bluuebunny/crossref_metadata_2025 huggingface dataset:
Enzymes in organic synthesis. 36. Synthesis of optically active civet constituent from an enzyme-generated chiral synthon
Authors: Jones J. Bryan , Hinks R. Scott
Published: April 2011

A synthesis of the civet constituent, (+)-(S,S)-(cis-6-methyltetrahydropyran-2-yl)acetic acid, is described, in which the key stereochemistry of the chiral starting material is set by an enantiotopically selective, porcine pancreatic lipase-catalyzed, hydrolysis of a meso diester.
A conductometric study of dissociation of picric acid in 2-methoxyethanol and 1,2-ethanediol from −10 to 80 °C
Authors: Franchini Giancarlo , Ori Elena , Preti Carlo , Tassi Lorenzo , Tosi Giuseppe
Published: April 2011

Dissociation constants of 2,4,6-trinitrophenol (picric acid) in 2-methoxyethanol and 1,2-ethanediol were measured by a conductance method at temperatures ranging from −10 to 80 °C. The determined constants in the two solvents are discussed and are compared on the basis of the physical properties of the solvents, which have been measured at the various temperatures. The dissociation constants show different behaviour with the temperature in the investigated range and they are fitted by equations of the type ln K = A + BT + C/T + D ln T. Furthermore, the thermodynamic functions have been evaluated and discussed.
Preparation of 8-methylberberines and aza analogues: Synthesis of (±)-alamaridine
Authors: Jahangir  , MacLean David B. , Holland Herbert L.
Published: April 2011

Methyllithium reacted with oxyberberine, affording on alkaline work-up a compound with an exomethylene group at C-8 that was transformed in acidic media into an 8-methylberberinium salt. The iminium salt was reduced to a mixture of racemic α- and β-8-methylcanadine. 5,6-Dihydro-8H-isoquino[2,1-b][2,7]naphthyridin-8-ones were similarly transformed into 8-exo-methylene and 8-methyl derivatives, which were subjected to further reduction. These reactions have been employed in a synthesis of the Alangium alkaloid, (±)-alamaridine.
13C magnetic resonance studies. 129. Homoenolization in the camphenilone system. Examination of some 7-substituted derivatives
Authors: Peiris Sriyawathie , Ragauskas Arthur J. , Stothers J. B.
Published: April 2011

The behavior of the 7,7-dimethyl, 7-spirocyclopropyl, and 7-isopropylidene derivatives of camphenilone under strongly basic conditions (t-BuO−/t-BuOH/185 °C) has been investigated to examine substituent effects on β-enolization in the [2.2.1] system. In each case, an equilibrium mixture of the initial ketone and its corresponding 5-substituted isomer was obtained, with the latter increasingly favored through the series. In each case the 1H/2H exchange processes were examined by experiments utilizing t-BuO−/t-BuOD to establish the individual sites of deuterium incorporation in these ketones with 2H and 13C magnetic resonance. These data provide the stereoselectivities and relative rates of the exchange processes. The 2H-induced effects in the 13C spectra are discussed.
Complexes of hybrid ligands. The synthesis of a thioanisole-substituted fluoro-alcohol and its complexes with Pd2+ and Pt2+; the structure of a palladium(II) complex containing alkoxide, phosphine, thioether, and chloride donors
Authors: Boeré René T. , Esser David E. , Willis Christopher J. , Stephan Douglas W. , Obal Taras W.
Published: April 2011

The compound 2-thioanisole-1,1,1,3,3,3-hexafluoro-2-propanol, CH3S—C6H4—C(CF3)2OH, HL2, has been prepared, and shown to act (in the ionized form) as a hybrid, chelating, ligand. Neutral bis-complexes M(L2)2 are formed with Pd2+ and Pt2+; the former reacts with PdCl42− to give the Cl-bridged dinuclear complex (L2)Pd(μ-Cl)2Pd(L2), which may in turn be cleaved by PPh3 or PPh2Me to give PdCl(L2)(PR3).A complete structural determination has been made for PdCl(L2)(PPh2Me); C23H20ClF6OPPdS. Crystals are monoclinic, space group P21/n, a = 15.526(5), b = 12.966(9), c = 12.900(8) Å, β = 101.84°, V = 2542(2) Å3, Z = 4. Least-squares refinement on F of 198 variables using 2801 observations converged at R1 = 0.0434, R2 = 0.0559. The complex consists of discrete square-planar molecules with phosphine and alkoxide trans-disposed. Bond lengths are Pd—O, 2.053(3); Pd—P, 2.242(1); Pd—Cl, 2.323(2); Pd—S, 2.252(2) Å. In the six-membered chelate ring, all atoms with the exception of Pd are close to coplanarity; there is a dihedral angle of 127.9° between the O—Pd—S plane and that of the aromatic ring.Multinuclear nmr measurements are used to show that both cis- and trans-forms of complexes PdCl(L2)(PR3) are present in solution, with inversion at coordinated sulfur occurring much more rapidly in the latter.
Densities and apparent molar volumes of aqueous NaCl–KBr mixtures at 298.15 K. An alternative treatment
Authors: Humffray Alan A.
Published: April 2011

In estimating properties of mixtures of electrolytes from those of the individual components, it is better to calculate the density directly using a mixing rule, rather than from the derived quantity, the apparent molar volume. Each component contributes to the density in proportion to its ionic strength fraction in the mixture; this simple mixing rule gives better agreement for NaCl–KBr mixtures than do Pitzer's equations, incorporating up to sixteen parameters. Some discrepancies in Kumar's published data (Can. J. Chem. 63, 3200 (1985)) on NaCl–KBr mixtures are discussed.
Cyclic nitronates from the diastereoselective addition of 1-trimethylsilyloxycyclohexene to nitroolefins. Starting materials for stereoselective Henry reactions and 1,3-dipolar cycloadditions
Authors: Brook Michael A. , Seebach Dieter
Published: April 2011

