no ‘through-conjugation’ of the π-donor substituent and the corresponding MeOCO group and an ‘on-state’ where this ‘through-conjugation’ is realized. The ‘through-conjugation’, i.e., conjugative interaction via the involved s-cis-butadiene substructure of the heptalene skeleton, is indicated by a strong enhancement of the intensities of the heptalene absorption bands I and II (cf. Tables 3-6). The most impressive examples are the heptalene-dicarboxylates 11a, representing the off-state, and 11b which stands for the on-state (cf. Fig.8).
    Additional Material: 12 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790819
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  • 144
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790101
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  • 145
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    Instructions to Authors (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1-19 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790131
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  • 146
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    Effect of non-planarity on the spin distribution in the radical anions containing the stilbene or the azobenzene π-system: An ESR and ENDOR Study (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 307-318 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The radical anions of 5H-dibenzo[a,d]cycloheptene (9), 5 H-dibenzo[c,f][1,2]diazepine (10), 5,6-dihydrodibenzo[a,e]cyclooctene (11), 5,6-dihydrodibenzo[c,g][1,2]diazocine (12), and (E)-2,2,5,5-tetramethyl-3,4-diphenylhex-3-ene (13) were characterized by ESR and ENDOR spectroscopy. Their hyperfine data were compared with those previously reported for radical anions also containing the stilbene or the azobenzene π-system. Whereas the π-spin distribution in the radical anions of the stilbene series is only moderately sensitive to deviations of the π-system from planarity, the radical anions of the azobenzene series respond to steric strain by shifting the π-spin population from the benzene rings to the azo group. This finding is impressively demonstrated by the similar hyperfine data for 9· - and 11· - which contrast with the strongly highly hindered 13· -. A. plausible interpretation is readily provided by the electron affinities of the constituent π-moieties in stilbene and azobenzene. While those of benzene and ethene are both comparatively low, the azo group has a considerably higher electron affinity.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790130
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  • 147
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    Theoretical Calculations of β-Lactam Antibiotics. Part VII. Influence of the solvent on the basic hydrolysis of the β-lactam ring (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 353-362 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We used semi-empirical and ab initio calculations to investigate the nucleophilic attack of the OH- ion on the β-lactam carbonyl group. Both allowed us to detect reaction intermediates pertaining to proton-transfer reactions rather than the studied reaction. We also used the PM3 semi-empirical method to investigate the influence of the solvent on the process. The AMSOL method predicts the occurrence of a potential barrier of 20.7 kcal/mol due to the desolvation of the OH- ion in approaching the β-lactam carbonyl group. Using the supermolecular approach and a H2O solvation sphere of 20 molecules around the solute, the potential barrier is lowered to 17.5 kcal/mol, which is very close to the experimental value (16.7 kcal/mol).
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790204
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  • 148
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    New Triterpene Saponins from Herniaria glabraDedicated to Prof. K. Hiller on the occasion of his 65th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 385-390 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three new saponins 1-3 were isolated from Herniaria glabra by means of prep. HPLC and TLC. The structures were established mainly by a combination of 2D-NMR techniques (COSY, TOCSY, ROESY, HMQC, and HMBC) as O-α-L-rhamnopyranosyl-(1→4)-O-β-D-glucopyranosyl-(1-→6)-O-[β-D-glucopyranosyl-(1→2)]-β-D-glucopyranosyl medicagen-28-ate (herniaria saponin 4; 1), O-β-D-glucopyranosyl-(1→3)-O-α-L-rhamnopyranosyl-(1→2)-O-[β-(3R)-D-apiofuranosyl-(1→3)]-β-D-4-O-acetylfucopyranosyl 3-O-(β-D-glucuronopyranosyl)-16α-hydroxymedicagen-28-ate (herniaria saponin 5; 2), and O-α-L-rhamnopyranosyl-(1→4)-O-β-D-glucopyranosyl-(1→6)-O-[β-D-6-O-acetylglucopyra nosyl-(1→2)]-β-D-glucopyranosyl medicagen-28-ate (herniaria saponin 6; 3).
    Additional Material: 2 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790207
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  • 149
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    Nucleotides. Part XLVIPart XLV: [1].. The synthesis of phospholipid conjugates of antivirally active nucleosides by the improved phosphoramidite methodology (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 426-438 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The application of the improved phosphoramidite strategy for the synthese of oligonucleotides using β-eliminating protecting groups to phospholipid chemistry offers the possibility to synthesize phospholipid conjugates of AZT (6) and cordycepin. The synthesis of 3′-azido-3′-deoxythymidine (6) was achieved by a new isolation procedure without chromatographic purification steps in an overall yield of 50%. Protected cordycepin ( = 3′-de-oxyadenosine) derivatives, the N6,2′-bis[2-(4-nitrophenyl)ethoxycarbonyl]cordycepin (12) and the N6,5′-bis[2-(4-nitrophenyl)ethoxycarbonyl]cordycepin (13) wre prepared by known methods and direct acylation of N6-[2-(4-nitrophenyl)ethoxycarbonyl]cordycepin (9), respectively. These protected nucleosides and the 3′-azido-3′-de-oxythymidine (6) reacted with newly synthesized and properly characterized lipid-phosphoramidites 21-25, catalyzed by 1H-tetrazole, to the corresponding nucleoside-phospholipid conjugates 26-38 in high yield. The deprotection was accomplished via β-elimination with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in aprotic solvents to give analytically pure nucleoside-phospholipid diesters 39-51 as triethylammonium or sodium salts. The newly synthesized compounds were characterized by elemental analyses and UV and 1H-NMR spectra.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790210
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  • 150
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    Ring-Opening Reaction of a Dihydrozirconafuran (= 2,5-Dihydro-1,2-oxazirconole): An equilibrium studied by NMR methods, and its chemical consequences (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 454-460 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The dihydrozirconafuran 3 (2,2-di(cyclopenta-2,4-dien-l-yl)-2,5-dihydro-5,5-dimethyl-3,4-bis(trimethylsilyl)-1,2-oxazirconole) is a valuable synthetic equivalent to the 14-electron fragment Cp2ZrThroughout this paper, Cp=η5-C5H5. The reactions of this metallacycle suppose an equilibrium with an ‘opened’ form [ZrCp2(RC≡CR)(acetone)], the latter paving the way to numerous ligand replacements. In a solution containing the dihydrozirconafuran and acetone, these reactions are reversible and degenerate, and can be studied by NMR methods: by two-dimensional spectroscopy and by observation of the isotope-induced chemical shifts that occur upon incorporation of (D6)acetone into the metallacycle. The findings give an indirect proof of the described equilibrium.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790213
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  • 151
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    Synthesis of (all-E,2R,2′R)-oscillol (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 499-506 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optically active (all-E,2R,2′R)-oscillol (= (all-E,2R,2′R)-3,4,3′,4′-tetradehydro-1,2,1′,2′-tetrahydro-ψ,ψ-carotene-1,2,1′,2′-tetrol; 1) was synthesized according to the C10 + C20 + C10 = C40 strategy, applying the Wittig reaction to couple the synthons 4 and 6. The chiral centre was introduced by a Sharpless dihydroxylation of 3-methylbut-2-enyl 4-nitrobenzoate (8).
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790217
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  • 152
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    Synthesis of Tenuilobine, a Bis-polyamine Alkaloid from Oncinotis tenuiloba, and Its Transamidation to Isotenuilobine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 541-547 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: From the leaves of Oncinotis tenuiloba STAPF, a novel polyamine alkaloid, tenuilobine (9), was isolated. This paper presents the synthesis of 9, as well as the base-catalyzed Zip reaction of 9, leading to the transamidation product isotenuilobine (10). The structure of 10 was further confirmed by 2D-NMR correlation spectroscopy. For analytical purposes, the bis-polyamines 9 and 10 were converted into their pentaacetyl derivatives 12 and 11, respectively, which were readily separable by reverse-phase HPLC.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790221
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  • 153
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    Synthesis of 1,6-Dideoxynojirimycin, 1,6-Dideoxy-D-allo-nojirimycin, and 1,6-Dideoxy-D-gulo-nojirimycin via Asymmetric Hetero-Diels-Alder Reactions (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 560-567 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Asymmetric Diels-Alder reaction of sorbaldehyde O-methyloxime 1d with chiral chloronitroso derivative 2 of D-mannose, followed by osmylation of the primary cycloadduct, led to diol 6a with excellent enantioselectivity (ee 〉 99%; Scheme 1). Catalytic hydrogenolysis of 6a gave 1,6-dideoxy-D-allo-nojirimycin (7a). Nucleophilic ring opening of cyclic sulfate 8 allowed a straightforward synthesis of 1,6-dideoxynojirimycin (11) and of 1,6-dideoxy-D-gulo-nojirimycin (12; Scheme 2).
