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  • kinetics  (17)
  • asymmetric synthesis  (16)
  • 1
    ISSN: 0947-6539
    Keywords: asymmetric synthesis ; chirality ; enolates ; hydrogen bonds ; structure elucidation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction of acetophenone with tetrabutylammonium hydroxide affords the tetrabutylammonium enolate of phenyl (2-hydroxy-2-phenyl)propyl ketone. The crystal structure of this chiral enolate shows intramolecular hydrogen bonding between the hydroxyl group and the enolate oxygen atom. Furthermore, the α-methylene units of the ammonium counterion form hydrogen bonds to the basic enolate C and O atoms and to the O atom of the hydroxy group. This three-point bonding occurs selectively on the Re,Re side, a phenomenon which may be responsible for the direction of diastereo-selectivity in the epoxide-forming reaction of the enolate with N-bromosuccinimide.
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  • 2
    ISSN: 0947-6539
    Keywords: iron complexes ; kinetics ; magnetic susceptibility ; Mössbauer spectroscopy ; spin crossover ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The thermal and light-induced spin transitions in [Fe(bpen)(NCS)2] and [Fe(bpen)(NCSe)2] have been investigated by means of magnetic susceptibility and Mössbauer measurements. The HS → LS transition in [Fe(bpen)(NCS)2] is detected at temperatures below 120 K (T1/2 [when the fraction of high-spin species γHS = 0.5] = 68 K) with a residual HS fraction of 8% at 20 K. The transition is fairly abrupt and no hysteresis is observed, in contrast to earlier studies (ref. [8]). The generation of metastable HS states in [Fe(bpen)(NCS)2] is achieved by rapid cooling of the sample and by irradiating the compound at low temperatures with light of appropriate wavelengths (LIESST). The HS → LS relaxation of the metastable spin states after rapid cooling is observed at temperatures between 35 and 42.5 K and shows strong deviations from single exponential behavior. The spin-transition behavior changes drastically on substitution of the thiocyanate ions by NCSe-. In [Fe(bpen)(NCSe)2] a gradual and complete spin transition between 140 and 250 K is observed (T1/2 = 181 K). Irradiation of the compound with green light at low temperatures does not result in formation of long-lived metastable HS states. The spintransition curves derived from magnetic susceptibility and Mössbauer studies show good agreement for each compound, and no hints of significantly different Lamb-Mössbauer factors for the HS and LS states are observed.
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  • 3
    ISSN: 0947-6539
    Keywords: electron transfer ; indene ; kinetics ; rhodium ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reduction of the indenyl complex [Rh(η5-C9H7)(η4-cod)] has been investigated in the context of structural effects induced by the transfer of one electron. The reduction of this complex occurs in two steps, leading first to the radical anion and then to the highly frangible dianion. Both species eliminate the indenyl anion. In the presence of free cyclooctadiene, the related cleavage leading to the indenyl anion and bis-cyclooctadiene rhodium fragments now follows a Michaelis-Menten-type mechanism involving precoordination of one extra COD ligand to the initial radical anion. These results suggest the modification of the hapticity of the indenyl ligand in connection with 17- and 19-electron metalcentered intermediates.
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  • 4
    ISSN: 0947-6539
    Keywords: bipyridine ligands ; helicates ; kinetics ; mass spectrometry ; self-assembly ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The self-assembly of the pentanuclear double helicates Hh, Ha and He from the corresponding oligobipyridine strands Lh, La and Le and Cu1 ions has been investigated by NMR and electrospray mass spectrometry (ESMS). Where as Hh is assembled rapidly (in less than 20 min), He (about 20 h) and especially Ha (about 60 h) form much more slowly. The rate decreases strongly with increasing steric bulk of the substituents in the 4,4′-positions on the bipyridine units; this indicates that the search processes (wrapping, unwrapping) that lead to the final helicate are strongly hindered by the size of the substituents. The ESMS data give information about the species present in solution under different conditions and allow the formulation of possible formation pathways, which may involve, in particular, helicates of hairpin type.