Induced by a stoichiometric excess of dichloro(diisopropoxy)-titanium, 1-trimethylsilyloxycyclohexene and (E)-nitroolefins (R—CH=CHNO2, R = C6H5, p-C6H4CH3, p-C6H4OCH3, p-C6H4CN, CH3) combine in CH2Cl2 solution at −90 °C preferentially with relative topicity ul, opposite to the corresponding reaction of enolates or enamines. The primary products are the epimeric bicyclic nitronates 4–6, which can be separated, and which are converted by KF in MeOH to the alkyl or aryl (nitroethyl)-substituted cyclohexanones 1 and 2 (Table 1). Instead of being solvolysed, the cyclic nitronates 4–6 can be used for nitroaldol additions to give the trisubstituted hexahydrobenzopyranols 15–19. Alternatively, 4–6 can be used in 1,3-dipolar cycloadditions with acrylates to furnish the tricyclic compounds 22–31. In either case, products with four asymmetric carbon atoms (not including acetal centres) are produced diastereoselectively. The addition described here is yet another example of an ul combination of trigonal centres induced by Lewis acids, overriding the influence of the configuration of the donor component.
Effect of protonation on the ground state properties of retinal analogs: an ab-initio study
Authors: Poirier Raymond A. , Yadav Arpita , Surján Péter R.
Published: April 2011

The ground state properties (bond lengths, bond orders, net atomic charge distribution) of various cis and trans isomers of retinal analogs have been studied at abinitio SCF and correlated levels. The effect of protonation on the properties of the retinal Schiff base analogs has also been studied. Convergence in various properties has been observed with increasing chain length justifying the use of smaller analogs mimicking retinal. Convergence is, however, slower for the protonated retinal Schiff base analogs. The protonated retinal Schiff base analogs show an increased conjugation reflected in the bond orders, bond lengths, and in the decrease in the HOMO–LUMO gap. The nodal characteristics of the HOMO of the protonated retinal Schiff base analog also indicate a strong conjugation in the vicinity of the protonated nitrogen. Based on the results of calculations at the correlated level, the sudden polarization charge transfer mechanism is shown to be an artifact of HF-based methods used in the calculations.
The synthesis, crystal and molecular structures of triphenyl(phenylthio)silane, Ph3SiSPh, and of bis(μ-thiophenolato)bis[tetracarbonyltungsten(I)], [W(CO)4(SPh)]2
Authors: Lucas C. Robert , Newlands Michael J. , Gabe Eric J. , Lee Florence L.
Published: April 2011

Preparations and X-ray structure determinations are described for Ph3SiSPh and [W(CO)4(μ-SPh)]2. The compound Ph3SiSPh crystallizes in space group Pcab with a = 15.260 Å, b = 15.968 Å, c = 16.757 Å, Z = 8, dcalc = 1.20 g cm−3 (MoKα1, λ = 0.70930 Å). The structure was solved with MULTAN and refined to the final R = 0.050 for 1933 significant reflections. The molecule has an unusual C—S—Si angle of 99.5° and a short Si—S bond length of 2.156 Å. The compound [W(CO)4(μ-SPh)]2 crystallizes in the space group P21/c with a = 9.164 Å, b = 12.054 Å, c = 11.163 Å, β = 111.63°, Z = 2, dcalc = 2.35 g cm−3 (MoKα1, λ = 0.70930 Å). The structure was solved with MULTAN and refined to the final R = 0.062 for 2748 significant reflections. The molecule is a dimer with a crystallographic center of symmetry. There is a short W—W distance (2.972 Å), a wide S—W—S′ angle (106.3°), a compressed W—S—W′ angle (73.67°) and there is distortion of a CO on each W due to the close approach of an ortho-H in a phenyl ring.
8H-Isoquino[2,1-b][2,7]naphthyridin-8-ones: synthesis of the Alangium alkaloids, alangimaridine and alangimarine
Authors: Jahangir  , Brook Michael A. , MacLean David B. , Holland Herbert L.
Published: April 2011

It is shown that 3,4-dihydroisoquinolines are activated toward nucleophilic attack by reaction with trimethylsilyl trifluoromethanesulfonate. The complex so formed reacts readily with the lithio derivative of 3-cyano-4-methyl-5-vinylpyridine, affording in a single step a tetracyclic amidine from which a lactam is obtained by hydrolysis. These reactions have been applied to the synthesis of the Alangium alkaloids, (±)-alangimaridine and alangimarine.
Previously unreported p-terphenyl derivatives with antibiotic properties from the fruiting bodies of Sarcodonleucopus (Basidiomycetes). A two-dimensional nuclear magnetic resonance study
Authors: Tringali C. , Piattelli M. , Geraci C. , Nicolosi G. , Rocco C.
Published: April 2011

Two previously unreported antibiotic metabolites, 2′,3′,4″-triacetoxy-3,4,5′,6′-tetrahydroxy-p-terphenyl (10) and 2′,3′-diacetoxy-3,4,5′,6′,4″-pentahydroxy-p-terphenyl (11), have been isolated from the basidiomycete Sarcodonleucopus. Their structures have been determined by chemical transformations and spectral analysis, including heteronuclear two-dimensional nuclear magnetic resonance methods.
Isolation, structure, and synthesis of combretastatin A-2, A-3, and B-2
Authors: Pettit George R. , Singh Sheo Bux
Published: April 2011

Further investigation of the South African tree Combretumcaffrum (Combretaceae) for murine P388 lymphocytic leukemia (PS) cell-growth inhibitory substances has led to discovery of three new active constituents designated combretastatins A-2 (5a, PS ED50 0.027 μg/mL), A-3 (5b, PS ED50 0.026 μg/mL), and B-2 (3b, PS ED50 0.32 μg/mL). Both combretastatins A-2 and A-3 were found to markedly inhibit tubulin polymerization. The structure of each combretastatin was firmly established by a combination of high resolution (400 MHz) 1H and 13C nuclear magnetic resonance and mass spectral analyses followed by total syntheses. The conversion of methyl gallate (7b) to combretastatin A-2 via intermediates 7c → 7d → 7e → 7a and 6a → 5a illustrates the practical synthetic route utilized for obtaining these substances. The Wittig reaction employed as the penultimate step in obtaining combretastatins A-3, afforded predominantly the natural Z isomer.
Substituent effects in cooxidation: Cr(VI) – oxalic acid – sulfoxides systems
Authors: Srinivasan C. , Jegatheesan P. Pandarakutty , Rajagopal S. , Arumugam N.
Published: April 2011

