bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxygen transfer reactions. 3. The oxidation of sulfides by sodium perruthenate
Authors: Lee Donald G. , Gai Huifa
Published: May 2006

The transfer of oxygen from transition metal oxidants to sulfur (MO4− + R2S → MO3− + R2SO) has been studied using the oxidation of alkylthio or arylthioacetic acids (RSCH2CO2H) by perruthenate as models. These reactions exhibit a first-order dependence on the concentrations of both the oxidant and the reductant, and are directly proportional to the basicity of the medium. The Hammet ρ value is negative (ρ = −0.66), a better correlation being obtained when σ, rather than σ+, substituent constants are used. The entropies of activation for the reactions are large and negative, indicating that the transition state is highly structured. A reaction mechanism involving an activating expansion of the ruthenium coordination shell through incorporation of a hydroxide ion is proposed. Oxygen transfer is then initiated by reaction of a nonbonding pair of sulfur electrons with either a vacant ruthenium d-orbital or a Ru=O π*-orbital. Keywords: perruthenate, sulfides, oxygen transfer, oxidation.
Ironcyclopentadienyl mediated 2-alkyl-2-arylphenylsulphonylacetonitrile synthesis
Authors: Abd-El-Aziz Alaa S. , Denus Christine R. de , Hutton Harold M.
Published: May 2006

A unique route to the synthesis of 2-alkyl-2-arylphenylsulphonylacetonitriles via the nucleophilic aromatic substitution (SNAr) of chloroarene cyclopentadienyliron complexes with 2-alkyl phenylsulphonylacetonitriles has been investigated. Reactions of chloroarene complexes (1a–d) with 2-alkyl phenylsulphonylacetonitrile (2a,b) in the presence of K2CO3 in DMF at room temperature led to the formation of complexes 3a–d and 4a,c,d in good yields. The use of alkylated phenylsulphonylacetonitriles as nucleophiles in the reactions with the p-dichlorobenzene complex (1e) allowed the formation of the disubstituted complexes (5,6). Photolytic demetallation provided an efficient route to the liberation of the arylated phenylsulphonylacetonitriles 7a–d, 8a,c,d, 9, and 10. Keywords: chloroarene, phenylsulphonylacetonitrile, nucleophilic substitution.
Generation of oxindole-2-thiones (thiols) via dilithiatedN-tert-Boc-anilines. Synthesis of a sulfur analog of MK886
Authors: Cervantes Alicia , Contreras Claudia A. , Guzman Angel , Vale Elena E. , Velarde Esperanza , Berthiaume Sylvie L. , Muchowski Joseph M.
Published: May 2006

The N,C-dilithiated derivatives of the N-tert-butoxycarbonyl-2-alkylthiomethylanilines 5a–c and the N-tert-butoxycarbonyl-2-alkoxycarbonylmethylanilines 11a, b were converted into the N-tert-butoxycarbonyl-oxindole-2-thiones(thiols) 7a–c and 14a, b, respectively, by sequential reaction with carbon disulfide and methyl iodide. Compound 7b was converted into 10d, the sulfur analog of MK886. Keywords: N-tert-butoxycarbonyl-2-alkylthiomethylanilines, dilithiation, N-tert-butoxycarbonyl-2-alkoxycarbonylmethylanilines, N-tert-butoxycarbonyloxindole-2-thiones.
Dependence upon temperature of the excess molar volumes of tetrahydropyran + n-alkane mixtures
Authors: Bravo R. , Pintos M. , Amigo A.
Published: May 2006

Excess molar volumes VE, at 288.15 and 308.15 K and normal atmospheric pressure, are reported for the binary mixtures tetrahydropyran + n-alkanes (C7–C9). From the results, the differential coefficient (∂VE/∂T)p was estimated over the entire range of mole fractions. The Prigogine–Flory–Patterson theory and its applicability in predicting VE is tested. Keywords: Excess molar volumes, digital densimeter, tetrahydropyran, n-heptane, n-octane, n-nonane, Prigogine–Flory–Patterson theory.
1994 E.W.R. Steacie Award Lecture Application of natural products chemistry to a biological problem
Authors: Ayer William A.
Published: May 2006

Aspen that bear a certain type of black gall have a lower incidence of heartwood rot (caused by the fungus Phellinustremulae) than do nearby non-gall trees. Efforts to determine the chemical nature of this black gall effect are described. The metabolites of some fungi associated with the black gall (Phomaetheridgei, Stachybotryscylindrospora), and of the rotting fungus Phellinustremulae, are described. Extracts of the black gall tissue have a very high concentration of benzoic acid and it is suggested that the benzoic acid may play a role in the protection of the galled trees. Keywords: fungal metabolites, aspen, benzoic acid, salicylic acid, black galls on aspen, Phellinustremulae.
The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 8: methanol–2-carene, and 1,4-dicyanobenzene
Authors: Arnold Donald R. , Du Xinyao , de Lijser Huub J.P.
Published: May 2006

The structure and reactivity of the radical cation of (+)-2-carene ((1S,6R)-3,7,7-trimethyl-cis-bicyclo[4.1.0]hept-2-ene (3)) have been studied. The radical cation was generated by photoinduced single electron transfer to the first electronically excited singlet state of 1,4-dicyanobenzene in acetonitrile–methanol (3:1). The 1:1:1 (methanol:2-carene:1,4-dicyanobenzene) adducts were formed: trans-3-(4-cyanophenyl)-4-(1-methoxy-1-methylethyl)-1-methylcyclohexene(14), and cis- (15) and trans-3-(4-cyanophenyl)-6-(1-methoxy-1-methylethyl)-3-methylcyclohexene (16) in a combined yield of 80%. The efficiency of the reaction and the yield of products were increased by the addition of biphenyl, serving as a codonor. These photo-NOCAS adducts formally result from cleavage of the three-membered ring of the 2-carene radical cation, at the C1—C7 bond, forming the tertiary carbocation and allylic radical. Reaction of the cation with methanol and coupling of the allylic radical with the 1,4-dicyanobenzene radical anion at the ipso position, followed by loss of cyanide ion, completes the sequence. There was no evidence for cleavage of the C1—C6 bond under these conditions; however, when the irradiation was carried out in acetonitrile (no methanol) the (+)-2-carene was partially racemized. Racemization is indicative of C1—C6 bond cleavage. The results of abinitio molecular orbital calculations (STO-3G) provide insight into the extent of C1—C7 bond cleavage in the radical cation. The calculated spin and charge distribution, on the 2-carene radical cation global minimum (3a+•), is consistent with the observed regiospecificity of adduct formation. Keywords: photoinduced electron transfer, radical ions, molecular orbital calculations, bond cleavage, 2-carene.
Reaction of γ-nonanolactone with deuterobromic acid
Authors: Granata Alessandro , Sauriol Françoise , Perlin Arthur S.
Published: May 2006

