bluuebunny/crossref_metadata_2025 huggingface dataset:
Solid-Phase Synthesis of Selectively Protected Peptides: Use of Thallous Alkoxide as Catalyst in the Transesterification Step
Authors: Savoie J. Y. , Barton Moira A.
Published: May 2006

Transesterification with 2-dimethylaminoethanol, in the presence of thallous alkoxide, was found to provide a useful method for the cleavage of protected peptides from the Merrifield resin after solid-phase synthesis. The extent of racemization, at the C-terminal residue, was low (< 1%) in the case of peptides containing C-terminal alanine or valine residues. The method was also found to provide good yields of the protected peptide derivative even in the case of the severely sterically hindered C-terminal valine peptides. A study of the α → β aspartyl transpeptidation, showed that the extent of this side reaction was low (< 1% β-isomer) in the case of the labile α-aspartylglycine sequence, when a short transesterification time was employed. The t-butyl ester group was shown to be stable under the conditions of transesterification and this group is recommended for the protection of carboxylic acid side chains. Hydrolysis of the 2-dimethylaminoethyl esters was accomplished by treatment with water or dilute base, to yield the selectively protected peptide acids, suitable for use as building units in the synthesis of large molecules by solid-phase or solution techniques.
Five-membered Heterocyclic Thiones. Part V.The Sulfurization of 5-Lithioisothiazoles
Authors: Horning Donald E. , Muchowski Joseph M.
Published: May 2006

5-Lithioisothiazoles reacted with sulfur at −70° to produce the corresponding unstable (in air) lithium isothiazole-5-thiolates. The salts could be utilized insitu to prepare 5-alkylthioisothiazoles or they could be transformed in low yield into stable mercuric thiolates.
The Electron Pair in Chemistry
Authors: Daudel R. , Bader R. F. W. , Stephens M. E. , Borrett D. S.
Published: May 2006

not available
Etude cinétique des interactions électrostatiques entre la phosphatase alcaline intestinale de veau et ses substrats
Authors: Julien Marius , Ouellet Ludovic
Published: May 2006

The role played by the ionic charge of the enzyme in the kinetics of the reactions catalyzed by alkaline phosphatases has been investigated. Data obtained from the hydrolysis of p-nitrophenyl phosphate (net charge −2) indicate that the repulsion between the negatively charged enzyme molecule and the substrate molecule is sufficient to take into account, quantitatively, the variation of the Michaelis constant with the pH of the system between pH 8.5 and 10.5. However, the results obtained with ethanolamine phosphate (net charge −1) and p- or o-carboxyphenyl phosphate (net charge −3) show that either the organic part of the substrate molecule is not involved in the formation of the enzyme substrate intermediate or the charge of the enzyme does not have the role suggested from the work with p-nitrophenyl phosphate.Further investigation of this reaction in solutions of various ionic strength and dielectric constants suggests that the active center of the enzyme molecule is positively charged, but that hydrophobic effects are important.
Concertedness in the Thermolysis of Some 1-Pyrazolines
Authors: Crawford Robert J. , Ohno Masatomi
Published: May 2006

A mixture of cis- and trans-3,5-divinyl-1-pyrazoline was synthesized and isolated at 0° by the cycloaddition of vinyldiazomethane to butadiene. The kinetics of the thermolysis in diphenyl ether at 35–65° were studied by measuring the rate of nitrogen evolution. The cis isomer gave log (k1/s−1) = (12.28 ± 0.9 ) − (22.8 ± 0.4 )/θ where θ = 2.303RT in kcal mol−1, for the trans isomer log(k2/s−1) = (13.9 ± 0.9 ) − (25.7 ± 0.4 )/θ was obtained. These data when compared with those of 1-pyrazoline and 3-vinyl-1-pyrazoline lead to the conclusion that both carbon–nitrogen bonds are being broken in the rate determining step. Thus the alkyl-1-pyrazolines are concerted whereas the azoalkanes are not.
Conformations of Bridged Diphenyls. VI. Substituent Effects and Internal Rotation in Triply Ortho-substituted Diphenyl Ethers
Authors: Benjamins Harm , Dar Fazal Haque , Chandler W. David
Published: May 2006