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790223
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  • 154
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    Nucleosides. Part LX.Part LIX: [1]. Synthesis and Characterization of Monomeric Cordycepin-Vitamin and Cordycepin-Lipid Conjugates Model Substances for Biodegradable Ester and Carbonate Linkages in Conjugates and Potential Inhibitors of HIV-1 ReplicationDedicated to Prof. Dr. h. c. Hans-Jürgen Bestmann on the occassion of his 70th birthday and in admiration to his interesting contribution to organic chemistry (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 609-618 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Monomeric 3′-deoxyadenosine (cordycepin) was modified at the 2′-O- (13-18) and 5′-O-position (25-29) by the vitamins E, D2, and A and by the two lipids 1,2-di-O-palmitoylglycerol and 1,2-di-O-hexadecylglycerol via succinate or carbonate linkages. The base-labile conjugates afforded protection groups like the 2-(4-nitro-phenyl)ethoxycarbonyl (npeoc) and monomethoxytrityl group (MeOTr) that are cleavable without harming the ester and carbonate bonds, respectively. Monomeric conjugates of cordycepin and vitamin E, vitamin D2, 1,2-di-O-palmitoylglycerol, and 1,2-di-O-hexadecylglycerol (see 13, 14, 17, 18, 25, 26, 28, and 29) inhibited HIV-1-induced syncytia formation 1.7 to 6.2 fold compared to 1.5-fold for cordycepin (see Table); IC50 values for 25 and 28 were 257 and 267 m̈M, respectively. In addition, the monomeric cordycepin-vitamin and -lipid conjugates inhibited HIV-1 RT activity 28-49% which compares with a 13% inhibition of HIV-1 RT observed for cordycepin. The minimal inhibition of HIV-1-induced syncytia formation and HIV-1 RT activity did not proceed by the activation of RNase L. The monomeric conjugates tested (13, 14) increased PKR expression.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790304
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  • 155
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    Anion Cryptates: Synthesis, Crystal Structures, and Complexation Constants of Fluoride and Chloride Inclusion Complexes of Polyammonium Macrobicyclic LigandsDedicated to the memory of Willy Simon (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 569-587 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three macrobicyclic octamines 1-3 and the macrotricyclic hexadecamine 14 have been synthesized. The octamines 1-3 bind anionic substrates when protonated. The stability constants of the complexes between the protonated forms of the macrobicyclic polyamines and halide anions have been determined by pH-metric measurements. The stability constants in H2O are very high; 1 in its hexaprotonated form binds F- with high selectivity (selectivity F-/Cl- 〉 108), while 3 exhibits strong stability constants for both F- and Cl-. Three X-ray structures have been obtained, one where F- is held inside the cavity of 1 · 6H+, one where Cl- is included in 3 · 6H+, and 3 · 6H+ where the cavity is empty.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790302
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  • 156
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    Perrhenat-katalysierte Umlagerung von Ethinyl-β-ionol (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 646-650 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Perrhenate-Catalyzed Rearrangement of Ethinyl-β-ionolThe rearrangement of 2-ethinyl-β-ionol (1) to α,β-unsaturated carbonyl compounds using tetraalkylammonium perrhenate catalysts was studied. It was found that the Rupe-Kambli product, ketone 2, is the main product (ca. 70%) of this rearrangement. The by-product 4 (ca. 15%) is formed in a ring-closure reaction.
    Additional Material: 1 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790307
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  • 157
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    ESR Study of Nitroxide Radicals Generated from Triaz-2-en-1-ols (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 663-669 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The triazenols 4-R1—C6H4—N=N—N(OH)—R2 (1), oxidized with t-BuO⋅2 radicals, produced nitroxide radicals R1—C6H4—N(O⋅)—N=N(R2) +O- (5). The suggested radical structure was confirmed by 15N-labeling. The reaction of triazenols 1 with PbO2 proceeded under N2 elimination, in which case nitroxides R1—C6H4—N(R2)—O⋅(2) were observed as the final radical products. The intermediate R1—C6H4⋅ radicals were identified by spin-trapping.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790310
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  • 158
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    Nucleotides. Part XLVII.Part XLVI: [1]. Synthesis of 3′-Deoxyadenylyl-(2′-5′)-3′-deoxyadenylyl-(2′-5′)-3′-O-(2-hydroxyethyl)adenosine and 3′-Deoxyadenylyl-(2′-5′)-3′-deoxyadenylyl-(2′-5′)-3′-O-{2-[(cholest-5-en-3β-yloxy)carbonyloxy]ethyl}adenosine: A New Type of (2′-5′)Adenylate Trimer Conjugate (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 718-726 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A suitably protected adenosine derivative bearing an acetyl-protected 2-hydroxyethyl moiety in 3′-O-position was attached to the 2′-terminus of a cordycepin (3′-deoxyadenosine) dimer. Coupling was performed by phosphoramidite chemistry using two alternative approaches - condensation of 5′-phosphoramidite 16 with 2′-OH cordycepin dimer 18 and condensation of dimeric cordycepin phosphoramidite 19 with 5′-OH adenosine derivative 15 - of which the latter synthesis worked best (→20). After cleavage of the acetyl protecting group (→21), cholesteryl carbonate was introduced into the OH function of the spacer (→24). Final deblocking of trimer 24 with, or trimer 21 without the cholesterol moiety afforded the modified cordycepin conjugates 25 and 23, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790314
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  • 159
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    Computations of 57Fe-NMR Chemical Shifts with the SOS-DFPT Method (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 742-754 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 57Fe Shielding tensors of substituted iron-carbonyl complexes have been computed employing the density-functional-based SOS-DFPT method (sum-over-states density-functional perturbation theory) with the IGLO (individual gauge for localized orbitals) choice of gauge origins and with large basis sets. The shieldings computed for [Fe(CO)5], [Fe(CO)3(H2C=CHCH=CH2)], [Fe(CO)3(cyclo-C4H4)], [Fe(CO)4(H2C=CHOMe)], [Fe(CO)4(H2C=CHCN)], [Fe(CO)3(H2C=CHCH=O)], and [Fe(CO)2(C5H5)R] (R = Me, Bu, i-Pr) correlate with the experimental δ(57Fe) values. However, the slope of the correlation line is 0.55 instead of 1, i.e., only about one half of the substituent effects on σ(Fe) is recovered in the calculations. Nearest-neighbor effects appear to be described qualitatively (cf. in the [Fe(CO)2(C5H5)R] series, whereas effects of more remote substituents, e.g., for [Fe(CO)4(H2C=CHX)] (X = MeO and CN)) are not reproduced. Dissociation energies of these species are discussed because of their relevance to experimental rate constants for substitution processes which are known to correlate with δ(57Fe). Even though the δ(13C) and δ(1H) data of ferrocene (7) are well reproduced theoretically, the computed σ(Fe) shielding of 7 deviates substantially from the σ(calc.)/δ(expt.) correlation, possibly indicating additional shortcomings in the theoretical description of this molecule.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790317
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  • 160
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    Syntheses of New Isocephems and Isodethiaoxacephems as Antimicrobial Agents (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 813-819 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The cis-configurated isocephem 26 as well as isodethiaoxacephems 21 and 33 were synthesized (Schemes 4 and 5). The key step involves chlorination of the corresponding carbanions of 23, 18, and 31 with CF3SO2Cl followed by internal alkylation. β-Lactams 3, 21, 26, and 33 were found to possess biological activity against several pathogenic microorganisms in vitro. Electronic activation of the lactam moiety in isodethiaoxacephem 33 remarkably enhanced its biological activity.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790323
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  • 161
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    An Analysis of the Bonding Properties of Benz[a]azulene by X-Ray, NMR, and Computational StudiesDedicated to Edgar Heilbronner on the occasion of his 75th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 837-854 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The X-ray crystal structures of 9-phenylbenz[a]azulene (4) and the corresponding non-benzannelated form, 4-phenylazulene (5), have been determined (cf. Fig.2). In contrast to 5, the skeleton of which shows nearly equal C,C bond lengths (cf. Table 1), the seven-membered ring of 4 exhibits clearly alternating C,C bond lengths (cf. Table 1). This is in agreement with a strong accentuation of the heptafulvene substructure in 4 by the [a] benzannelation. The alternating bond lengths of 4 and of its parent structure 3 are also reflected in the corresponding variations of the 3J(H,H) and 1J(13C,13C) values of these benz[a]azulenes (cf. Tables 4 and 5). Computations on the MP2/6-31G* level as well as on the BP86/6-31G* level for azulene (6), benz[a]azulene (3), and heptafulvene (7) are in good agreement with the experimental values (cf. Tables 6-8).