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  • 5
    ISSN: 0947-6539
    Keywords: alkaloids ; asymmetric catalysis ; asymmetric synthesis ; indoles ; Mannich reactions ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Indolo[2,3-a]quinolizines have been prepared in enantiomerically pure form by a very short and efficient synthetic sequence consisting of a) formation of imines of tryptophan esters, b) their enantioselective reaction with substituted silyloxydienes mediated by a chiral or an achiral boron Lewis acid, and c) subsequent ring closure initiated by conversion of the generated vinylogous amides into vinylogous imidoyl chlorides. With this strategy various substituents can be incorporated directly into the 1-position of the heterocyclic framework of complex indole alkaloids by the choice of an appropriate silyloxydiene, so that subsequent derivatization of the alkaloid precursor at this position is rendered unnecessary.
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  • 6
    ISSN: 0947-6539
    Keywords: chemoselectivity ; enzyme inhibitors ; glycosidases ; kinetics ; pyridazines ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (3,4-trans-4,5-trans)-4,5-dihydroxy-3-hydroxymethylhexahydropyrida-zine (16) was synthesized in four steps from 2,4-pentadienol (22) and 4-phenyl-triazolin-3,5-dione (18) in an overall yield of 32%. In the first step a Diels-Alder reaction between 18 and 22 gave (π)-2-hydroxymethyl-8-phenyl-1,6,8-triazabicyclo[4.3.0]non-3-ene-7,9-dione (23c) in 88% yield. Epoxidation of 23c with trifluoromethyl(methyl)dioxirane, generated in situ, gave the trans epoxide 24c in 62% yield. Hydrolysis of the epoxide with perchloric acid gave stereoselectively (2,3-trans-3,4-trans)-3,4-dihydroxy-2-hydroxy-methyl-8-phenyl-1,6,8-triazabicyclo[4.3.0]-nonane-7,9-dione (26) in 73% yield. In the fourth and final step, hydrazinolysis of 26 gave 16 in 84% yield. Pyridazine 16 was found to be a potent inhibitor of α-and β-glucosidase, isomaltase and glyco-gen phosphorylase, while galactosidases and α-mannosidase were not inhibited. The inhibition of β-glucosidase is independent of pH, and was found to be due to unprotonated 16.
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  • 7
    ISSN: 0947-6539
    Keywords: autocatalysis ; coiled coil ; kinetics ; peptides ; self-replication ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A 32-residue α-helical peptide with a sequence similiar to that of the GCN4 leucine zipper region is shown to catalyze its own formation by accelerating the amide bond formation of a 17-residue peptide, preactivated as a thiobenzyl ester, and a 15-residue peptide with a N-terminal cysteine. The self-replication process displays parabolic growth characteristics as revealed by a detailed kinetic analysis. Control reactions with single-mutant peptides strongly support a mechanism in which a ternary and/or quaternary complex of the product with both peptide fragments act(s) as the catalytically active intermediate(s). Furthermore, these experiments reveal a remarkable sequence selectivity, as evidenced by the loss of autocatalytic activity as a result of a single replacement of leucine or valine residues with an alanine at the recognition interface.