Series of isomeric monohaloalkanoic acids may be prepared from γ- or δ-alkanoic lactones by reaction with hydrogen halides. For example, γ-nonanolactone in 48% hydrogen bromide under reflux affords a 1:1.5:1.7:2.2:2.4 mixture of 4-, 5-, 6-, 7-, and 8-bromononanoic acids. In an examination of possible ways whereby such products are formed, it has been found that the use of 48% deuterium bromide leads to the introduction of up to 14 atoms of the isotope per bromoacid. According to 1H, 2H, and 13C nuclear magnetic resonance observations, however, the major components of the isomer mixture contain 8–10 atoms of deuterium confined mainly to carbons 4–8, i.e., close to full deuteration of each of the carbons. This indicates that a prominent mechanism governing the formation of the bromoacid mixture involves repeated elimination and addition of HBr (2HBr) along a strictly limited segment of the carbon chain. Initiated at the C-4 bridge of the lactone and characterized, hypothetically, by a succession of carbocation intermediates, the halogenation reactions appear to include a repetitive migration of transient alkene functions from C-4 through C-8, and an accompanying series of "zipperlike" hydride (deuteride) shifts in the inverse direction. Keywords: deuterohaloalkanoic acids, carbocation rearrangements.
Taxus canadensis taxanes: structures and stereochemistry
Authors: Zamir Lolita O. , Nedea Maria E. , Zhou Zhen-Hua , Bélair Sophie , Caron Gaétan , Sauriol Françoise , Jacqmain Emile , Jean France I. , Garneau François-X. , Mamer Orval
Published: May 2006

The structures and stereochemistry of 14 taxanes isolated from Taxuscanadensis have been rigorously established by high-resolution NMR techniques and mass spectrometry. Taxuscanadensis is the only yew that accumulates 9-dihydro-13-acetylbaccatin III (also called 7,9-deacetylbaccatin VI) as the most abundant taxane. Five 9-dihydrotaxanes derived from the Canadian yew are novel natural products not reported in the needles of other species of yew (8–11, 14, Fig. 1). Keywords: Taxuscanadensis, Taxaceae, taxanes, 9-dihydro-13-acetylbaccatin III, taxol.
Evidence for reversible dissociative adsorption in the reaction of molecular chlorine with gallium arsenide
Authors: Wong Kin-Chung , Ogryzlo Elmer A.
Published: May 2006

The reaction of molecular chlorine with the (100) face of a gallium arsenide crystal has been studied at pressures of Cl2 between 0.10 and 9.0 Torr and in the temperature range from 90 to 110 °C. In contrast to an earlier report, the etch rate was found to be half order with respect to Cl2. The similarity of these results to those obtained for the reaction of molecular chlorine and bromine with silicon points to a mechanism in which the gaseous halogen molecule is first physisorbed on the semiconductor surface and then dissociates into chemisorbed atoms. The data indicate that both steps occur reversibly at higher pressures, where the composite half-order rate constant can be represented by the Arrhenius equation:[Formula: see text]At low pressures the first-order rate constant is given by the equation:[Formula: see text]Keywords: etching, gallium arsenide, molecular chlorine, kinetics.
Synthesis of nitropolychlorinated dibenzo-p-dioxins (NPCDDs) and their photochemical reaction with nucleophiles
Authors: Merica Simona G. , Bunce Nigel J.
Published: May 2006

A series of nitropolychlorodibenzo-p-dioxins (NPCDDs) was synthesized by condensation between catechols and 2,6-dinitrohalobenzene derivatives. In the presence of sodium ethoxide in anhydrous ethanol, these underwent photochemical SN2Ar* substitutions meta to the nitro group in high chemical yield and moderate quantum yield. Both ring-opening and chloride replacement reactions were observed. The reactions involved the triplet excited state of the NPCDD, and showed a linear relationship between Φ−1and [nucleophile]−1. Analogous reactions with KCN in methanol showed similar behaviour, but the products could not be isolated. Keywords: photosubstitution, SN2Ar*, dibenzo-p-dioxins.
Fragmentation of 2-(dialkylamino)ethyl phosphate derivatives. Mechanism change as a function of substituents at phosphorus
Authors: Pienaar Andre , Modro Tomasz A.
Published: May 2006

The products and the first-order rate constants for the fragmentation of six phosphonic diesters (2) and two phosphonic monester chlorides (3), all containing the 2-(dialkylaminoethyl) group as one of the ester functions, have been determined. On the basis of different reaction products, different rates of decomposition, and different activation parameters, a new mechanism for the fragmentation of ester chlorides (3) is proposed: fragmentation involving release of the chloride ion and the formation of the 1,1-dialkylaziridinium ion and metaphosphonate as the reactive intermediates. Keywords: fragmentation of phosphates and phosphorochloridates, "metaphosphate" mechanism of phosphates, activation entropy in mechanistic change, 2-(dialkylamino)ethylphosphates.
Synthesis and structure of titanium diolate complexes derived from CpTiCl3
Authors: Huang Yujin , Stephan Douglas W.
Published: May 2006