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790326
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  • 162
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    Synthesen von 2,3-Dioxoalkylphosphonaten und anderer neuartiger β-Ketophosphonate sowie eines Phosphinopyruvamids ( = (Alkyloxyphosphinyl)pyruvamids)Herrn Kollegen H.W. Roesky, Göttingen, mit den besten Wünschen zum 60. Geburtstag gewidmet (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 895-912 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Syntheses of 2,3-Dioxoalkylphonates and Other Novel β-Ketophosphonates As Well As of a Phosphinopyruvamide ( = (Alkyloxyphosphinyl)pyruvamide)The new 3-(diethoxyphosphoryl)-2-oxopropanoates 5 and 6 and -propanamides 1-4 with various amino substituents at C(3) were prepared (Scheme 2). These compounds exist, depending on N-substitution, as pure (E)-enols (in the case of1 and 5) or as a mixture of three tautomeric forms (in the case of 1-4 and 6). The configuration could be unambiguously assigned from the 1H-, 13C-, and 31P-NMR spectra. Phosphinopyruvamide ( = (alkyloxyphosphinyl)pyruvamide) 9 was prepared in a similar manner in spite of the instability of phosphinate derived carbanions. Some 3-(ethoxyimino)-2-oxobutylphosphonates, 11-13 (Scheme 5), and various 3,3-dimethoxy-2-oxoalkylphosphonates, 19-23 and 26-33 (Scheme 6), were available from the reaction of lithioalkylphosphonates with 2-(ethoxyimino)propanoates and 2,2-dimethoxyalkanoates, respectively. The 3,3-dimethoxybutylphosphonates 20, and 26-30 were cleaved to give 2,3-dioxobutylphosphonates 34-39 (Scheme 6). This method provides easy access to a new class of potentially pharmaceutically useful compounds.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790329
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  • 163
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    Oncinotine-Type Spermidine Alkaloids from Oncinotis tenuiloba. Transformation of N-acetyloncinotin-12-one to N,N′-diacetylinandenin-12-one (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 973-981 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: From extracts of Oncinotis tenuiloba STAPF, two novel polyamine alkaloids, oncinotin-11-one (5) and oncinotin-12-one (6), were isolated. Peracetylation of 6 provided the N-acetyl derivative 11 as well as N,N′-diacetylinandenin-10-en-12-one (12) due to a β-elimination-type side reaction resulting in ring enlargement of 11 (Scheme 1). Deuteration of 12 yielded 13, showing the same retention time as N,N′-diacetylinandenin-12-one (14), when co-HPLC was performed together with different keto-isomeric N,N′-diacetylinandeninones. Structure elucidation was extended by Schmidt degradation of 6 and N,N′-diacetyl(10,11-2H2)inandenin-12-one (13); the degradation products were identified by GC and ESI-MS. The structure of 5 was proposed on the basis of spectroscopic means. Comparison of the spectroscopic data of 5 with those obtained from synthetic material as well as co-HPLC of the N-acetyl derivative 20 together with the corresponding synthetic compound revealed the identity of the substances and confirmed the structure of 5. Additionally, oncinotine (2) and neooncinotine (3) were isolated, separated, and identified with authentic samples by co-HPLC of their N-acetyl derivatives 8 and 9, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790405
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  • 164
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    Metal Complexes with Macrocyclic Ligands. Part XLIIPart XLI: [1].. Tetraazamacrocyclic nickel(II) complexes with a methylthio or a methoxy pendant chain as model for cofactor F430 (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1011-1020 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 14-membered macrocyclic Ni2+ complexes of 1 and 2, with a methylthio pendant chain, and those of 3 and 4, with a methoxy pendant chain, have been synthesized and their chemistry has been studied. Solution spectra in H2O, MeCN, and DMF indicate no participation of the side-chain donor group in metal coordination. This is also the case in the solid state as shown by the X-ray structures of the Ni2+ complexes with 1 and 2, in which a tetrahedrally distorted square-planar geometry around the Ni2+ results by the coordination of the four N-atoms of the macrocycle. Cyclic voltammetry of these complexes in MeCN reveals that Ni2+ is reversibly reduced to Ni+ between -0.7 and -0.8 V vs. SCE. For the complexes with 1 and 2, the thioether bond is cleaved at more negative potentials, whereby a thiol group is formed. This thiol group is then oxidized at ca. +0.7 V vs. SCE, when a glassy C electrode is used, or at ca. 0 V vs. SCE at a dropping Hg electrode. No cleavage of the ether bond in the complexes with 3 and 4 is observed under similar conditions. Reduction of the Ni2+ complexes of 1 and 2 with Na-amalgam in DMF produces small amounts of methane only in the case of 1, indicating the importance of the proximity between the NiI centre and the MeS group.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790410
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  • 165
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    Preparation of α,α-Dibromocyclobutanones from Olefins: A simple procedure for the regioselective functionalization of olefinsFor a preliminary communication, see [1]. (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1075-1084 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Dibromoketene is prepared by in-situ Zn dehalogenation of tribromoacetyl bromide in the presence of POCl3. Under these conditions, the cycloaddition of dibromoketene with olefins 5 yields α,α-dibromocyclobutanones 2, which are cleaved with alkoxides to yield β-dibromomethyl esters 4. Several examples are given. For monosubstituted olefins, the functionalization is regioselective, the methoxycarbonyl group being introduced at the terminal C-atom.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790416
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  • 166
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    Theory-enforced Re-investigation of the Origin of the Large Metal-Carbon Bond Strength in PdCH2I+ and its reactions with unsaturated hydrocarbonsIn [1], a preliminary account of the reaction of ground-state Pd+ with CH3I is given.Dedicated to Edgar Heilbronner on the occasion of his 75th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1110-1120 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: State-of-the-art ab initio studies demonstrate that the reaction Pd+ + CH3I → PdCH2I+ + H. is endothermic by ca. 20 kcal/mol, which translates into a bond dissociation energy (BDE) of ca. 83 kcal/mol for the Pd+—CH2I bond. This figure is in agreement with an experimental bracket of 68 kcal/mol 〈 BDE(Pd+—CH2I) 〈 92 kcal/mol. Based on these findings, the previously studied Pd+/CH3I system was re-investigated, and double-resonance experiments demonstrate that the formation of PdCH2I+ occurs stepwise via PdCH3+ as a reactive intermediate. Further, ion/molecule reactions of PdCH2I+ with unsaturated hydrocarbons are studied, which reveal the formation of carbon-carbon bonds in the gas phase.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790419
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  • 167
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    The Synthesis and Characterization of New Optically Active ‘dimeric’ ‘pineno’-[4,5]-fused 2,2′-bipyridines linked without spacer or by small spacer groups (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1192-1202 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Linked chiral bipyridines 2-4 are prepared by combining two optically active ‘pineno’-[4,5]-fused 2,2′-bipyridines in a stereoselective reaction (Scheme 1). These potential ligands are new members of the ‘chiragen’ family, and are characterized by NMR spectroscopy and, in the case of 2 and 3 by single-crystal X-ray analysis. A new synthesis of ‘dipineno’-[4,5;4′,5′]-fused 2,2′ -bipyridine 8 is described, which, when coupled, gives additional four chiral centres to the analogous ‘chiragen’ series ( → 9). Analysis of the CD spectra allowed conformational information about the solution species to be determined.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790425
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  • 168
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    Mitteilungen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1240-1240 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790430
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  • 169
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    Singlet-Oxygen (1Δg) Production by Ruthenium(II) complexes containing polyazaheterocyclic ligands in methanol and in water (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1222-1238 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the context of our studies on ruthenium(II) complexes containing polyazaheterocyclic ligands, we have determined the rate constants of quenching by molecular oxygen (kq) of the metal-to-ligand charge-transfer-excited state of a series of homoleptic [RuL3] complexes (where L stands for 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 2,2′-bipyrazine (bpz), 4,7-diphenyl-1,10-phenanthroline (dip), diphenyl-1,10-phenanthroline-4,7-disulfonate (dpds), and 1, 10-phenanthroline-5-octadecanamide (poda)) in H2O and in MeOH. These compounds are singlet-oxygen (O2(1Δg)) sensitizers, and quantum yields of singlet-oxygen production (ΨΔ) in both solvents are also reported. Values of kq and ΨΔ depend on the nature of the ligand L and on the solvent, ΨΔ values showing a large range of variation (0.2 to 1.0). In MeOH, the only pathway for quenching of the excited [RuL3] complexes by molecular oxygen is energy transfer: the fraction of quenched excited states yielding singlet oxygen (ƒΔT) is unity for all compounds in the series investigated. Changing from MeOH to H2O has several remarkable effects: higher kq and lower ΨΔ values are observed; ƒΔT drops to ca. 0.5 except for [Ru(bpz)3]2+. In fact, [Ru(bpz)3]2+ is by far the weakest reductant in the series and behaves differently from the other complexes, with lowest kq and ΨΔ values and a ƒΔT equal to 1 in both solvents. Results are interpreted on the basis of the role played by charge-transfer interactions between the sensitizer excited state and molecular oxygen in the quenching mechanism. RuII Complexes based on the 4,7-diphenyl-1, 10-phenanthroline (dip) ligand are very efficient and stable singlet-oxygen sensitizers with ΨΔ values close to unity in air-saturated MeOH.