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  • 8
    ISSN: 0947-6539
    Keywords: kinetics ; peroxyl radicals ; pulse radiolysis ; radicals ; superoxide radicals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hydroxyl radicals were generated radiolytically and reacted with a number of benzene derivatives (PhH). In the presence of oxygen, the hydroxycyclohexadienyl radicals thus formed were converted into their corresponding peroxyl radicals. Pulse radiolysis has shown the oxygen addition (forward reaction, f) to be reversible (reverse reaction, r) [Eq. (1)]. The peroxyl radicals can eliminate HO2· to yield phenols, alongside some ring-fragmentation products (product-forming reaction, p). The rate constants for the forward and reverse reactions (kf and kr) and the corresponding stability constants K (= kf/kr) were determined for the hydroxy-cyclohexadienyl radicals derived from anisole, toluene, fluorobenzene, benzene, chlorobenzene, benzyl chloride, benzoate ion, phenylalanine, and terephthalate ion. The constants kf lie between 8 × 108 (anisole) and 1.6 × 107 dm3 mol-1 s-1 (terephthalate ion), and kr between 7.5 × 104 (toluene) and 3.4 × 103 s-1 (terephthalate ion). The stability constants lie between 2.6 × 104 (benzene) and 3.3 × 103 dm3 mol-1 (phenylalanine). The rate constants for the product-forming reactions kp are between 5.5 × 103 (anisole) and 3.4 × 102 s-1 (benzoate). For the peroxyl radical derived from phenylalanine, a bond dissociation energy of 5.5 kcal mol-1 has been derived. A number of hydroxy-cyclohexadienyl radicals (e.g., those derived from benzoic acid, ethylbenzoate, benzonitrile, and nitrobenzene) react too slowly to allow the equilibrium constant to be determined by means of pulse radiolysis. These reactions have rate constants kf in the order of 5 × 106 dm3 mol-1 s-1, except for nitrobenzene where the reaction is too slow for measurement. The rate constants kr are below 500 s-1, and the product-forming reaction is too slow to be detected by pulse radiolysis. γ-Radiolysis of N2O/O2(4:1)-saturated aqueous solutions of benzonitrile gave dimeric compounds (e.g., dicyanobiphenyls) in low yield, alongside the three isomeric phenols; this again proves the low reactivity of its hydroxycyclohexadienyl radical toward oxygen.
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  • 9
    ISSN: 0947-6539
    Keywords: amino acids ; asymmetric synthesis ; BNCT ; carboranes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two α-amino acids containing the 1,2-dicarba-closo-dodecaborane (12) cage, namely, 5-(1,2-dicarba-closo-dodecaboran (12)-1-yl)-2-aminopentanoic acid (1) and 5-(2-methyl-1,2-dicarba-closo-dodecaboran (12)-1-yl)-2-aminopentanoic acid (2), were prepared by asymmetric synthesis (e.p. 〉 98%) by using the chiral glycine equivalent, imidazolidinone 3, introduced by Seebach, and Oppolzer's camphor-derived sultam derivative 4. The dextrorotatory enantiomers (sodium D line in methanol) of the amino acids 1 and 2 were both shown to have (S) configuration.
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  • 10
    ISSN: 0947-6539
    Keywords: antitumor agents ; DNA ; kinetics ; platinum complexes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The kinetics of the reactions between the GG-containing double-stranded oligonucleotide d(TTGGCCAA)2 (II) and the platinum complexes cis-[Pt-(NH3)2(H2O)2]2+ (1) and [Pt(NH3)3-(H2O)]2+ (2) were studied and compared with those already determined for the reactions of the single-stranded octanucleotide d(CTGGCTCA) (I).[1] The results were as follows: i) Complex 1 reacted faster than 2 with both I and II. ii) Both complexes 1 and 2 reacted faster with II than with I. This acceleration was greater for 1 (x 13) than for 2 (x4) and only due to the increase of the platination rate of the 5′-G of the GG sequence. iii) For both I and II, the first platination by 1 and 2 was faster on the 5′-G than on the 3′-G. This difference was more significant for the platination of II (k5′/k3′ = 12 for 1 and 5 for 2) than of I (k5′/k3′ ≤ 2). iv) The cyclization reaction of the monoadducts (G*) of 1 to yield the GG cis-Pt(NH3)2+2 chelate (G*G*) was considerably slowed down in the duplex. This rate decrease was significantly larger for the chelation of the 5′-G* (factor of 16) than of the 3′-G* (factor of 4) monoadducts. v) The intrastrand chelation of the 3′-G* monoaducts (k3′c) was faster than that of the 5′-G* monoadducts (k5′c), both for I and II (k3′c/k5′c = 3 and 13, respectively). vi) In addition to the intrastrand G*G* crosslink, we also observed the interstrand crosslink d(GG*CC)-d(GG*CC) between the two 3′-Gs of the central tetranucleotide. The rate constant for the interstrand crosslinking (k3′i) was half that of the intrastrand chelation (k3′c). vii) The 5′ monoadduct, which was formed faster (k5′ 〈 k3′) and was chelated more slowly (k5′c 〉 k3′i 〈 k3′c), exhibited a half-life of 3.2 h under our experimental conditions.
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