The compounds [Cl2CpTi]2(OCH2CH2CH2O) 1, [Cl2CpTi]2(OCH2CMe2CH2O) 2, and [Cl2CpTi]2-(OCHMeCH2CHMeO) 3 were prepared from the reactions of CpTiCl3 and the bis-trimethylsilyl ethers of the diols. Alternatively, these species could be made from the reactions of the diol, imidazole, and CpTiCl3 employing the appropriate stoichiometry. In the presence of excess diol, loss of CpH is observed. In the case of 1,3-propanediol the species [TiCl(HOCH2CH2CH2O)(OCH2CH2CH2O)]24 was isolated. Recrystallization of 4 from THF led to the isolation of [TiCl2(OCH2CH2CH2O)(THF)]25, albeit in very low yield. This chemistry is described and the implications are considered. Compounds 1, 3, 4, and 5 were characterized crystallographically; the parameters are summarized as follows: 1: C2/c (No. 15), a = 12.587(2) Å, b = 9.632(2) Å, c = 15.824(3) Å, β = 107.88(1)°, V = 1811.3(5) Å3, Z = 4; 3: C2/c (No. 15), a = 12.855(2) Å, b = 10.772(2) Å, c = 15.665(2) Å, β = 109.26(1)°, V = 2047.7(5) Å3, Z = 4; 4: P21/n (No. 14), a = 8.911(3) Å, b = 7.821(2) Å, c = 13.986(2) Å, β = 102.56(2)°, V = 951.3(3) Å3, Z = 2; 5: P21/n (No. 14), a = 8.693(3) Å, b = 9.286(2) Å, c = 13.411(2) Å, β = 101.18(2)°, V = 1062.1(4) Å3, Z = 2. Keywords: titanium alkoxides, structure, synthesis.
Synthesis and structural characterization of an ether-bridged cobalta-bis(dicarbollide): a model for Venus flytrap cluster reagents
Authors: Harwell David E. , Nabakka Juliet , Knobler Carolyn B. , Hawthorne M. Frederick
Published: May 2006

The synthesis of bis-(1-carboranylmethyl) ether was first published in 1963; however, its subsequent complexation with a transition metal was never reported. We now report the complexation of cobalt and the corresponding X-ray structure for the triphenylmethylphosphonium salt of the cobalta-bis(carboranylmethyl) ether. The cobalt complex crystallized in the monoclinic space group C2/c with a = 21.729(6) Å, b = 9.845(2) Å, c = 35.565(9) Å, β = 105.363(9)°, V = 7336 Å3, and Z = 8. Data were collected using CuKα radiation, to a maximum 2θ = 115°, giving 4123 unique reflections, and the structure was solved by heavy atom methods. The final discrepancy index was R = 0.093, Rw = 0.100 for 1919 independent reflections with I > 3σ(I). This metala-bis(carboranylalkyl) ether is the first in a new class of Venus flytrap compounds (VFC), containing an ether linkage, to be synthesized as model compounds for the development of reagents suitable for use in the radioimmunodetection and radioimmunotherapy of cancer. Keywords: cobalt, radioimmunodetection, radioimmunotherapy, Venus flytrap.
Activated cycloheptenone dienophiles. A versatile approach to 6,7-fused ring targets
Authors: Liu Hsing-Jang , Yeh Wen-Lung , Browne Eric N.C.
Published: May 2006

Several 2-carbalkoxy-2-cyclohepten-1-ones were prepared and their Diels–Alder characteristics examined in order to develop a facile general approach to the synthesis of cycloheptane ring-containing polycyclic natural products. Substitution pattern on the seven-membered ring was found to greatly affect the Diels–Alder behavior. In addition, some extraordinary facial stereoselectivity was observed. Keywords: Diels–Alder reaction, 2-carbalkoxy-2-cyclohepten-1-ones, 6,7-fused ring system.
Metal complexes of biologically important ligands, LXXVIII. Synthesis of palladium complexes of ferrocenyl-substituted amino acid derivatives
Authors: Freiesleben Doris , Polborn Kurt , Robl Christian , Sünkel Karlheinz , Beck Wolfgang
Published: May 2006

Schiff bases from ferrocene aldehyde and α-amino acidates react with PdCl2 or Na2PdCl4 to give N,O-chelate complexes (2a–e, 3). The related N-ferrocenylmethylen-α-amino acid esters give, with Pd(OAc)2, cyclopalladated acetate-bridged complexes (4a,b). N,O-chelate complexes (5–7) are obtained from Na2PdCl4 with N-ferrocenylmethyl prolinate and-alaninate as well as with N,N-dibenzyl glycinate. The ethyl ester of N,N-dibenzylglycine and Pd(OAc)2 form the cyclopalladated acetate-bridged complex 8, while the ester of N-monobenzylglycine and Na2PdCl4 affords trans-PdCl2(L)2 with a monodentate N-coordinated ligand. The structures of 2a, 4a, 5, 7, and 9 were determined by X-ray diffraction. Keywords: ferrocene, amino acids, palladium(II) complexes, crystal structures.
Comparison of calcium complexation of some carboxylic acids derived fromD­glucose andD-fructose
Authors: Bazin Hélène , Descotes Gérard , Bouchu Alain , Petit-Ramel Michelle
Published: May 2006

The aim of this work was to compare calcium sequestering behaviour of 11 carboxylic acids derived from carbohydrates, and to study the influence of molecular structure on the calcium complexation. For this purpose, various carboxylic acids derived from methyl D-glucopyranoside, methyl D-fructopyranoside, and methyl D-fructofuranoside were synthesized and studied using an ion selective electrode to determine calcium complex formation constants. Complexation sites of carbohydrate skeletons were determined using13C NMR. Keywords: Ca(II) complexation, carboxylic acids, fructofuranosides, fructopyranosides, glucopyranosides.
1H and13C nuclear magnetic resonance and molecular orbital studies of the internal rotational potential and of spin–spin coupling transmission in phenylallene
Authors: Schaefer Ted , Kroeker Scott , McKinnon David M.
Published: May 2006