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    URL: http://dx.doi.org/10.1002/hlca.19960790428
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  • 170
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    Three-Component Reactions with Sterically Crowded 2,2,4,4-Tetramethyl-3-thioxocyclobutanone, Phenyl Azide, and Electron-Deficient C,C-DipolarophilesPresented by G.M. at the ‘17th International Symposium on the Organic Chemistry of Sulfur’, Tsukuba, Japan, July 1996.Dedicated to Professor Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1305-1314 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In order to trap ‘thiocarbonyl-aminides’ A, formed as intermediates in the reaction of thiocarbonyl compounds with phenyl azide, a mixture of 2,2,4,4-tetramethyl-3-thioxocyclobutanone (1), phenyl azide, and fumarodinitrile (8) was heated to 80° until evolution of N2 ceased. Two interception products of the ‘thiocarbonylaminide’ A (Ar=Ph) were formed: the known 1,4,2-dithiazolidine 3 (cf. Scheme 1) and the new 1,2-thiazolidine 12 (Scheme 2). The structure of the latter was established by X-ray crystallography (Fig.1). The analogous ‘three-component reaction’ with dimethyl fumarate (9) yielded, instead of 8, in addition to the known interception products 3 and 6 (Scheme 1), two unexpected products 15 and 16 (Scheme 3), of which the structures were elucidated by X-ray crystallography (Fig.2). Their formation is rationalized by a primary [2 + 3] cycloaddition of diazo compound 18 with 1 to give 19, followed by a cascade of further reactions (Scheme 4).
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790507
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  • 171
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    Synthesis of Oncinotin-11-one, a Macrocyclic Polyamine Alkaloid from Oncinotis tenuiloba (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1379-1386 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This paper presents a short synthesis of oncinotin-11-one (11), a minor alkaloid of Oncinotis tenuiloba (Apocynaceae). Based on a disconnection approach, the spermidine portion of the key intermediate 6 was constructed consecutively by simple N-alkylations starting from ethyl piperidine-2-carboxylate (1). Treatment of 6 with in situ lithiated 2-[(10-bromodecyl)oxy]tetrahydropyran resulted in the formation of the keto moiety under simultanous deprotection of the lactam N-atom to give the amino ketone 7 in 71% yield. Cleavage of the tetrahydro-2H-pyran-2-yl(Thp) portion and Jones oxidation of the resulting alcohol 8 gave the amino acid 9 which was cyclized. Final N-debenzylation of 10 provided the natural alkaloid 11. Only two protective groups were needed in this synthesis. The reaction of N-alkyl-lactams with organometallic reagents is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790511
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  • 172
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    The Diels-Alder Reactivity of 2,2′-Ethylidenebis[3,5-dimethylfuran] and Exploratory Chemistry of the Mono-adducts (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1415-1427 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the presence of Me3Al, 1-cyanovinyl acetate added to 2,2′-ethylidenebis[3,5-dimethylfuran] (1) to give a 20:10:1:1 mixture of mono-adducts 4,5,6, and 7 resulting from the same regiocontrol (‘para’ orienting effect of the 5-methyl substituent in 1). The additions of a second equiv. of dienophile to 4-7 were very slow reactions. The major mono-adducts 4 (solid) and 5 (liquid) have 2-exo-carbonitrile groups. The molecular structure of 4(1RS,1′RS,2SR,4SR)-2-exo-cyano-4-[1-(3,5-dimethylfuran-2-yl)ethyl-7-oxabicyclo[2.2.1]hept-5-en-2-endo-yl acetate) was determined by X-ray single-crystal radiocrystallography. Mono-adducts 4 and 5 were saponified into the corresponding 7-oxanorbornenones 8 and 9 which were converted with high stereoselectivity into (1RS,1′SR,4RS,5RS,6RS)-4-[1-(3,5-dimethyl furan-2-yl)ethyl]-6-exo-methoxy-1,5-endo-dimethyl-7-oxabicyclo [2.2.1]heptan-2-one dimethyl acetal (12) and its (1′RS-stereoisomer 12a, respectively. Acetal hydrolysis of 12a followed by treatment with (t-Bu)Me2SiOSO2CF3 led to silylation and pinacol rearrangement with the formation of (1RS,1′RS,5RS,6RS)-4-[(tert-butyl)dimethy lsilyloxy]-1-(3,5-dimethylfuran-2-yl)ethyl]-5-methoxy-6-methyl-3-methylidene- 2-oxabicyclo[2.2.1]heptane (16). In the presence of Me3Al, dimethyl acetylenedicarboxylate added to 12 giving a major adduct 19 which was hydroborated and oxidized into (1RS,1′RS,2″RS,3″RS,4SR,4″RS,5 SR,6SR)-dimethyl 5-exo-hydroxy-4,6-endo-dimethyl-1-[1-(3-exo,5,5-trimeth oxy-2-endo,4-dimethyl-7-oxabicyclo[2.2.1]hept-2-yl)ethyl]-7-oxabicyclo [2.2.1]hept-2-ene-2,3-dicarboxylate (20). Acetylation of alcohol 20 followed by C=C bond cleavage afforded (1′RS,1″SR,2RS,2′″SR,3RS, 3″SR,4RS,4″SR,5RS)-dimethyl {3-acetoxy-2,3,4,5-tetrahydro-2,4-dimethyl-5-[1-(3-exo,5,5-trimethoxy -2-endo,4-dimethyl-7-oxabicyclo[2.2.1]hept-1-yl)-ethyl]furan-2,5-diyl} bis[glyoxylate] (24).
    Additional Material: 1 Ill.
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    Lipase-Catalysed Regioselective Deacetylation of androstane derivatives (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 999-1004 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of acetoxy derivatives of androstane was deacetylated in organic solvents by several lipases. The most satisfactory results were obtained with lipase from Candida cylindracea (CCL) and Candida antarctica (CAL). In some derivatives, CCL and CAL showed an overwhelming regioselectivity towards the removal of the 3β- or the 17β-acetyl group (see Table 2). Three new steroid derivatives were obtained through this approach. A hypothetical rationale for the behaviour of these enzymes is given.