The kinetics of cooxidation of several substituted phenyl methyl sulfoxides and oxalic acid with Cr(VI) have been carried out in the presence of perchloric acid. The reaction is first order each in sulfoxide, oxalic acid, Cr(VI), and H+. The products of oxidation are sulfones and carbon dioxide. Electron-releasing groups in the phenyl ring accelerate the rate while electron-withdrawing groups retard it. The Hammett correlation yields a reaction constant of −0.927 ± 0.08 (r = 0.994) at 313 K. Addition of aluminium nitrate prevents the occurrence of cooxidation. Based on the kinetic information a suitable mechanism has been proposed. Diaryl sulfoxides behave in an analogous manner in the cooxidation.
The gas phase ion chemistry and proton affinity of hexamethyldisiloxane studied by high pressure mass spectrometry
Authors: Li Xiaoping , Stone John Alfred
Published: April 2011

The ion chemistry of hexamethyldisiloxane ((CH3)3SiOSi(CH3)3, HMDS) has been studied under chemical ionization conditions at ion source temperatures of 300–600 K and pressures of 2–4 Torr. Highly exothermic proton transfer to HMDS from CH5+ and C2H5+ leads mainly to loss of CH3 but with decreasing exothermicity the yield of HMDSH+ increases such that transfer from t-C4H9+ (ΔH0 = −7.5 kcal mol−1) yields almost exclusively HMDSH+. Although HMDSH+ transfers (CH3)3Si+ rather than a proton to most reference bases, the proton affinity of HMDS has been determined from van't Hoff plots using the equilibrium method with methylaromatics as reference bases. PA(HMDS) = 203.4 kcal mol−1 is in excellent accord with an earlier estimate of 203 kcal mol−1 obtained by the bracketing method. The rate constants for these proton transfer reactions show very large negative temperature coefficients in the exothermic directions which are consistent with the reactions of charge delocalized ions and/or reactions in which considerable loss of rotational freedom occurs along the reaction coordinate. The rate constants in the reverse directions have a positive temperature coefficient only when the endothermicity is significant (>2.5 kcal mol−1).
Ion-pair interactions between the methyl tris(1-pyrazolyl)gallato molybdenum, tricarbonyl anion, [MeGapz3]Mo(CO)3−, and the Na+ cation, and the reactivity of the anion toward RX molecules (where RX = HCl, DBr, MeI, or EtBr)
Authors: Onyiriuka Emmanuel C. , Storr Alan
Published: April 2011

Ion-pair interactions between the carbonylate anion, [MeGapz3]Mo(CO)3−, and the Na+ cation in THF solution have been demonstrated using ir spectroscopy. The carbonylate ion has been protonated and the resulting hydride, [MeGapz3]Mo(CO)3H, characterized. The reaction of MeI with [MeGapz3]Mo(CO)3− proceeds via a detectable σ-methyl derivative to the η2-acyl complex [MeGapz3]Mo(CO)2(η2-COMe), whereas an analogous reaction using EtBr leads to the formation of an unstable σ-ethyl derivative, [MeGapz3]Mo(CO)3Et, with no evidence for the (η2-COEt) insertion product.
Infrared spectra of unannealed and of annealed Cu4(OH)6(NO3)2
Authors: Secco Etalo A. , Worth Glen G.
Published: April 2011

The infrared spectra over the range 4000–150 cm−1 of Cu4(OH)6(NO3)2 along with site group and factor group analysis are reported for the first time. The observed absorption frequencies associated with OH and NO3 modes show distinct individual spectral features in the unannealed and annealed Cu4(OH)6(NO3)2. Three distinct types of OH groups involving different degrees of H-bonding are evident. The multiple frequencies involving Cu—O—H bending modes further reflect the varying degrees of H-bonding and OH coupling in Cu(OH)4O2 and Cu(OH)5O entities paralleling their behavior in Cu4(OH)6SO4. The NO3 internal optic modes' activities, fundamentals along with overtones and combination bands, illustrate distinct interaction effects (OH and Cu) with respect to specific modes relative to halo-nitrates with same site symmetry. The presence of CuO in the sample after the anneal effected a striking change in ν3. The Cu—O, O—Cu—O, and OH torsional mode frequencies show consistent behavior with Cu4(OH)6SO4.
Regioselective synthesis of 2-pyrazolines and pyrazoles from ethyl α-cyanocinnamates
Authors: Hassaneen Hamdi M. , Mousa Hiyam A. H. , Ezmirly Saleh T. , Shawali Ahmad S.
Published: April 2011

The cycloaddition of nitrilimines 1a–e to ethyl α-cyanocinnamate 6a furnished the 2-pyrazoline derivatives 11a–e respectively. Similar reactions of 1a–e with ethyl α-cyano-p-nitrocinnamate 6b yielded 1,3-disubstituted 4-(p-nitrophenyl)-5-ethoxycarbonylpyrazoles 14a–e, via elimination of hydrogen cyanide from their precursors, the 2-pyrazoline derivatives 13a–e, respectively. The structures of the cycloadducts 11 and 14 were supported by spectral (13C and 1H nuclear magnetic resonance and infrared) and analytical data.
Fused heterocycles from o-acylbenzenethiol derivatives
Authors: McKinnon David M. , Lee Kingsley R.
Published: April 2011

The oximes of 2-acylthioanisole derivatives may be conveniently converted into 1,2-benzisothiazoles by acetic anhydride in pyridine. 3-(2-Methylthiophenyl)-1,2-benzisothiazole, prepared by this method, has been further converted into the 1,2-benzisothiazolo[2,3-b]-1,2-benzisothiazolium system. The phenylhydrazones of certain 2-acylthioanisoles are also cyclized by polyphosphoric acid to 2-(2-methylthio)phenylindoles, which are further converted into benzo[b]thieno[3,2-b]indoles by demethylation and oxidation.
Tautomerism of bis(2-benzothiazolyl)methanes
Authors: Avendaño Carmen , Ramos María Teresa , Elguero José , Jimeno María Luisa , Bellanato Juana , Florencio Feliciana
Published: April 2011