The1H nuclear magnetic resonance spectra of phenylallene, diluted in acetone-d6and benzene-d6, yield long-range coupling constants over as many as eight formal bonds between the ring and side-chain protons. These are discussed in terms of σ- and π-electron spin–spin coupling mechanisms, which are sensitive to the torsion angle between the allenyl and phenyl fragments. The torsion angle is assessed by means of molecular orbital computations of the internal rotational potential, whose height is calculated as 16.0 kJ/mol at the MP2/6-31G* level of correlation-gradient theory. Comparison with experimental and theoretical internal rotational potentials for styrene suggests that steric repulsions in the planar form of styrene amount to about 4 kJ/mol. In a field of 7.0 T, phenylallene is partially aligned, entailing a positive dipolar coupling constant between the methylene protons, from which absolute signs of the spin–spin coupling constants involving these protons can be inferred. Such coupling constants over seven and eight bonds, to the meta and para protons, are taken as being mediated by the extended π-electron system, providing a measure of π-electron contributions to coupling constants between meta protons and those in side chains (spin correlation). Some coupling constants between protons and13C nuclei in the side chain, as well as between ring protons and these13C nuclei, are also discussed in terms of spin coupling mechanisms. Solvent perturbations of one-bond proton–carbon coupling constants in the allenyl group do not follow the usual pattern in which an increase in polarity of the solvent is associated with an increase in the magnitude of the coupling constant. Keywords:1H NMR, phenylallene;1H NMR, long-range spin–spin coupling constants in phenylallene; phenylallene, internal rotational potential, molecular orbital computations; molecular orbital calculations, an internal rotational potential in phenylallene.
Action of tetrabutylammonium tribromide with para-substituted chalcones in protic and aprotic media
Authors: Berthelot Jacques , Benammar Yamina , Desmazières Bernard
Published: May 2006

Bromination of the double bond of para-substituted chalcones under mild conditions in aprotic solvents is accomplished with high yields using tetrabutylammonium tribromide (TBABr3). In methanol, the main reaction is (α-β) bromomethoxylation. Stereoselectivity, regioselectivity, and chemioselectivity of this bromomethoxylation reaction are described. Keywords: bromination, bromomethoxylation, tetrabutylammonium tribromide, (α-β) dibromodihydrochalcones, α-bromo, β-methoxydihydrochalcones.
On the Baylis–Hillman reaction of acrylate, acrylonitrile, and acrolein with some non-enolizable α-dicarbonyl compounds: synthesis of phytotoxic bipolaroxin models
Authors: Strunz George M. , Bethell Richard , Sampson Glen , White Peter
Published: May 2006

The Baylis–Hillman reaction of acrylonitrile, methyl acrylate, and acrolein with several cyclic α-dicarbonyl compounds was investigated. Whereas acrylonitrile reacted with most of these ketones, giving good yields of the expected 1′-cyanovinyl carbinols, the more sterically demanding methyl acrylate failed to undergo the reaction. Attempted Baylis–Hillman reaction of acrolein with the 1,2-dicarbonyl substrates usually resulted in polymers but, in two cases, the desired α-substituted acroleins were obtained. An alternative route to such compounds was developed, employing a Grignard – allylic oxidation sequence. In bioassays, some of the products, embodying the functionality believed responsible for the phytotoxicity of the sesquiterpene bipolaroxin, inhibited germination of lettuce seeds. This activity diminished with time. Keywords: Baylis–Hillman, acrylonitrile, acrylate, acrolein, α-dicarbonyl, bipolaroxin, phytotoxicity.
Reactive collisions in quadrupole cells. 6. H/D exchange reactions of protonated alkylbenzenes with D2O, CH3OD, and C2H5OD
Authors: Ni Jinsong , Harrison Alex. G.
Published: May 2006

The H/D exchange reactions occurring on collision of protonated alkylbenzenes with C2H5OD, CH3OD, and D2O at low collision energies in the quadrupole collision cell of a hybrid tandem mass spectrometer have been studied for 22 alkylbenzenes. The exchange reactions with D2O are much less efficient than the exchange reactions with C2H5OD or CH3OD; this difference is rationalized on energetics grounds. The H/D exchange of the protonated alkylbenzenes with C2H5OD or CH3OD leads, typically, to 40–70% deuterium incorporation in the protonated alkylbenzene and, for most cases, to significant ion signals corresponding to exchange of the added hydrogen and the aromatic hydrogens. Thus, the method is a feasible approach to counting the number of aromatic hydrogens in alkylbenzenes. The extent of exchange decreases with increasing size of the alkyl group in monoalkylbenzenes for reasons that are not readily understood. Keywords: protonated alkylbenzenes, H/D exchange, reactive collisions.
Redox reactions of 3,5-di-tert-butyl-1,2-benzoquinone. Implications for reversal of paper yellowing
Authors: Jovanovic S.V. , Kónya K. , Scaiano J.C.
Published: May 2006

One- and two-electron transfer reactions of 3,5-di-tert-butylcatechol and the corresponding quinone were studied by fast kinetic spectroscopy coupled with laser photolysis and pulse radiolysis, and by cyclic voltammetry in aqueous solutions. Photoionization of catechol yields the same semiquinone radical as the formate-radical-induced quinone reduction in the pulse radiolysis experiment. The neutral semiquinone radical deprotonates at pKr = 6.0 ± 0.1, as deduced from the pH induced changes in the radical spectra. The two-electron reduction potentials of quinone and catechol were measured by cyclic voltammetry in aqueous solutions containing 25% methanol in the pH range 3–14. The pH dependence has two linear parts with slopes of 0.056 and 0.03 V/pH, intersecting at pKa = 10.0. This is in excellent agreement with pKa = 10.05 ± 0.05 obtained spectrophotometrically. The reduction potential of 3,5-di-tert-butyl quinone, E7 = 0.01 ± 0.04 V, was obtained from the one-electron transfer equilibrium of the quinone with the oxygen/superoxide redox couple by pulse radiolysis. The rate constant of the reaction of quinone with the superoxide radical, k = 1.2 × 109 M−1 s−1, is higher than those of the superoxide reduction by catechols and phenols (typically ~ 105 M−1 s−1); thus, lignin o-quinones could be efficient scavengers of superoxide under the conditions relevant for the photo-induced yellowing of lignin-rich paper. The reaction with [Formula: see text] effectively bleaches yellow quinone and generates metastable furanone, which hydrolyses to muconic acid, thus completely eliminating yellow quinone. 3,5-Di-tert-butylquinone also undergoes rapid bleaching with ascorbate, k = 600 ± 100 M−1 s−1, in methanol. The reaction has a 1:1 stoichiometry and leads to complete reduction of quinone to catechol with concomitant oxidation of ascorbate to dehydroascorbate. This unusual selectivity and the fact that the reaction of the milder oxidant 3,5-di-tert-butylquinone is an order of magnitude faster than that of stronger oxidant p-benzoquinone (k = 60 ± 10 M−1 s−1) suggest that a nucleophilic attack of quinone at the ascorbate double bond initiates the reaction. Keywords: superoxide, lignin, photochemistry, quinone.
Chemically modified pullulans II. New hydrophobically substituted derivatives
Authors: Carpov A. , Mocanu G. , Chapelle S. , Merle L. , Muller G.
Published: May 2006