    Additional Material: 3 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790408
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  • 174
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    Synthesis and Circular Dichroism of Both Enantiomers of 2-deuterofluoroacetic acid (= Fluoro[2H1]acetic acid) (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1021-1025 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Sharpless epoxidation of (E)-1-(trimethylsilyl)[1-2H1]oct-1-en-3-o1 (3a) yielded (1S,2S,3S)- and (1R,2R,3R)-1-(trimethylsilyl)-1,2-epoxy[1-2H1]octan-3-ols (4a and 4b, resp.) which were converted in three steps into (S)- and (R)-fluoro[ 2H1]acetic acid (7a and 7b, resp.) in good yields. Their high isotopic and optical purity was established by 1H- and 19F-NMR, mass, and circular-dichroism spectroscopy.
    Additional Material: 1 Ill.
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  • 175
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    Nucleotides. Part XLVIIIPart XLVII: [1].. Synthesis of 2′-amino-2′-deoxyarabinonucleoside phosphoramidite building blocks (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1067-1074 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Chemical syntheses of 2′-amino-2′-deoxyarabinonucleosides of uracil, thymine, cytosine, adenine, and guanine and their conversion into suitably protected 3′-phosphoramidite building blocks 24-28 for oligonucleotide synthesis are described. The 2-(4-nitrophenyl)ethoxycarbonyl (npeoc) group was used for protection of the aglycon and the 2′-amino functions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790415
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  • 176
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    Synthesis of 1,3,4,5-Tetrahydro-2-benzoxepin Derivatives as Conformationally Restricted Analogues of cyclamenaldehyde-type compounds and as intermediates for highly odour-active homologues (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1095-1109 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nine 1,3,4,5-tetrahydro-2-benzoxepin derivatives have been prepared as mimics of the folded (gauche) conformation of cyclamenaldehyde (1) and related compounds, but none of them showed the typical lily-of-the valley (muguet) odour activity. However, conversion of these substances to previously unknown analogues of 1 having a Me substituent on the aromatic ring in an ortho position to the side chain led to new fragrance substances with remarkable properties. The results indicate that the extended (anti) conformation is more likely to be the ‘bioactive’ one at the receptor site(s).
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790418
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  • 177
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    The Degradation Mechanism of an Oral Cephalosporin: Cefaclor (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1793-1802 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The degradation of cefaclor (1), an oral cephalosporin antibiotic, was studied at 37° in a neutral aqueous medium by HPLC and 1H-NMR. Under these conditions, 1 underwent intramolecular aminolysis by the 7-side-chain NH2 group on the β-lactam moiety to give a piperazine-2,5-dione. The most prominent peak in the HPLC profile of a degradation solution from 1 was isolated by prep. HPLC. Mechanistically, the formation of this degradation product cis-11 from 1 involves the contraction from a six-membered cephem ring to a five-membered ring, which presumably takes place via a common episulfonium ion intermediate 9 (see Scheme). Loss of the Cl-atom from 3-chloro-3-cephem is a general reaction subsequent to β-lactam ring opening.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790705
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  • 178
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    Detection of Noncovalent Complexes of Hydroxamic-Acid Derivatives by means of electrospray mass spectrometry (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1701-1709 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The complexes of isolated benzoxazinone derivatives with FeIII were investigated by means of electrospray ionization mass spectrometry. The hydroxamic-acid derivatives could be differentiated from the lactam or the methyl-hydroxamate derivatives by the formation of FeIII complexes with two or three ligands. These complexes or adducts of them have been identified in the mass spectra. Moreover, the mass-spectral behavior of these complexes was not markedly influenced by the presence of a β-D-glucopyranosyloxy substituent.
    Additional Material: 5 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790620
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  • 179
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    Polymer- and Dendrimer-Bound Ti-TADDOLates in Catalytic (and Stoichiometric) Enantioselective Reactions: Are pentacoordinate cationic Ti complexes the catalytically active species? (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1710-1740 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: α,α,α′,α′-Tetraaryl-1,3-dioxolane-4,5-dimethanols (TADDOLs), containing styryl groups either at C(2) of the heterocyclic ring or in the α-position, were prepared in the usual way (18-22, 24, 25). These compounds were copolymerized with styrene and divinylbenzene in a suspension, yielding polymers (33-40, Scheme 3) as beads with a rather uniform particle-size distribution (150-45 μm), swellable in common organic solvents. HOCH2- and BrCH2-substituted TADDOLs were also prepared and used for attachement to Merrifield resin or to dendritic molecules (23, 26-32). The TADDOL moieties in these materials are accessible to form Ti (and Al) complexes (Scheme 4) which can be used as polymer- or dendrimer-bound reagents (stoichiometric) or Lewis acids (catalytic). The reactions studied with these new chiral auxiliaries are: enantioselective nucleophilic additions to aldehydes (of R2Zn and RTi(OCHMe2)3; Scheme 5, Table 1) and to ketones (of LiAlH4, Table 2); enantioselective ring opening of meso-anhydrides (Scheme 6); [4+2] and [3+2] cycloadditions of 3-crotonyl-1,3-oxazolidin-2-one to cyclopentadiene and to (Z)-N-benzylidenephenylamine N-oxide ( → 48, 49, Scheme 7, Tables 3, 4, and Fig. 5). The enantioselectivities reached with most of the polymer-bound or dendritic TADDOL ligands were comparable or identical to those observed with the soluble analogs. The activity of the polymer-bound Lewis acids was only slightly reduced as compared with that encountered under homogeneous conditions. Multiple use of the beads (up to 10 times), without decreased performance, has been demonstrated (Figs. 3 and 4). The poorer selectivity in the Diels-Alder reaction (Scheme 7a), induced by the polymer-bound Cl2Ti-TADDOLate as compared to the soluble one, is taken as an opportunity to discuss the mechanism of this Lewis-acid catalysis, and to propose a cationic, trigonal-bipyramidal complex as the catalytically active species (Fig. 6). It is suggested that similar cations may be involved in other Ti-TADDOLate-mediated reactions as well.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790621
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  • 180
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    Synthesis, Separation, and Characterization of Optically Pure C76 Mono-Adducts (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1741-1756 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Nucleophilic Bingel cyclopropanation of D2-C76 with bis[(S)-1-phenylbutyl] 2-bromomalonate in toluene in the presence of base yielded three constitutionally isomeric pairs of diastereoisomeric mono-adducts together with one other constitutional isomer. All seven mono-adducts were isolated in optically pure form by prep. HPLC on a (S,S)-Whelk-O1 chiral stationary phase. They represent the first optically pure adducts of an inherently chiral fullerene. Characterization by UV/VIS, CD, 13C- and 1H-NMR spectroscopy allowed identification of pairs of stereoisomers and symmetry assignments: the two pairs of diastereoisomers which were isolated as the major product possess C1 symmetry, whereas the third pair of diastereoisomers, which is a minor product, is C2-symmetrical. The circular dichroism spectra of the optically active C76-adducts showed very pronounced Cotton effects resulting from strong chiroptical contributions of the chiral fullerene chromophore with the maximum observed Δε values being twice as high than those previously measured for optically active adducts of achiral fullerenes with a chiral addition pattern. Whereas the regioselectivity of mono-additions to C70 correlates with the degree of local bond curvature and the regioselectivity of multiple Bingel cyclopropanations of C60 with electronic parameters such as coefficients of the lowest unoccupied molecular orbital (LUMO), no such simple predictive correlations exist for the nucleophilic addition to C76. Despite full spectral characterization, an unambiguous structural assignment of the isolated compounds was not possible, except for the two C2-symmetrical isomers. Based on considerations of local bond curvature and the previous experiences with the chemistry of C70, the structures of the C2-symmetrical stereoisomers were assigned as (S,S,fC)-3 and (S,S,fA)-3, resulting from addition to the polar α-type C(1)—C(6) bond.