Tautomerism of dibenzothiazolylmethane (1) and its C-methyl derivative (2) has been studied by 1H nuclear magnetic resonance, ultraviolet, and infrared spectroscopy using C,C-dimethyl (3) and N-methyl (4) derivatives as model compounds of the "CH" and "NH" forms, respectively. X-ray diffraction analysis of the "fixed" N-methyl derivative 4 shows that it corresponds to the Z-sE isomer 4b2. The CH tautomers are unstable in solution and they slowly isomerize into a mixture of NH tautomers that depends on the solvent and on the C-substituent (H or CH3).
Photochemistry of α-chloro- and α-bromoacetophenone. Determination of extinction coefficients for halogen–benzene complexes
Authors: McGimpsey W. G. , Scaiano J. C.
Published: April 2011

The photochemical dehalogenation of α-haloacetophenones (Cl, Br) in benzene solution has been examined under conditions of continuous and pulsed laser irradiation. Product and quantum yield studies indicate that carbon-halogen bond cleavage occurs with quantum yields of 0.88 and 0.41 for α-chloro- and α-bromoacetophenone, respectively. These values are based on trapping studies in which the PhC(O)ĊH2 radicals produced photochemically are scavenged by hydrogen donors such as benzenethiol. Laser flash photolysis studies lead to intense transient signals due to benzene–halogen complexes. Combination of transient data and quantum yields lead to extinction coefficients of 1 800 and 23 700 M−1 cm−1 for the chlorine (490 nm) and bromine (550 nm) complexes, respectively.
Theoretical and experimental data on the internal rotational potential in isopropylbenzene
Authors: Schaefer Ted , Sebastian Rudy , Penner Glenn H.
Published: April 2011

Geometry-optimized STO-3G MO computations of the internal rotational potential about the [Formula: see text] bond in isopropylbenzene yield V (kJ/mol) as 14.1(3) sin2 ψ − 2.8(3) sin2 2ψ + 1.4(3) sin2 3ψ; ψ is the angle by which the α C—H bond deviates from the benzene plane. This result provides no support for the very low internal barrier once hypothesized from microwave data for isopropylbenzene in the vapor phase, nor for the local minimum at ψ = 90° inferred from hyperfine coupling constants in π electron radicals of isopropylbenzene derivatives. An alternative explanation in the literature for such microwave spectra is therefore a more likely one. Similar MO computations with the 6-31 G basis at three values of ψ can be fit by 13.40 sin2 ψ − 2.61 sin2 ψ for the energies in kJ/mol. In CS2 and acetone-d6 solutions, the long-range proton–proton coupling constants imply the independence of the rotational potential of solvent polarity and also that the barrier is rather lower than that computed for the free molecule. An apparent twofold barrier of 6.5 kJ/mol is consistent with the coupling constants but so is a combination of twofold and fourfold barrier components amounting to 9.7, −3.5 and 7.2, −1.5 kJ/mol. The coupling constants do not allow discrimination among these possibilities. The internal rotational potential is not altered significantly in the presence of a 4-hydroxyl substituent in isopropylbenzene.
2-Picolyl thioesters; a useful synthon for the preparation of 1-β-alkyl carbapenem intermediates
Authors: Martel Alain , Daris Jean-Paul , Bachand Carol , Corbeil Jacques , Menard Marcel
Published: April 2011

In this study, a highly stereoselective preparation of (3S,4S)-3-[(1R)-1-tert-butyldimethylsilyloxyethyl]-4-[(1R)-1-carboxy-ethyl]-azetidin-2-one (2) is reported. It involves a Lewis acid mediated condensation of (3S,4R)-4-acetoxy-3-[(1R)-1-tert-butyldimethylsilyloxyethyl]azetidin-2-one (3) with simple O-silylenol ethers of thiopropionates. From all the examples reported in this paper the 2-picolyl thiopropionate or similar arrangements were found to be essential for this stereoselection. Finally, a mechanism involving chelation control seems to be operative.
The substrate- and stereoselectivity of microbial reductions of 1,4-dithiaspiro[4.5]decanones and 1,5-dithiaspiro[5.5]undecanones
Authors: Takemura Tetsuo , Hosoya Yoshiko , Mori Nobuo
Published: April 2011

The title spirocyclic ketones, possessing possible substitution patterns 1,4-dithiaspiro[4.5]decan-6-one (1a), -7-one (3a), and -8-one (5a), and 1,5-dithiaspiro[5.5]undecan-7-one (2a), -8-one (4a), and -9-one (6a), have been reduced with new strains of yeast (Saccharomycescerevisiae, JCM 1819 and JCM 2214). Reductions of the prochiral ketones 1a–4a occur with high enantiofacial selectivity on the preparative scale (up to 99% ee). The product alcohols (1,4-dithiaspiro[4.5]decan-6-ol (1b) and -7-ol (3b) and 1,5-dithiaspiro[5.5]undecan-7-ol (2b) and -8-ol (4b)) have the S configuration, as confirmed by 2D COSY experiments of the (S)-MTPA esters of 1b–4b using the configuration correlation model of Mosher. Keywords: microbial reduction, Saccharomycescerevisiae, dithiaspiro ketones, substrate- and stereoselectivity, H,H-COSY spectra.
Conformational isomerism in squaraines: saturation transfer NMR studies on hydroxy squaraines
Authors: Kazmaier Peter M. , Hamer Gordon K. , Burt Richard A.
Published: April 2011

The proton and 13C NMR of azulene and dialkylaminohydroxyphenyl squaraines have been investigated. The four-membered ring carbon resonances occur in the 180–190 ppm range and are attributed to the cyclobutenediylium character of this structure. Some charge delocalization from the central ring to the pendent aromatic substituents was also observed, indicative of a substituent effect of the squaraine moiety on aminoaromatic systems equivalent to that of a formyl or acetyl group. Hydroxy squaraines 10 and 11 show the presence of two isomeric squaraine species in the proton NMR in deuterochloroform. Saturation transfer experiments confirm that these species are readily interconverted, and cis/trans isomerism [Formula: see text] is consistent with these observations. The free energy difference between the two isomers at 318 K is calculated to be 3.0 and 3.2 kJ/mol for hydroxysquaraines 10 and 11 respectively. Keywords: squaraines, saturation transfer, NMR.
Kinetics of the reduction of the tropylium and xanthylium cations by 1,4-dihydropyridine derivatives
Authors: Bunting John W. , Conn M. Morgan
Published: April 2011