New hydrophobically substituted pullulans were synthesized, such as (i) neutral derivatives obtained by direct esterification of pullulan with adenine and thymine butyric acid and (ii) ionic derivatives obtained by amidation of carboxymethyl pullulan with aromatic amines (aminopyrene and adenine). Rheological measurements in dilute solutions allowed us to test the influence of the hydrophobic aromatic substituents and of the ionicity of the parent support on the nature of the hydrophobic associations in dilute solutions. The rheological studies on the pullulan derivatives revealed mainly intramolecular hydrophobic associations for both ionic and non-ionic derivatives, suggesting a more compact conformation than that of the precursor. Keywords: hydrophobically modified pullulans, esterification, amidation, condensed aromatic ring, size exclusion chromatography, multi-angle laser light scattering.
Unusual polyketides from the wood-decay fungus Sistotrema raduloides
Authors: Amegadzie Albert K. , Ayer William A. , Sigler Lynne.
Published: May 2006

The metabolites of the wood rot decay fungus Sistotremaraduloides (P. Karst) Donk have been investigated and a new type of norpentaketide has been discovered. The compounds sistodiolynne (1), sistolynone (5), and sistopyrone (6) represent new carbon skeletons among natural products. The compounds are very unstable and readily polymerize in the presence of air to give black insoluble material. It has been shown by 13C NMR labelling experiments that they arise from five acetate units, and that a methyl carbon of one of the acetate units is lost during the biosynthesis. The structures were determined by spectroscopy methods, mainly NMR, and the absolute configuration of sistodiolynne was established by circular dichroism methods. Keywords: fungal metabolites, Sistotremaraduloides, norpentaketides, biosynthetic studies, polyacetylenes.
Comportement photochimique du 4-nitrosophénol en solution aqueuse
Authors: Pilichowski Jean-François , Boule Pierre , Billard Jean-Philippe
Published: May 2006

In acidic medium (pH ≈ 1)4-nitrosophenol irradiated in the range 296–366 nm is mainly transformed into benzoquinone (75–80%). The quantum yield of this reaction is wavelength dependent: it is about 6 times higher at 365 nm than at λ ≤ 313 nm. This phenomenon is attributed to an equilibrium between two tautomeric forms, nitroso and quinone monooxime, the former being much more photoreactive than the latter. In unbuffered aqueous solution (pH = 5–6) the formation of an intermediate is observed. Its HPLC retention time is just a little shorter than that of benzoquinone and the maximum of its absorption band is located at 235 nm. The rate of transformation of this intermediate increases with decreasing pH. Keywords: photochemistry, 4-nitrosophenol, wavelength effect.
A cycloaddition approach to tricyclic taxoid skeletons
Authors: Lu Yee-Fung , Fallis Alex G.
Published: May 2006

A cycloaddition approach to the functionalized tricyclo[9.3.1.03,8]pentadecene skeleton contained in Taxol® is described. The cyclohexenone 13 was converted as illustrated to the nitrile-aldehyde 24 to which the diene and acetylenic side chains were attached by sequential nucleophilic additions. Removal of the trimethylsilyl protecting group and Dess–Martin oxidation afforded the triene 35. Microwave-assisted thermal cyclization stereoselectively generated the tricyclic ketone 36 whose structure was further established by conversion to the aromatic system 37 upon treatment with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). An epoxidation sequence was developed to introduce the epimeric C13 alcohol 47 as required for this cycloaddition strategy. Alternatively, an allylic oxidation (CrO3, 3,5-dimethylpyrazole) afforded the C13 ketone 49. Keywords: Taxol®, Diels–Alder, synthesis, diterpene, alkaloid.
The asymmetric synthesis of aryltetralin lignans: (−)-isolariciresinol dimethyl ether and (−)-deoxysikkimotoxin
Authors: Coltart Don M. , Charlton James L.
Published: May 2006

The total asymmetric syntheses of (−)-isolariciresinol dimethyl ether (6) and (−)-deoxysikkimotoxin (7) have been carried out, in an attempt to exploit a synthetic strategy recently developed for the synthesis of (−)-deoxypodophyllotoxin (1, R1 = -CH2−, Ar = 3,4,5-trimethoxyphenyl). In so doing, a generalized method for the synthesis of aryltetralin lignans has been developed that should be applicable to a variety of substitution patterns and stereochemistries. A one-pot, 100% regio-selective reduction–lactonization procedure has been developed for the conversion of the ester 18b to (−)-deoxysikkimotoxin, which gave 93% isolated yield in that step. Key words: ortho-quinodimethanes, lignans, Diels–Alder, asymmetric, mandelate.
The selective complexation of adamantane nitriles by tungsten pentacarbonyl
Authors: Jefford Valerie J. , Schriver Melbourne J. , Zaworotko Michael J.
Published: May 2006