    Additional Material: 10 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790702
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  • 181
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    Synthesis and Structural Characterization of the Trimeric Furoxan (= Furazan 2-Oxide) System, a New Potent Vasodilating Moiety (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1803-1817 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis, structural characterization, and NO donor properties of a series of terfuroxan (= terfurazan trioxide) derivatives 1a-h and 2a-j are reported (Schemes 1 and 2). Structural assignments were confirmed principally by mass and 13C- and 1H-NMR spectroscopy. The extent and the initial rate of NO release in the presence of thiol cofactor was evaluated for each derivative. Vasodilator effects of all the terfuroxan derivatives were evaluated on endothelium-denuded strips of rat aorta precontracted with noradrenaline. Concentration-response curves were also evaluated in the presence of 10 m̈M oxyhemoglobin (HbO2), a well known NO scavenger. The whole series displays high vasodilating potency; the most active terfuroxans (1a, b, g and 2i) are 5-10 times as potent as glyceryl trinitrate taken as reference (see Table 3). The potency decrease observed in the presence of HbO2 agrees with the involvement of NO in the vasorelaxing action. The 4,3′:4′,4″ connection (series 1; furoxan numbering) gives rise to the most potent compounds. The conformational factors seem to play important roles in the activity. No clear relationship between physico-chemical properties of the substituents and potencies of derivative emerges.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790706
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  • 182
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    Probing the Helical Secondary Structure of Short-Chain β-PeptidesDedicated to Professor Teruaki Mukaiyama, a dear friend and revered colleague on the occasion of his 70th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2043-2066 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structural prerequisites for the stability of the 31 helix of β-peptides can be defined from inspection of models (Figs. 1 and 2): lateral non-H-substituents in 2- and 3-position on the 3-amino-acid residues of the helix are allowed, axial ones are forbidden. To be able to test this prediction, we synthesized a series of heptapeptide derivatives Boc-(β-HVal-β-HAla-β-HLeu-Xaa-β-HVal-β-HAla-β-HLeu)-OMe 13-22 (Xaa = α- or β-amino-acid residue) and a β-depsipeptide 25 with a central (S)-3-hydroxybutanoic-acid residue (Xaa = -OCH(Me)CH2C(O)-) (Schemes 1 3). Detailed NMR analysis (DQF-COSY, HSQC, HMBC, ROESY, and TOCSY experiments) in methanol solution of the β-hexapeptide H(-β-HVal-β-HAla-β-HLeu)2-OH (1) and of the β-heptapeptide H-β-HVal-β-HAla-β-HLeu-(S,S)-β-HAla(αMe)-β-HVal-β-HAla- β-HLeu-OH (22), with a central (2S,3S)-3-amino-2-methylbutanoic-acid residue, confirm the helical structure of such β-peptides (previously discovered in pyridine solution) (Fig.3 and Tables 1-5). The CD spectra of helical β-peptides, the residues of which were prepared by (retentive) Arndt-Eistert homologation of the (S)- or L-α-amino acids, show a trough at 215 nm. Thus, this characteristic pattern of the CD spectra was taken as an indicator for the presence of a helix in methanol solutions of compounds 13-22 and 25 (including partially and fully deprotected forms) (Figs.4-6). The results fully confirm predicted structural effects: incorporation of a single ‘wrong’ residue ((R)-β-HAla, β-HAib, (R,S)-β-HAla(α Me), or N-Me-β-HAla) in the central position of the β-heptapeptide derivatives A (see 17, 18, 20, or 21, resp.) causes the CD minimum to disappear. Also, the β-heptadepsipetide 25 (missing H-bond) and the β-heptapeptide analogs with a single α-amino-acid moiety in the middle (13 and 14) are not helical, according to this analysis. An interesting case is the heptapeptide 15 with the central achiral, unsubstituted 3-aminopropanoic-acid moiety: helical conformation appears to depend upon the presence or absence of terminal protection and upon the solvent (MeOH vs. MeOH/H2O).
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    URL: http://dx.doi.org/10.1002/hlca.19960790802
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    Nucleosides. Part LXI.Part LX: [1]. A Simple Procedure for the Monomethylation of Protected and Unprotected Ribonucleosides in the 2′-O- and 3′-O-Position Using Diazomethane and the Catalyst Stannous Chloride (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2114-2136 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Intensive studies on the diazomethane methylation of the common ribonucleosides uridine, cytidine, adenosine, and guanosine and its derivatives were performed to obtain preferentially the 2′-O-methyl isomers. Methylation of 5′-O-(monomethoxytrityl)-N2-(4-nitrophenyl)ethoxycarbonyl-O6-[2-(4-nitrophenyl)ethyl]-guanosine (1) with diazomethane resulted in an almost quantitative yield of the 2′- and 3′-O-methyl isomers which could be separated by simple silica-gel flash chromatography (Scheme 1). Adenosine, cytidine, and uridine were methylated with diazomethane with and without protection of the 5′ -O-position by a mono- or dimethoxytrityl group and the aglycone moiety of adenosine and cytidine by the 2-(4-nitrophenyl)ethoxycarbonyl (npeoc) group (Schemes 2-4). Attempts to increase the formation of the 2′-O-methyl isomer as much as possible were based upon various solvents, temperatures, catalysts, and concentration of the catalysts during the methylation reaction.
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  • 184
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    Rarisetenolide, Epoxyrarisetenolide, and Epirarisetenolide, New-Skeleton Sesquiterpene Lactones as Taxonomic Markers and Defensive Agents of the Marine Ciliated Morphospecies Euplotes rariseta (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2180-2189 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Rarisetenolide (3) and epoxyrarisetenolide (4), new-skeleton α,β-conjugated sesquiterpene lactones, were isolated from various strains of the unicellular ciliated protist Euplotes rariseta collected from marine coasts widely far apart from each other: northern and southern Australia, southern Brazil, and Canary Islands. A strain of E. rariseta from New Zealand gave epirarisetenolide (5) instead, revealing a subtle variability in secondary metabolism for this ciliated morphospecies. Nonetheless, these metabolites - which are the first non-aldehydic terpenoids so far isolated from ciliates - represent a unique whole that constitutes a reliable taxonomic tool at the morphospecies level. Epirarisetenolide (5) and rarisetenolide (3), in this order, showed higher toxicity towards nonproductive ciliates than the chemically more reactive natural epoxide 4 and the semisynthetic aldehyde/protected-aldehyde froms 8/7a/7b. This inverse trend of biological vs. chemical effectiveness suggests that these cytotoxic agents interact noncovalently with membrane receptors.
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    URL: http://dx.doi.org/10.1002/hlca.19960790813
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    A Tricyclic Template Derived from (2S,4R)-4-Hydroxyproline for the Synthesis of Protein Loop Mimetics (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2235-2248 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A tricyclic diketopiperazine, formally derived by coupling (2S,4S)-4-aminoproline (Pro(NH2)) and (2S,4R)-4-(carboxymethyl)proline (Pro(CH2COOH)), is synthesized starting from readily available (2S,4R)-4-hydroxyproline. The resulting tricyclic template has carboxy and amino groups to which a peptide chain may be attached. The Fmoc-protected template 5 is incorporated into the cyclic molecule cyclo(-Ala1-Asn2-Pro3-Asn4-Ala5-) (6) where Pro(NH2)7 = Pro(CH2COOH)8 represents the template, using solid-phase peptide synthesis with cyclization in solution. The molecule is shown by NMR and dynamic simulated annealing methods to adopt a preferred conformation in aqueous solution, which includes an extended backbone at the residues Asn2-Pro3-Asn4, and a type-Iβ-turn at . These studies show that this novel template may be used in the synthesis of cyclic peptide and protein mimetics having defined secondary structure in aqueous environments.