The pH-dependences of the apparent second-order rate constants [Formula: see text] for the reduction of 2,4,6-cycloheptatrien-1-ol and 9-xanthydrol by each of 1-benzyl-1,4-dihydronicotinamide (BNH) and 10-methyl-9,10-dihydroacridine (MAH) have been measured in 20% acetonitrile – 80% water, at 25 °C and ionic strength 1.0. For each of these reactions, the pH-dependence of [Formula: see text] is only consistent with reduction occurring via the aromatic cation (either tropylium or xanthylium) that is present in equilibrium with these alcoholic species. The relative second-order rate constants [Formula: see text] for reductions by these two reducing agents (1700 for tropylium and 770 for xanthylium) are similar for these two cations. These ratios are also similar to those observed for a variety of nitrogen heteroaromatic hydride acceptors, even though the absolute magnitudes of these rate constants vary by 1010-fold. The second-order rate constants for the reductions of the tropylium and xanthylium cations are predicted reasonably well by their [Formula: see text] values, with the latter cation being (7 × 105)-fold more reactive than its π-isoelectronic N-methyl acridinium cation. The xanthylium cation has the greatest [Formula: see text] ratio yet observed for any heteroaromatic cation, and this value further extends the known range of this ratio as a function of reactivity. Keywords: hydride transfer, kinetics of reduction, 1,4-dihydropyridine derivatives, tropylium cation, xanthylium cation.
Rhodium induced titanium–sulfur bond cleavage: crystal and molecular structure of ((COD)Rh(μ-SMe))2
Authors: Wark Teresa A. , Stephan Douglas W.
Published: April 2011

Reactions of Ti(III) and Ti(IV) thiolates with Rh complexes have been investigated. In the reaction of Cp2Ti(SMe)2 and [(COD)2Rh]BF4 or [(COD)Rh(sol)2]PF6, thiolate abstraction yields ((COD)Rh(μ-SMe))2, 1. Reaction of (Cp2Ti(μ-SMe))2 with ((COD)Rh(μ-Cl))2 results in ligand exchange affording (Cp2Ti(μ-Cl))2 and 1. The complex 1 crystallizes in the monoclinic space group P21/n, with a = 8.551(2) Å, b = 10.058(3) Å, c = 22.187(4) Å, β = 92.54(1)°, Z = 4, and V = 1906(1) Å3. The structural data show a relatively short approach between the Rh centres (2.948 Å) and between the bridging sulfur atoms (2.888 Å). The implications of these structural features in terms of metal–metal and sulphur–sulfur bonding are discussed. In addition, the implications of these results with respect to the formation of thiolato-bridged, early–late heterobimetallics is considered. Keywords: thiolate abstraction, rhodium thiolate bridged dimer.
Intercalation of basic molecules into niobyl-vanadyl phosphate
Authors: García-Ponce L. , Moreno-Real L. , Jiménez-López A.
Published: April 2011

Mixed niobyl-vanadyl phosphate (V0.14Nb0.86)OPO4•2.7H2O, VNbP, is a moderately acid solid that intercalates strong basic molecules such as branched alkylamines, allylamine, cyclohexylamine, piperidine, and diethylamine, giving rise to bilayers of inclined molecules. In the case of molecules of medium basicity, such as pyridine, the intercalated amounts are small at room temperature and they present a planar orientation. A special behaviour was found with ethylenediamine and butylenediamine, strong basic diamines, which also present a planar disposition in the interlayer space. Keywords: mixed niobyl-vanadyl phosphate, layered compounds, phosphates, intercalation, pyridine.
Synthèse stéréosélective de (±) boschnialactone, (±) 7-épiteucriumlactone et (±) 7-épiisoiridomyrmécine. Étude de la stéréochimie par spectroscopie de résonance magnétique nucléaire
Authors: Hanquet B. , Tabyaoui B. , Caille J.-C. , Farnier M. , Guilard R.
Published: April 2011

The stereoselective syntheses of (±) boschnialactone 1, (±) 7-epiteucriumlactone 2, and (±) 7-epiisoiridomyrmecine 3 are described. Their preparation involved Stetter's reaction followed by nucleophilic addition of lithium enolates of suitable esters. Silylated reagents are used in the lactonisation step and the observed yields are between 63 and 78%. The proposed structural analysis is not in accord with the results of a previous study. The nuclear magnetic resonance data are determined using ID and 2D proton and carbon NMR experiments. Keywords: stereoselective synthesis, boschnialactone, 7-epiteucriumlactone, 7-epiisoiridomyrmecine, 1H and 13C NMR.
The reaction of aluminum atoms with methane
Authors: Yu Hengtai , Goddard John D.
Published: April 2011

Abinitio SCF and CISD calculations with split valence and larger basis sets have been carried out for several possible reactions of ground state (2P) aluminum atoms with methane. In particular, the barriers to insertion of Al into a CH bond to give a CH3AlH intermediate and the abstraction of a hydrogen by Al to give AlH and CH3 have been studied. At the SCF level, transition state structures have been located and verified by vibrational analyses. In addition, intrinsic reaction coordinates (IRC) have been followed from the transition state geometries down to reactants and to products. Calculated vibrational frequencies and lowest excitation energy for the CH3AlH intermediate are in good agreement with matrix isolation experimental results. As anticipated on the basis of MO natural correlation symmetry arguments, the energy barrier to the insertion reaction is high (~50 kcal mol−1 at the CI level). The energy requirement for the abstraction reaction is similar.[Formula: see text]Keywords: Al, CH4, HAlCH3, insertion and abstraction reactions.
Heteroatom-directed lateral lithiation: synthesis of isoquinoline derivatives from N-(tert-butoxycarbonyl)-2-methylbenzylamines
Authors: Clark Robin D. , Jahangir  , Langston James A.
Published: April 2011