Adamantyl-1,3,4-oxathiazol-2-one is usually prepared as a mixture with 1-adamantanecarbonitrile. To separate these two compounds the mixture is reacted with thf•W(CO)5, which selectively forms a complex with the nitrile. The resulting mixture can then be readily separated into pure compounds by sublimation. Characterization data are presented, including the X-ray crystal structure of the nitrile complex, which can be prepared directly from the reaction of the adamantyl nitrile and thf•W(CO)5. (Crystal data for C16H15NO5W: orthorhombic, space group Pmcn, a = 10.5869(19) Å, b = 14.0622(22) Å, c = 23.342(4) Å, V = 3475.0(11) Å, Z = 8, R = 0.042.) The nitrile can be recovered from the complex by reaction with P(C6H5)3 followed with separation by sublimation. The reaction of the related 1-cyano-3-(1,3,4-oxathiazol-2-on-5-yl)-adamantane with thf•W(CO)5 yields a complex in which the site of coordination is shown spectroscopically to be the nitrile moiety. Semi-empirical calculations at the PM3 level indicate that the oxathiazolone heterocycle may be a poor ligand due to the influence of the exo- and endo-cyclic oxygen atoms. Key words: oxathiazolone, nitrile, tungsten, complexation, selectivity, adamantane.not available
Modified method of synthesis of N-substituted dithioesters of amino acids and peptides in the Pinner reaction
Authors: Neugebauer Witold , Pinet Eric , Kim Munsok , Carey Paul R.
Published: May 2006

An improved method for the synthesis of dithioesters of amino acids and peptides has been developed. The syntheses have been carried out from the nitriles. The addition of thiol to the nitrile derivative in the Pinner step of dithioester synthesis was activated with hydrogen fluoride. A few examples of dithioester synthesis using liquid HF are described. Some novel dithioesters, which are model compounds for resonance Raman spectroscopic studies of dithioacylpapain intermediates, are described. Key words: dithioesters, amino acids, Pinner reaction, HF, isotopes.
Use of19F NMR as a direct probe of Δ9desaturase cryptoregiochemistry: a feasibility study
Authors: Buist Peter , Marecak Dale , Dawson Brian , Black Bruce
Published: May 2006

19F NMR can be used to monitor the relative efficiency with which various fluorinated aromatic sulfides are oxidized by the Δ9 desaturating system of Saccharomyces cerevisiae. Thus the sulfoxy acids produced from methyl S-(4-(trifluoromethyl)benzyl)-8-mercaptooctanoate (3d), methyl S-(4-(trifluoromethyl)phenyl-2-ethyl))-8-mercaptooctanoate (3e), and methyl S-(4-(trifluoromethyl)benzyl)-9-mercaptononanoate (4c) were observed in the supernatant of S. cerevisiae cultures at concentrations of ~140, ~45, and ~10 μM, respectively. This paper lays the methodological basis for a novel, in vitro "thia test" of cryptoregiochemistry and provides further experimental evidence that the yeast Δ9 desaturase initiates oxidation of stearoyl CoA at C-9. Key words:19F NMR, desaturase, chiral sulfoxidation, hydrocarbon activation.
Substituted tetra-2,3-pyrazinoporphyrazines. Part I. Angular annelation of tetra-2,3-quinoxalinoporphyrazine
Authors: Kudrevich Svetlana V. , Lier Johan E. Van , Galpern Maria G. , Luk'yanets Evgeny A.
Published: May 2006

Several derivatives of 2,3-dicyanopyrazine were prepared via the condensation of o-quinones with diaminomaleodinitrile. Benzo[f]quinoxaline-2,3-dinitrile was obtained from 1,2-naphthoquinone, and a series of isomeric, di-tert-butyl substituted 5,6-(9,10-phenanthro)-2,3-dicyanopyrazines were prepared from the corresponding di-tert-butyl-9,10-phenanthrenequinones. Complexation of benzo[f]quinoxaline-2,3-dinitrile, and unsubstituted 5,6-(9,10-phenanthro)-2,3-dicyanopyrazine, with an appropriate metal salt yielded metal complexes of tetra-2,3-(benzo[f]quinoxalino)porphyrazine and tetra-2,3-[5,6-(9,10-phenanthro)]porphyrazine, respectively. Metal-free tetra-2,3-[5,6-(9,10-phenanthro)porphyrazine was obtained from the dilithium derivative by demetallation in HCl. These compounds have limited solubility in organic solvents, such as quinoline, and are aggregated in solutions. To eliminate the aggregation phenomenon and to determine the spectral properties of angularly annelated naphthalocyanine aza analogs, we prepared several isomeric tetra-2,3-[5,6-(di-tert-butyl-9,10-phenanthro)pyrazino]porphyrazines. These octa(tert-butyl) substituted complexes were synthesized via complexation of di-tert-butyl substituted 5,6-(9,10-phenanthro)-2,3-dicyanopyrazines with metal salts in the presence of urea, quinoline, and tri(n-butyl)amine, and purified by silica gel chromatography. They are soluble in chloroform and substantially monomerized in solutions. A hypsochromic shift of the Q-band of octaaza naphthalocyanines versus their carbocyclic analogs was observed for all aza analogs, with the extent of the shift depending on the composition of the aromatic macrocycle. Thus, the first angularly annelated benzo ring addition causes a hypsochromic shift (∼25 nm) of the Q-band of tetra-2,3-quinoxalinoporphyrazine, whereas addition of a second condensed benzo ring has little effect. Key words: phthalocyanine, aza analog, tetra-2,3-quinoxalinoporphyrazine, angular annelation.
Intramolecular 1,2-alkyl shifts in unsymmetric dialkoxycarbenes studied by very low vapour pressure (VLVP) pyroiysis – mass spectrometry
Authors: Suh Dennis , Pole David L. , Warkentin John , Terlouw Johan K.
Published: May 2006