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    URL: http://dx.doi.org/10.1002/hlca.19960790817
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996) 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790701
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    Synthesis and Enzymatic Evaluation of Substrates and Inhibitors of β-Glucuronidases (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1757-1784 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The phosphono and the tetrazolyl analogues 4 and 5 of 4-methylumbelliferyl β-D-glucuronide (=(4-methyl-2-oxo-2H-1-benzopyran-7-yl β-D-glucopyranosid)uronic acid; 6) were synthesized and evaluated as substrates of β-glucuronidases. Similarly, the phenylcarbamate 7 and its phosphono analogue 8 were prepared and evaluated as inhibitors. To examine the diastereoselectivity of the phosphorylation, we also synthesized the protected L-ido-D-gluco-, and D-galacto-configurated phospha-glycopyranuronates 12, 13, 21, 22, 34 and 35. Two strategies were followed. In the first one, the glucuronic acid 19 was decarboxylated to 11 and further transformed, via 20, into the trichloroacetimidate 10 (Scheme 2). Phosphorylation of 10 with (MeO)3P yielded the diastereoisomers 12 and 13, the diastereoselectivity depending on the solvent. In MeCN, 12 and 13 were obtained in a ratio of 1:1, while in non-participating solvents the L-ido 12 was by far the major diastereoisomer. The acetate 11 was inert to (MeO)3P, but reacted with (PhO)3P to the anomeric mixture 21/22, in keeping with a stabilizing 1,3-interaction in the intermediate phosphonium salt. Similarly, the phospha-galacturonates 34 and 35 were prepared from the galactoside 23 via the enol ether 26, the lactone 27, and the acetates 28/29 that were also transformed into the trichloroacetimidate 33 (Scheme 3). In the second, higher-yielding strategy, phosphorylation of the pentodialdehyde 39 to 40/41 was followed by hydrolysis and acetylation to the phospha-glucuronates 43/44 (Scheme 4). Transesterification to 45/46, selective deacetylation to 48/49, and formation of the trichloroacetimidates 50/51 were followed by glycosidation and deprotection to 4. The tetrazole 5 was prepared from the lactones 54/55 via the N-benzylamides 57/58 that were treated with TfN3 to give the N-benzyltetrazoles 59/60 (Scheme 4). These were transformed into the trichloroacetimidates 63/64, glycosylated to 65, and deprotected. The O-carbamoylhydroximo-lactone 7 derived from the glucuronate 67/68, and the phosphonate analogue 8 were prepared by established methods. The phosphonate 4 is slowly hydrolyzed by the E. coli β-glucuronidase, but neither 4 nor the tetrazole 5 are affected by the bovine liver β-glucuronidase (Table 4). The phenylcarbamate 7 of D-glucarhydroximo-1,5-lactone, but not its phosphonate analogue 8, is an inhibitor (KI = 8 m̈M) of the E. coli β-glucuronidase. The bovine liver β-glucuronidase is inhibited strongly by 7 (IC50 = 0.2 m̈M) and weakly by 8 (IC50 = 2mM).
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790703
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  • 188
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    Protein-Loop Mimetics: A Diketopiperazine-Based Template to Stabilize Loop Conformations in Cyclic Peptides Containing the NPNA and RGD Motifs (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1825-1842 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We explore here an approach to mimic the structures and biological functions of protein loops in small synthetic molecules, by grafting the loop of interest onto an organic template comprising a bicyclic diketopiperazine, prepared by the formal coupling of (2S,4S)-4-aminoproline (Pro(NH2)) and aspartic acid (Asp). The Fmoc-protected template 4 is used to prepare cyclo(-Ala1-Asn2-Pro3-Asn4-Ala5- Ala6-Temp-) (5) and cyclo(-Ala1-Arg2-Gly3-Asp4-Temp-) (6) (where Temp = template derived from 4), containing the Asn-Pro-Asn-Ala (NPNA) and Arg-Gly-Asp (RGD) motifs. The conformational properties of these molecules are studied in aqueous solution by NMR and simulated-annealing methods. The NPNA motif, an immunodominant epitope on the circumsporozoite surface protein of the malaria parasite Plasmodium falciparum, is shown to adopt a stable type-I β-turn in 5. The template in 5 adopts a preferred conformation with Pro(NH2)χ1 ≍ -35° and the Asp moiety χ1 ≍ 70°. A different template conformation is inferred for 6, with Pro(NH2)χ1 ≍ 0°, but the ARGD loop appears by NMR to undergo rapid conformational averaging. Solid-phase binding assays reveal that 6 displays modest antagonist activity towards both the integrin αIIbβ3 and αvβ3 receptors.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790708
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  • 189
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    Synthesis, Molecular Recognition, and Enzymology of Oligonucleotides Containing the Non-standard Base Pair between 5-Aza-7-deazaisoguanine and 6-Amino-3-methylpyrazin-2(1H)-one, a Donor-Acceptor-Acceptor Purine Analog and an Acceptor-Donor-Donor Pyrimidine Analog (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1881-1898 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A 6-aminopyrazin-2(1H)-one (pyADD), when incorporated as a pyrimidine-base analog into an oligonucleotide chain, presents a H-bond acceptor-donor-donor pattern to 5-aza-7-deazaisoguanine (puDAA), the complementrary donor-acceptor-acceptor purine analog. Reported here are the syntheses of the phosphoramidite of the 2′-deoxyribonucleoside bearing the puDAA base, oligonucleotides containing this nucleoside unit, the enzyme-assisted synthesis of oligoribonucleotides containing the pyADD ribonucleoside, and the molecular-recognition properties of this non-standard base pair in an oligonucleotide context. A series of melting experiments suggests that the pyADD · puDAA base pair contributes to the relative stability of a duplex structure approximately the same as an A · T base pair, and significantly more than mismatches between these non-standard bases and certain standard nucleobases. The pyADD nucleoside bisphosphate is accepted by T4 RNA ligase, but the triphosphate of the pyADD nucleoside was not incorported by T7 RNA polymerase opposite the puDAA nucleobase in a template. Oligonucleotides containing the pyADD base slowly undergo a reversible first-order reaction, presumably an epimerization process to give the α-D-anomer. These experiments provide the tools for laboratory-based use of the pyADD · puDAA base pair as a component of an oligonucleotide-like molecular-recognition system based on an expanded genetic alphabet.
    Additional Material: 9 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790711
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  • 190
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    Stereoselektive Synthese von 2′-O-(2-Methoxyethyl)ribonucleosiden: Nachbargruppenbeteiligung der Methoxyethoxy-Gruppe bei der Ribosylierung von Heterocyclen (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1930-1938 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stereoselective Synthesis of 2′-O-(2-Methoxythyl)ribonucleosides: Neighboring-Group Participation of the Methoxythoxy Group in the Ribosylation StepA new access to 2′-O-(2-methoxyethyl)ribonucleosides, building blocks for second-generation antisense oligonucleotides, is presented. The influence of various reaction parameters on the coupling reaction of 2-O-(2-methoxyethyl)-D-ribose derivatives with heteroaromatic bases as the key step was investigated, and reaction conditions were optimized with regard to formation of the desired β-D-anomers. With Sn2+ salts as promotors in polar solvents, these β-D-anomers were formed with a high degree of steroselectivity.
    Additional Material: 4 Tab.
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    URL: http://dx.doi.org/10.1002/hlca.19960790716
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  • 191
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    Synthesis of 4′-C-Acylated Thymidines (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1980-1994 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two synthetic pathways towards 4′-C-acylthymidines are presented. These modified mononucleosides are precursors of the 2′-deoxyribonucleotide 4′-C-radical. They were converted into their corresponding 3′-O-[(2-cyanoethyl) N,N-diisopropylphosphoramidites] 3a-c and incorporated in oligonucleotides by solid-phase synthesis. The structure of some modified nucleosides was revealed by X-ray crystal-structure analysis.