Methodology for the preparation of isoquinoline derivatives from N-(tert-butoxycarbonyl)-2-methylbenzylamines (1) was developed. Conversion of 1 to the dilithio species followed by condensation with DMF afforded Boc-3-hydroxy-1,2,3,4-tetrahydroisoquinolines 3, which could be dehydrated to 1,2-dihydroisoquinolines 4. Hydrogenation of dihydro compounds 4 afforded the corresponding tetrahydroisoquinolines 5. Treatment of the dilithio species from 1 with N-methoxy-N-methylamides afforded ketones 14, which were converted to 3-substituted dihydro-isoquinoline 15, tetrahydroisoquinolines (16, 17), or isoquinolines (20).
Isolation and characterization of an UDPG-dependent glucosyltransferase activity from Rauwolfia serpentina Benth. cell suspension cultures
Authors: Lutterbach Ralf , Ruyter Carl Michael , Stöckigt Joachim
Published: April 2011

From cell suspension cultures of Rauwolfia serpentina Benth. a new enzyme activity was isolated and its properties determined. The enzyme is a soluble protein and catalyzes the transfer of a glucose moiety from UDPG to a wide variety of phenolic compounds with p-nitrophenol as one of the best substrates (Km = 1.21 mM, UDPG = 0.54 mM). In contrast to the membrane-bound UDPG: vomilenine-21-OH-β-D-glucosyltransferase from Rauwolfia serpentina cells, this enzyme is not able to glucosylate indole alkaloids. The enzyme activity has been detected in 14 callus cultures belonging to 10 different plant families.
Some routes to C2-alkoxymethyl hex-2-enopyranosides
Authors: Wong Gordon , Fraser-Reid Bert
Published: April 2011

Several procedures for installing a C2 alkoxy group at C2 of a hex-2-enopyranoside have been investigated. The one that is most convenient for large-scale preparation begins with a 2-keto pyranoside, which is converted into the corresponding exo methylene derivative. An SN2′ rearrangement is effected with thionyl chloride, and the resulting primary allylic chloride is displaced with sodium benzylate.
Preparation of (2R,3S)-β-hydroxy-α-amino acids by use of a novel Streptomyces aldolase as a resolving agent for racemic material
Authors: Herbert Richard B. , Wilkinson Barrie , Ellames George J.
Published: April 2011

A unique aldolase, which was isolated from Streptomyces amakusaensis, is used to catalyse a reverse aldol reaction on representative racemic β-hydroxy-α-amino acids (3–6) to give samples of the (2R,3S)-(D-threo)-enantiomers with excellent enantiomeric purity.
Biosynthesis of the indolizidine alkaloid cyclizidine: incorporation of singly and doubly labelled precursors
Authors: Leeper Finian J. , Shaw Susan E. , Satish Padma
Published: April 2011

Incorporation of CH313C18O2Na and CD3CH213CO2Na into the indolizidine alkaloid cyclizidine 1, produced by Streptomyces species NCIB 11649, shows that the oxygen attached to C-2 is derived intact from acetate and that the cyclopropyl ring is derived from a single intact propionate unit. However, the level and stereochemistry of the incorporation of deuteriated sodium propionate indicates that it undergoes unexpected modification during incorporation into the cyclopropyl ring.
Stereoselective total synthesis of the marine sesterterpenoid (±)-palauolide
Authors: Piers Edward , Wai John S.M.
Published: April 2011

A total synthesis of the antimicrobial, structurally unusual sesterterpenoid (±)-palauolide (1) is described. Methylenecyclohexane annulation of 3,6-dimethyl-2-cyclohexen-1-one (4) afforded a mixture of the bicyclic ketones 7 and 9 in a ratio of 94:6. Stereoselective conversion of this mixture into the phosphorodiamidate 46 was achieved via a five-step reaction sequence. Reduction of 46 with Li in MeNH2 gave the bicyclic substance 47, which was readily transformed in seven steps into the α,β-unsaturated aldehyde 54. Subjection of 54 to a Julia olefin synthesis, employing reagent 56, gave the furan 58. Photooxygenation of 58 provided (±)-palauolide (1).
Overexpression of a plant cysteine synthase gene and biosynthesis of a plant specific metabolite, β-(pyrazol-1-yl)-L-alanine, in Escherichia coli
Authors: Saito Kazuki , Kanda Reiko , Kurosawa Makoto , Murakoshi Isamu
Published: April 2011

Cysteine synthase (EC 4.2.99.8) in higher plants is responsible for biosynthesis of not only cysteine but also some nonprotein amino acids such as β-(pyrazol-1-yl)-L-alanine. The cDNA of a cysteine synthase from spinach (Spinacia oleracea) was inserted into pET8c (=pET3d) under the transcriptional control of strong T7 promoter to yield an overexpression vector pCEK1. The amount of the exogenous cysteine synthase was increased up to 40% of the total soluble protein of Escherichia coli transformed with pCEK1. β-(Pyrazol-1-yl)-L-alanine, a specific metabolite in plants of the Cucurbitaceae, was biosynthesized by overexpressed cysteine synthase from pyrazole in the presence of O-acetyl-L-serine and serine, in vitro and in vivo, respectively. The present study provides the system for mechanistic investigation of biosynthesis of cysteine and biogenetically related β-substituted alanines at molecular genetic level.
Heterocyclic analogs of nucleosides: synthesis and biological evaluation of some 1-(3-thianyl)uracil and 9-(3-thianyl)adenine derivatives
Authors: Hultin Philip G. , Szarek Walter A.
Published: April 2011

The 1-(3-thianyl)uracil (9) and 9-(3-thianyl)adenine (14) nucleoside analogs have been prepared from the key intermediate, (±)-(3β,5β)-3-amino-5-(hydroxymethyl)thiane (6). Analog 9 was converted into a mixture of diastereomeric sulfoxides (10) that afforded, by a Pummerer reaction, a mixture of (±)-1-{(2′β,3′β,,5′β)-2′-acetoxy-5′-(acetoxymethyl)thian-3′-yl}-2,4(1H,3H)-pyrimidinedione (11a) and its 6′-β isomer (11b). The EI mass spectra of the nucleoside analogs are discussed. The uracil nucleoside analogs have been evaluated also for their anti-HIV and antitumor activities.
Syntheses of methyl glycosides of 6-deoxyheptoses
Authors: Aspinall Gerald O. , McDonald Armando G. , Sood Ramesh K.
Published: April 2011