Methoxy-(2,2,2-trifluoroethoxy)carbene radical cations, CH3O-C-OCH2CF3•+, 1•+, are cleanly generated by the dissociative electron ionization (EI) of 2-methoxy-5,5-dimethyl-2-(2,2,2-trifluoroethoxy)-Δ3-1,3,4-oxadiazoline I. Neutralization–reionization (NR) mass spectrometry of the neutral carbene 1, generated by one-electron reduction of 1•+, shows no recovery ion signal and thus 1 is not a viable species within the μs time scale of the experiment. Very low vapour pressure (VLVP) pyrolysis – mass spectrometry of I in conjunction with (multiple) collision experiments shows that 1 completely isomerizes, via a 1,2-trifluoroethyl shift, into methyl 3,3,3-trifluoropropionate, CF3CH2C(=O)OCH3, 1a. This technique was also used to study the related dialkoxycarbenes C2H5O-C-OCH2CF3, 2, CH3O-C-OC2H5, 3, and CH3O-C-OCH(CH3)2, 4, generated from the corresponding 2,2-dialkoxy-5,5-dimethyl-Δ3-1,3,4-oxadiazolines. The pyrolytically generated carbene 2 behaves analogously to 1 and completely isomerizes to ethyl 3,3,3-trifluoropropionate, 2a. The neutral carbenes 3 and 4 undergo only a partial isomerization via 1,2-alkyl shifts in which the ethyl and isopropyl groups show a slightly greater migratory aptitude, respectively, than the methyl group. The differences in migratory aptitude are explained in terms of a transition state model similar to that of a 1,2-H shift in carbenes, with development of negative charge in the migrating group. The greater migratory aptitude of CF3CH2, as compared to CH3 and CH3CH2, is attributed to the stabilization of negative charge in the transition state by strongly electron-withdrawing β-fluorines whereas the differences in migratory aptitude between the alkyl groups in 3 and 4 are largely due to the greater polarizability of isopropyl and ethyl groups, as compared to the methyl group. Key words: dialkoxycarbenes, pyrolysis, tandem mass spectrometry.
Polyhalide anions in crystals. Part 1. Triiodides of the Me4N+, Me4P+, quinuclidinium, 1-azoniapropellane, and 1,4-diazoniabicyclo[2.2.2]octane (DabcoH22+) cations, and 1,10-phenanthrolinium(1+) tribromide
Authors: Bakshi Pradip K. , James Margaret A. , Cameron T. Stanley , Knop Osvald
Published: May 2006

Crystal-structure determinations are reported for Me4NI3(4MI3, Pnnm, a = 10.107(2), b = 14.141(2), c = 8.252(5) Å, Z = 2), quinuclidinium I3(QI3, Pnnm, a = 10.290(2), b = 14.011(2), c = 8.813(2) Å,Z = 2), Me4PI3(4MPI3,P42/m, a = 15.098(4), c = 13.770(4) Å, Z = 10), 1-azoniapropellane I3(API3, P21/c, a = 15.131(7), b = 28.453(6), c = 22.848(8) Å, β = 108.53(3)°, Z = 24), 1,4-diazoniabicyclo[2.2.2]octane (I3)2(Da2I3, P21/m, a = 9.671(6), b = 8.127(2), c = 11.144(3) Å, β = 96.60(3)°, Z = 2), and 1,10-phenanthrolinium(1+) Br3(phenBr3, [Formula: see text] a = 8.370(2), b = 10.410(4), c = 8.171(2) Å, α = 107.52(2)°, β = 97.35(1)°, γ = 94.84(2)°, Z = 2). The nearly isostructural 4MI3 and QI3 are of a distorted anti-NiAs type. The 4MPI3 and API3 structures can be regarded as consisting of distorted CsCl-type coordination cuboids; both are unexpectedly complex, 4MPI3 (not isostructural with 4MI3) containing cations of two and anions of three types, API3 containing cations of six and anions of seven types. API3 thus represents an extreme case of a frustrated CsCl structure. The ion packing in Da2I3 is related to that in CaF2. The distortion of one of the two nonequivalent I3−anions in Da2I3 is one of the largest, if not the largest on record (I—I = 2.793(2) and 3.167(2) Å, I-I-I = 172.47(4)°), and attributable to the involvement of the anion in a strong N—H … I bond, H … I = 2.78 Å. The bifurcated H-bond in which the protonated cation nitrogen in phenBr3is engaged has an intra-cation, N—H(N)… N, branch and an inter-ion branch, N—H(N)… Br. Key words: crystal structure, hydrogen bond, polyhalide anions, tribromides, triiodides.
Effect of ligand structure, solvent, and temperature on the electrochemical behavior of polyarene–iron complexes
Authors: Abd-Ei-Aziz Alaa S. , Denus Christine R. De , Epp Karen M. , Smith Simone , Jaeger Richard J. , Pierce David T.
Published: May 2006

The electrochemical investigation of a number of polyarene–iron complexes ([3]2+–[9]5+) containing etheric, sulphide, and sulphone bridges indicated that there were various degrees of interaction based on the nature of the bridging heteroatoms. While the electrochemical investigation of all etheric complexes showed that the metallic moieties behaved as isolated redox centers, it was found that there was electronic communication (ca. 70–80 mV) for the isomeric sulphide complexes [4]2+ and [6]2+. The rate constant of the following chemical reaction (kf) was calculated for some of these complexes and it was found that these rates were affected by the nature of the solvent, the bridging ligand, and the temperature. At various temperatures, kf indicated a higher degree of stability for complexes containing sulphide bridges than for those containing etheric bridges, especially at room temperature. The effect of a strong coordinating solvent, such as acetonitrile, on the kf of complex [3]2+ indicated that the substitution of the arene ligand with acetonitrile molecules proceeded as a dissociative mechanism. Controlled potential coulometry was also used to verify the transfer of two electrons in the first reduction process of the di-iron complexes. Key words: cyclopentadienyliron, cyclic voltammetry, arene complexes, isolated and interacting redox centers.
The chemistry of thujone. XVIII Homothujone and its derivatives
Authors: Kutney James P. , Chen Yong-Huang , Rettig Steven J.
Published: May 2006

The monoterpene thujone (1) is ring expanded regioselectively to provide homothujone (4) in order to explore its potential as a chiral starting material. Stereoselective Robinson annulation of homothujone provides an enone intermediate (5), further derivatization of which was undertaken to furnish functionalized naphthalenones useful for synthesis of the antifeedant (−)-polygodial and the ambergris fragrance (−)-Ambrox®. Key words: thujone, homothujone, synthesis, naphthalenones, polygodial, Ambrox.
Topological properties of the electronic structures of the reactants, transition states, and products of the reactions of the hydroxyl radical with the series C2HnF6−n, n = 1–6
Authors: Martell Jaime M. , Tee James B. , Boyd Russell J.
Published: May 2006