    Additional Material: 2 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790720
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  • 192
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    Oligosaccharide Analogues of Polysaccharides. Part 9. Synthesis and Thermolysis of Acetylenosaccharide-Derived 1,2-Dialkynylbenzenes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2004-2022 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermolysis of the 1,2-bis(glucosylalkynyl)benzenes 6 and 16 was studied to evaluate the effects of intramolecular H-bonding on the activation energy of the Bergman-Masamune-Sondheimer cycloaromatization, and to evaluate the use of the cycloaromatization for the synthesis of di-glycosylated naphthalenes. The dialkynes were prepared by cross-coupling of the O-benzylated or O-silylated glucosylalkynes 1 and 4 (Scheme 1). Thiolysis of the known 1, or acetolysis of 1, followed by deacetylation (→2→3) and silylation gave 4. Cross-coupling of 1 or 4 with iodo- or 1,2-diiodobezene depended upon the nature of the added amine and on the protecting group, and led to the mono- and dialkynylbenzenes 5 and 6, or 12, 13, and 15, respectively. The benzyl ethers 5 and 6 gave poor yields upon acetolysis catalyzed by BF3 · OEt2, while Ac2O/CoCl2 · 6 H2O transformed 6 in good yields into the regioselectively debenzylated 10. Desilylation of 7 and 13 gave the alcohols 8 and 14, respectively. Thermolysis of 6 in PhCl gave 22 and 23, independently of the presence or absence of 1,4-cyclohexadiene; 23 was formed from 22 (Scheme 2). Acetolysis of 22 gave the hexaacetate 24 that was completely debenzylated by thiolysis, yielding the diol 26 and trans-stilbene, evidencing the nature and position of the bridge between the glucosyl moieties (Scheme 3). Thiolysis of 22 yielded the unprotected 2,3-diglucosylnaphthalene 28, a new type of C-glycosides. Depending upon conditions, hydrogenation of 22 led to 29 (after acetylation), 30, or 32. NMR and particularly NOE data evidence the threo-configuration of the bridge. The structure of 23 was confirmed by hydrolysis to the diol 34 and diphenylacetaldehyde, and by correlation of 23 with 22 via the common product 31. Formation of 22 is rationalized by a Bergman cyclization to a diradical, followed by regioselective abstraction of a H-atom from the BnO—C(2) group, and diastereoselective combination of the doubly benzylic diradical (Scheme 4). While thermolysis of 3 in EtOH sets in around 140°, 16 did not react at 160° and decomposed at 180-220°. No evidence for intramolecular H-bonds of 16, as compared to 14, were found.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790722
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  • 193
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    Synthesis and Crystal Structure of Tricaesium Heptaphosphide-Ammonia(1/3) Cs3P7·3 NH3 (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2083-2087 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Recrystallization of Cs3P7 from liquid NH3 yields the triammoniate Cs3P7·3 NH3, which loses the weakly bound NH3 of crystallization below 253 K. A low-temperature crystal-structure analysis shows that Cs3P7· NH3 consists of a framework of heptaphosphanortricyclane anions P73- and Cs+ cations with NH3 molecules completing the coordination of the cations. The framework is built from Cs3P7 layers connected by only few Cs…P interactions, the interlayer gap being filled by a two-dimensional network of NH3. The Cs7P7 part of the structure completes a family of alkali-metal-polyphosphide substructures which range from ∞1[RbP7]2- or ∞1[CsP11]2- chains over ∞2[Cs2Pn]- layers (n = 7, 11) to now ∞3[Cs3P7] frameworks.
    Additional Material: 3 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790805
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  • 194
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    Electrospray-Ionization Mass Spectrometry. Part 2.Part 1: [1]. Neighboring-Group Participation in the Mass-Spectral Decomposition of 4-Hydroxycinnamoyl-spermidines (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2137-2151 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The three mono substituted N-[(E)-3-(4-hydroxyphenyl)prop-2-enoyl]spermidines 1-3 have been studied by positive-ion electrospray-ionization tandem mass spectrometry (ESI-MS/MS). Because of the neighboring-group participation, the MS/MS of [1 + H]+ and [2 + H]+ are essentially similar, while compound 3 can be easily distinguished from 1 and 2 because of the characteristic ions at m/z 218. However, with the source collision-induced dissociation (source-CID) MS/MS technique, the compounds 1 and 2 can be unambiguously distinguished by the signal of the pyrrolidinium ion (m/z 72) from their daughter ion (m/z 275). The source-CID MS/MS of the labeled compound N-(4-aminobutyl)-N-(3-aminopropyl)-N-[3-(4- hydroxyphenyl)prop-2-en[15N]amide] ([15N(4)]-2) provide more information on the decomposition mechanisms and proved the occurrence of a partial transamidation reaction 2→1 during the measurement.
    Additional Material: 4 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790809
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  • 195
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    Synthesis of Lipid Derivatives of Colchicine (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 2346-2352 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of glycero-lipids linked to colchicine derivatives is reported. The lipid structures are designed to perform two-dimensional crystallization experiments with tubulin, the structural subunit protein of microtubules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790821
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  • 196
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    Synthesis and Characterization of the (5′ → 5″)-Ester of Adenosine 5′-diphosphate with α-D-ribofuranose cyclic 1″,2″-phosphate: A NAD derivative produced during tRNA splicing (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1005-1010 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis and spectroscopic properties of the (5′ → 5″)-ester of adenosine 5′-diphosphate with ribofuranose cyclic 1″, 2″-phosphate 3, a recently discovered metabolite produced during tRNA splicing, are reported.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790409
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  • 197
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    Efficient Preparation and X-Ray Structure Analyses of (2R)-N-pyruvoyl- and (2R)-N-(phenylglyoxyloyl)bornane-10,2-sultamDedicated to Prof. Vladimir Prelog on the occasion of his 90th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1059-1066 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An efficient synthesis of the two title compounds is reported, as well as their X-ray crystal-structure analyses. A discussion based on stereoelectronic considerations rationalizes the first example of a crystalline SO2/C(O) syn-periplanar conformer of a N-acylbornane-10,2-sultam.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790414
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  • 198
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    Unparalleled, Enormous Metal-Carbon Bond Strength in PdCH2I+Dedicated to Albert Eschenmoser on the occasion of his 70th birthday (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 1-5 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermalized Pd+ cations activate methyl iodide by selective cleavage of a C—H bond under formation of PdCH2I+ and an H-atom. This finding implies that the interaction energy between the metal cation and the CH2I fragment and thus the metal-carbon bond strength exceeds 103 kcal/mol. Theory predicts that the energetically most favorable isomer of this ion exhibits the Pd+—CH2—I structure, which is stabilized by an unprecedented bridging interaction between the two heavy atoms Pd and I.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/hlca.19960790102
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  • 199
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    Regioselektive 1,3-Dipolare Cycloadditionen eines ‘Thiocarbonyl-methanids’ ((Alkylidensulfonio)methanids) mit aromatischen SulfinenAls ‘Sulfine’ werden Thiocarbonyl-S-oxide bezeichnet (s.z.B. [1]). (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 31-40 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Regioselective 1,3-Dipolar Cycloadditions of a ‘Thiocarbonyl-methanide’ ((Alkylidenesulfonio)methanide) with Aromatic SulfinesReaction of the spirocyclic 2,5-dihydro-1,3,4-thiadiazole 7 and thiobenzophenone S-oxide (6a) in THF at 45° yielded the spirocyclic 1,3-dithiolane 1-oxide 8, thiirane 9, and the diazane derivative 10 in a ratio of 61:15:23 (Scheme 2). The formation of 8 is rationalized by a 1,3-dipolar cycloaddition of ‘thiocarbonyl-methanide’ 1, generated from 7 by thermal elimination of N2, and the C=S bond of sulfine 6a. Cyclization of intermediate 1 leads to thiirane 9. Under the same conditions, 7 and adamantane-2-thione S-oxide (6b) or 2,2,4,4-tetramethyl-3-thioxocyclobutanone S-oxide (4) reacted to give only 9 and 10 but no cycloadduct of type 8 (Scheme 4). With the aim to favor the formation of 8, a mixture of 6a and 1.1 equiv. of 7 was heated to 45° without any solvent in a sealed tube. The ratio of products was only slightly different from that of the thermolysis in THF. An analogous experiment with 7 and 9H-fluorene-9-thione S-oxide (6c) yielded cycloadduct 13 and 9 (Scheme 5). It is most interesting that the 1,3-dipolar cycloadditions of 1 and the sulfines 6a and 6c proceeded with different regioselectivity. A reaction mechanism for the unexpected formation of 10 is proposed in Scheme 7. The key step is the base-catalyzed ring opening of 7 and the nucleophilic addition of the thereby formed thiolate 21 onto the sulfonium ion 19.
    Additional Material: 1 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790105
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  • 200
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    The Chemistry of Stable Carbenes. Part 2Part 1: [15].. Benzoin-type condensations of formaldehyde catalyzed by stable carbenes (1996)
    New York, NY : Wiley-Blackwell
    Helvetica Chimica Acta 79 (1996), S. 61-83 
    Details
    ISSN: 0018-019X
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Stable carbenes derived from thiazole, 1H-imidazole, and 4H-1,2,4-triazole are efficient catalysts for benzointype condensations of formaldehyde. Catalysts derived from N-substituted thiazolium salts trimerize formaldehyde to dihydroxyacetone (II). Catalysts based on 1,4-disubstituted 4H-1,2,4-triazol-1-ium salts give glycolaldehyde (I) as the main product and no II, whereas N,N′-disubstituted 1H-imidazol-3-ium salts yield mixtures of both products. The isolation of several intermediates in the catalytic cycle provide a better insight into the reaction mechanism.
    Additional Material: 6 Ill.
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    URL: http://dx.doi.org/10.1002/hlca.19960790108
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