Methyl α-D-glycopyranosides of 6-deoxy-D-altro-heptose, 6-deoxy-D-manno-heptose, and 6-deoxy-D-talo-heptose have been prepared. Displacements of methyl 2,3,4-tri-O-benzylhexopyranoside 6-trifluoromethanesulfonates with potassium cyanide, followed by reduction of the resulting heptopyranosidurononitriles with diisobutylaluminum hydride, hydrolysis of the imine, further reduction with sodium borohydride, and catalytic O-debenzylation, give the corresponding methyl 6-deoxyheptopyranosides. Configurational change at C-4 of methyl 6-deoxy-7-O-tert-butyldiphenylsilyl-α-D-manno-heptopyranoside to give the talo isomer was effected by oxidation followed by stereoselective reduction. 1H nuclear magnetic resonance data of the glycosides, and gas chromatography of acetylated glycosides of (R)- and (S)-2-butanol serve to establish ring and enantiomeric configurations of the parent sugars when these are encountered as constituents of lipopolysaccharides or extracellular carbohydrate polymers, as in Campylobacter species.
Facial selectivity in the addition of nucleophiles to the radical cations of substituted 2-methyleneadamantanes
Authors: Swansburg Susan , Janz Kristin , Jocys Gloria , Pincock Alexandra , Pincock James
Published: April 2011

The diphenylmethylenecyclohexane, 2, and the substituted 2-methyleneadamantanes, 3-7, have been prepared. The radical cation of each was generated by photochemical oxidation using 1,4-dicyanobenzene as the sensitizer, and their reactivity was examined in methanol-acetonitrile mixtures with added tetraethylammonium cyanide. Although compound 2 reacted only by tautomerization, the other compounds all gave addition products. For the monoaryl alkenes 3, 4, and 5, comparable yields of both methanol and HCN addition products were obtained, resulting from nucleophilic attack at either end of the alkene radical cation. For the diphenylalkenes 6 and 7, the regioselectivity favored nucleophilic attack at the adamantyl carbon, and the addition products were predominantly those of HCN addition. For the 5-methoxy compound 7, HCN addition was facially selective with a syn:anti ratio of 58:42.Key words: methyleneadamantanes, facial selectivity, radical cations.
A1H NMR study of cyclodextrin - hydrocarbon surfactant inclusion complexes in aqueous solutions
Authors: Wilson Lee D , Verrall Ronald E
Published: April 2011

A1H NMR chemical shift ( delta ) study of a homologous series of hydrocarbon (hc) (CxH2x+ 1CO2Na, x = 5, 7, 9, 11, 13) surfactants (S) has been carried out in water and in binary solvent (D2O + cyclodextrin (CD)) systems at 22°C. Complementary1H NMR chemical shift ( delta ) data of the cyclodextrins in binary (D2O + S) systems containing hc surfactants have also been obtained. Complex induced shift (CIS) values for selected host or guest protons were found to increase as the alkyl chain (Cx) length of the surfactant increased. The CIS values are found to depend on the following factors: (i) the magnitude of the binding constant (Ki, i = 1:1, 2:1), (ii) the chain length of the surfactant, (iii) the mole ratio of the host to guest species, (iv) the host-guest stoichiometry, and (v) the host-guest inclusion geometry. The CIS values of the CD-S systems have been analyzed using equilibrium models in which 1:1 complexes, 1:1 plus 2:1 complexes, and uncomplexed species are present. Differences in the binding affinity, stoichiometry, and inclusion geometry of the complexes formed between a given hc surfactant and the various cyclodextrins were observed.Key words: cyclodextrin, surfactant, NMR, chemical shift, complex, binding constant.
Article
Authors: Avilov Sergey A , Drozdova Olga A , Kalinin Vladimir I , Kalinovsky Anatoly I , Stonik Valentin A , Gudimova Elena N , Riguera Ricardo , Jiménez Carlos
Published: April 2011

Frondoside C (1) is a new sulfated nonholostane triterpene glycoside obtained (with the glycosides closed by aglycone structure as impurities) from the sea cucumber Cucumaria frondosa. Its structure has been elucidated on the basis of spectral data (NMR and MS) of compound 1 and of its desulfated derivative (2) obtained by solvolysis. Frondoside C (1) is just the seventh glycoside reported from sea cucumbers, having a lanostane-type aglycone devoid of the typical 18(20)- lactone ring.Key words: Cucumaria frondosa, sea cucumbers, frondosides, triterepene glycosides, antitumor activity.
Article
Authors: Barnett Steven M , Vlcek, Jr. Antonín , Butler Ian S
Published: April 2011

Pressure-tuning infrared and Raman spectroscopic studies were performed on (2,2'-bipyridine)tetracarbonyl-chromium(0), Cr(CO)4(bpy). Previous bpy ring mode assignments were confirmed with new assignments offered by examination of the pressure-induced alterations in the vibrational spectra. The electron-density distribution of the bipyridyl ligand (and consequently, the electron-donating ability to the central metal) is modulated by the application of pressure. The pressure dependence of the carbonyl moieties reveals that the bipyridyl ligand can accommodate a significant portion of the molecular compressibility at higher pressures.Key words: infrared, Raman, pressure, Cr(CO)4(bpy), metal carbonyl.
On the relationship between the rate constants for racemization and isotopic exchange
Authors: Scott John MW , Summers Danny
Published: April 2011

The relationship between the rate constants that describe the halide ion catalysis of the racemization (kr) of optically alkyl halides and the related isotopic halide exchange reaction (ke) is shown to be valid in two distinct cases. The first is when the racemization and exchange mechanisms follow the classical and conventional SN2 path characterized by a symmetrical trigonal (sp2) transition state. The second envisages the trigonal species characteristic of the SN2 reaction as an intermediate instead of a transition state. The latter mechanism is also shown to be characterized by the relationship kr = 2ke, and hence this diagnostic test is seen to be a necessary but not a sufficient condition for invoking the SN2 mechanism.Key words: racemization, isotopic exchange, kinetics, bimolecular substitution, inversion.
Novel syntheses of 4-alkyl-2(5H)-thiophenones and 8-phenylimino-1,2,4-triazolo[5,1-c]- 1,4-thiazines
Authors: Katritzky Alan R , Feng Daming , Lang Hengyuan , Qi Ming , Steel Peter J
Published: April 2011