Properties of the bond critical points, including the electron density and its Laplacian, and distances to bonded nuclei, for all species (reactants, transition states, and products) in the reactions of the hydroxyl radical with the series C2HnF6−nn = 1–6 were calculated using Bader's atoms-in-molecules methodology. The electron density and its Laplacian at the bond critical points correlate with bond strength, as measured by bond dissociation energies. The positions of the bond critical points vary with the electronegativity of surrounding atoms. Charge development in the course of the reactions was monitored using Mulliken population analysis at the HF/6-31G(d), HF/6-311G(d,p)//HF/6-31G(d), MP2/6-311G(d,p)//HF/6-31G(d), and HF/6-311G(d,p)//MP2/6-31G(d,p) levels of theory, and natural population analysis and Bader population analysis at the highest common level of theory, MP2/6-311G(d,p)//HF/6-31G(d). In general, there is a buildup of charge in the transition states, concentrated near the reaction centre, which dissipates somewhat as the reaction proceeds to products. The description of charge transfer varies somewhat with the three methods. Key words: topological properties, atomic charges, charge development, reactions of the hydroxyl radical with fluorinated ethanes.
1995 Syntex Award Lecture Correlation and prediction of rate constants for organic reactions
Authors: Guthrie J. Peter
Published: May 2006

Marcus theory provides a framework for correlating rate and equilibrium constants, and thus the possibility of predicting rate from structure. This had been successful for a number of simple reactions, including the addition of hydroxide to carbonyl compounds, enolate formation from carbonyl compounds, and aldol addition and elimination reactions. The theory can be extended, as Multidimensional Marcus Theory, to cover concerted reactions such as E2 eliminations, the water-mediated proton switch, or the breakdown of the tetrahedral intermediate in amide hydrolysis. We have now found that Marcus theory can be applied not only to inter-molecular reactions but also to intra-molecular reactions. This is illustrated for aldol condensation and ester/lactone formation. Key words: Marcus theory, concerted reactions, intramolecular reactions, aldol condensation, ester formation, proton transfer.
Gaussian- and Slater-type bases for ground and certain low-lying excited states of positive and negative ions of the atoms H through Xe based on the generator coordinate Hartree–Fock method
Authors: Silva A.B.F. da , Trsic M.
Published: May 2006

We applied a discretized version of the generator coordinate Hartree–Fock (GCHF) method to generate Gaussian- and Slater-type functions for mono positive and mono negative ions of the atoms H through Xe. The basis sizes for Slater-type functions are (12s, 10p, 10d) for positive ions, and (13s, 11p, 10d) for negative ions. In the case of Gaussian-type functions the bases are (18s, 12p, 11d) for both positive and negative species. Ground and excited state Hartree–Fock energies are calculated with these bases and the results compared with the best atom-optimized calculations and numerical HF results available. A discussion on the role of weight functions in the evaluation of electronic energies emphasizes the integral character of the GCHF method. Key words: Slater-type bases, Gaussian-type bases, generator coordinate Hartree–Fock, atomic ions.
Substituted tetra-2,3-pyrazinoporphyrazines. Part II. Bis(tri-n-hexylsiloxy)silicon derivatives
Authors: Kudrevich Svetlana V. , Van Lier Johan E.
Published: May 2006

Dichlorosilicon complexes of substituted tetra-2,3-pyrazinoporphyrazines were obtained via condensation of 2,3-dicyanopyrazine, 2,3-dicyano-5,6-diphenylpyrazine, 2,3-dicyanoquinoxaline, 2,3-dicyano-benzo[f]quinoxaline, and 2,3-dicyano-dibenzo[f,h]quinoxaline with silicon tetrachloride in the presence of urea, quinoline, and tri-n-butylamine. Hydrolysis of the Si-Cl bond in concentrated H2SO4, followed by treatment with 0.01 N NaOH and aqueous NH3, afforded the corresponding dihydroxides, which were converted to the bis(tri-n-hexylsiloxy)silicon derivatives via reaction with tri(n-hexyl)silane in 3-picoline (2,4,6-collidine) in the presence of tri-n-butylamine. The axial tri-n-hexylsiloxy substituents at the central silicon atom prevent aggregation in organic solvents, permitting detailed studies on the effects of structural modifications on the electronic spectra of tetraazaphthalocyanines. Our data show that each benzo ring addition, angularly condensed to the tetra-2,3-quinoxalinoporphyrazine, induces a hypsochromic shift (~ 10–15 nm) of the main absorption maximum. Key words: phthalocyanine, aza analog, bis(tri-n-hexylsiloxy)silicon complex.
A study of the vacuum pyrolysis of 11 -oxatricyclo[6.2.1.02,7]undeca-2,9-diene. The HeI ultraviolet photoelectron spectrum of 1,2-cyclohexadiene
Authors: Werstiuk Nick Henry , Roy Chandra Deo , Ma Jiangong
Published: May 2006

A newly developed ultraviolet photoelectron spectrometer – CO2, laser apparatus that utilizes a 50-watt CW CO2, laser as a directed heat source is used to study the vacuum pyrolysis of 11-oxatricyclo[6.2.1.02,7]undeca-2,9-diene (4). We report the HeI photoelectron spectrum of the strained cyclic allene 1,2-cyclohexadiene (1) that correlates with the HAM/3 ionization energies calculated with the optimized C2, equilibrium structure obtained with AM1 and the molecular orbital energies of the optimized C2, equilibrium structure calculated at the ab initio HF/6-31G** level of theory. Key words: 11-oxatricyclo[6.2.1.02,7]undeca-2,9-diene, vacuum pyrolysis, HeI ultraviolet photoelectron spectrum, 1,2-cyclohexadiene.
Secondary ion mass spectroscopy (SIMS) and kinetics of the catalysis of bisulphite oxidation by aqueous iron
Authors: Martin R.R. , Li Jinjiang
Published: May 2006

The catalytic activity of Fe(II) and Fe(III) in the aqueous oxidation of bisulphite by molecular oxygen at bisulphite concentrations similar to those in cloud water has been studied. Secondary Ion Mass Spectroscopy (SIMS) of insoluble sulphates derived from bisulphite after oxidation by 18O2 gives different 18O/32S ratios for Fe(II) and Fe(III), indicating that the oxidation mechanism is different in these two species. Key words: oxidation, bisulphite, iron, secondary ion mass spectroscopy.