1,2,4-Triazole stabilized allene anions are intermediates for the synthesis of 4-alkyl-2(5H)-thiophenones and 8-phenylimino-1,2,4-triazolo[5,1-c]-1,4-thiazines. Structures are confirmed by X-ray crystallography.Key words: allene anion, 4-alkyl-2(5H)-thiophenones, 8-phenylimino-1,2,4-triazolo[5,1-c]-1,4-thiazines, X-ray crystallography.
Alpha-Effect nucleophiles and azide ion: effective charge studies of displacement reactions at esters
Authors: Colthurst Matthew J , Kanagasooriam Angeline JSS , Wong Michael SO , Contini Christian , Williams Andrew
Published: April 2011

The bimolecular reactions of azide, 2-iodosobenzoate, and acetohydroxamate ions and methoxyamine nucleophiles with substituted phenyl acetates possess Yukawa-Tsuno type correlations with substantial resonance interaction. The rate constants for hydroperoxide ion and the oxyanion of N,N-diethylhydroxylamine obey simple Hammett equations. The reactivities of azide, acetohydroxamate, hydroperoxide, and 2-iodosobenzoate ion nucleophiles exhibit modest alpha -effects when the reactivity in substitution at 4-nitrophenyl acetate is compared with that of phenoxide ions of similar pKa values. The N,N-diethylhydroxylamine oxyanion is less reactive than its phenoxide ion analogue due to steric requirements, and the reactivities of the anionic nucleophiles exhibit changes in effective charge on the leaving oxygen similar to those for regular nucleophiles of similar pKa. Methoxyamine possesses only a small enhancement compared with the reactivity of a normal primary amine of similar pKa.Key words: alpha -effect, nucleophile, Yukawa-Tsuno, Leffler, Hammett, phenyl ester, effective charge, Brønsted.
Article
Authors: Ferro Vito , Weiler Larry , Withers Stephen G , Ziltener Herman
Published: April 2011

The synthesis of N-glycosyl phosphonamidates has been accomplished via the coupling of peracetylated glycosylamines with an appropriate phosphonochloridate in the presence of pyridine. The resulting glycosyl phosphonamidate esters are dealkylated with bromotrimethylsilane and then deacetylated to give the target compounds, which are potential transition-state analogue inhibitors of glycopeptidases and may prove useful as haptens for generating catalytic antibodies with glycopeptidase activity.Key words: enzyme inhibition, PNGase, phosphonamidate, amide hydrolysis.
Article
Authors: Walker Richard A , MacDonald Aaron C , MacGillivray Tanya , Marangoni D Gerrard , Smith-Palmer Truis
Published: April 2011

Capillary zone electrophoresis (CZE) was used to study the preparation of 1,2-decanedisulfate from 1,2-decanediol and determine the stability of the two-headed surfactant and the corresponding hydroxydecanesulfates under both acidic and basic conditions. The optimum buffer for the CZE determination was found to be diethylbarbiturate. Resolution of the hydroxydecanesulfate constitutional isomers was facilitated by the addition of magnesium ions. Decanedisulfate was stable in base but hydrolyzed in acid. The hydroxysulfates hydrolyzed much more rapidly, and in both acid and base. Mechanisms were postulated based on differential rates of formation and hydrolysis of the hydroxydecanesulfate constitutional isomers.Key words: capillary zone electrophoresis, decanedisulfate, hydroxydecanesulfate, hydrolysis, surfactant.
The decarbonylation of ionized β-hydroxypyruvic acid: the hydrogen-bridged radical cation [CH2=O...H...==C-OH].+studied by experiment and theory
Authors: Fell Lorne M , Burgers Peter C , Ruttink Paul JA , Terlouw Johan K
Published: April 2011

The intriguing gas-phase ion chemistry of β-hydroxypyruvic acid (HPA), HOCH2C(==O)COOH, has been investigated using tandem mass spectrometry (metastable ion (MI) and (multiple) collision-induced dissociation (CID) experiments, neutralization-reionization mass spectrometry (NRMS),18O and D isotopic labelling on both the acid and its methyl ester) in conjunction with computational chemistry (ab initio MO and density functional theories). HPA does not enolize upon evaporation, but it retains its keto structure. When ionized, decarbonylation occurs and, depending on the internal-energy content, this dissociation reaction proceeds via two distinct routes. The source-generated, high-energy ions lose the keto C==O, not via a least-motion extrusion into ionized glycolic acid, HOCH2COOH.+, but via a rearrangement that yields the title H-bridged radical cation CH2==O...H...O==C-OH.+for which Δ Hf0= 99 ± 3 kcal/mol. The long-lived low-energy ions enolize prior to decarbonylation and lose the carboxyl C==O. Again, this is not a least-motion extrusion (which would produce the most stable isomer, HOC(H)==C(OH)2.+Δ Hf0= 73 kcal/mol) but a rearrangement yielding the ion-dipole complex HOC(H)C==C==O.+/H2O. The methyl ester of HPA behaves analogously, yielding CH2==O...H...O==C-OCH3.+and HOC(H)C==C==O.+/ CH3OH upon decarbonylation of the high- and low-energy ions, respectively. Decarboxylation into the ylidion CH2OH2.+characterizes the dissociation chemistry of both the title H-bridged ion and its glycolic acid isomer HOCH2COOH.+. A computational analysis of this reaction (which satisfies the experimental observations) leads to the proposal that the decarboxylation of the acid occurs via CH2-O(H)...H...==C==O.+as the key intermediate, whereas the title H-bridged ion follows a higher energy route that involves ion-dipole rotations leading to the ionized carbene HO(H2)CO-C-OH.+and the distonic ion H2O-C(H2)-O-C==O.+as key intermediates.Key words: tandem mass spectrometry, hydrogen-bridged radical cation, hydroxypyruvic acid, ab initio calculations, keto-enol tautomerization,18O labelling.