Experimental evidence is presented to support the hypothesis that, in triply ortho-substituted diphenyl ethers, conformer interconversion involves both rings. The free energy barrier, measured by p.m.r., is sensitive to substituent effects arising in the ring having only one ortho substituent. The exceptionally high barrier found when an ortho iodine is present is consistent with the existence of a substituent-inside intermediate occurring during conformer conversion.
Anhydro Sugar Formation in Acid and Base Hydrolyses of 3,4-Di-O-methylsulfonyl-D-mannitol: a Rapid Route to 1,4:3,6-Dianhydro-D-iditol(D-Isoidide)
Authors: Hicks David Roy , Fraser-Reid Bert
Published: May 2006

Brief acid hydrolysis of 1,2:5,6-di-O-isopropylidene-3,4-di-O-methylsulfonyl-D-mannitol (1a), removes the isopropylidene groups giving the disulfonated hexitol, 2a. Upon continued acid hydrolysis of 2a, one sulfonate group is lost with formation of a sulfonated monoanhydro hexitol, 5a, then the second ester group is lost to give 1,4:3,6-dianhydro-D-iditol (D-isoidide, 3a). If the disulfonate, 2a, is treated with base, an isomeric dianhydro hexitol, the bisoxirane 4, is formed. Under similar basic conditions, the monoanhydro hexitol, 5a, is stable. On acid hydrolysis, the bisoxirane, 4, gives hexitols and only 20% of D-isoidide, which indicates that 4 cannot be an intermediate in the conversion of 2a to 3a. These results indicate that, in 2a at least, five-membered anhydro rings are formed preferentially in acid hydrolyses and three-membered rings in saponification.The stage and course of hydrolysis of 2a are readily monitored by observing the τ 4–6 region in the n.m.r. spectra of D2O samples of the hydrolysate.
The Synthesis and Properties of 4-Acetoxy-2,3-dihydro-3-oxo-4H-naphth[1,2-b] [1,4]oxazine, an Analog of Known Carcinogenic Hydroxamic Acids
Authors: Coutts R. T. , Wohllebe M.
Published: May 2006

A synthesis of 4-acetoxy-2,3-dihydro-3-oxo-4H-naphth[1,2-b][1,4]oxazine (4a) from 2-nitroso-1-naphthol is described. This compound (4a) is substituted by various nucleophiles at position 5, with loss of the N-acetoxy group; in contrast, it did not react at physiological pH with the biological nucleophile methionine. On heating, 4a rearranges to the 2- and 5-acetoxy isomers,
Reaction of Acetaldehyde and Malononitrile. 2,2,4,4-Tetracyano-3,5-dimethylcyclohexylmalononitrile
Authors: Krajewski John J. , Moyer James W.
Published: May 2006

The condensation of acetaldehyde and malononitrile is reported to yield 1,1,3,3-tetracyano-2,4-dimethylcyclobutane. The n.m.r. spectrum of this material and several of its derivatives, however, was found to best fit the title compound. A reaction scheme for the formation of the title compound is suggested.
On the Origin of the Bohlmann Bands
Authors: Wolfe Saul , Schlegel H. Bernhard , Whangbo Myung-Hwan , Bernardi Fernando
Published: May 2006

A perturbational molecular orbital analysis has been performed of the strengths of the CH bonds of methylamine and methanol in their staggered conformations. This analysis leads to the prediction that a CH bond anti-coplanar to a directed lone pair is stronger than a gauche CH bond, and is in disagreement with experimental observation. The origin of the disagreement is to be found in the underestimation of the role of the nuclear–nuclear contribution to the bond strengths. Abinitio computation of the gauche and anti stretching force constants of methylamine provides quantitative theoretical support for the view that these differ because of a nuclear-dominated effect. It is suggested that effects, analogous to those observed in the Bohlmann bands, may be seen even in the absence of nonbonded electron pairs.
Syntheses of Some Phosphino- and Phosphinatoazoles
Authors: Fischer Sven , Peterson Louis K. , Nixon John F.
Published: May 2006

The following N-substituted azoles have been prepared: (a) diphenylphosphine derivatives of pyrazole (1a), 3,5,-dimethylpyrazole (1b), 3(5)-methylpyrazole (isomers 2a and 2b), pyrrole (7), imidazole (8), and 1,2,4-triazole (9); (b) diphenylphosphinatopyrazole (4b); (c) dichloro-phosphinopyrazole (5a) and dichlorophosphino-3,5-dimethylpyrazole (5b); (c) 1,1-phenyl-phosphinobis(pyrazole) (3); (d) tris(pyrazolyl)phosphine (6a) and tris(3,5-dimethylpyrazolyl) phosphine (6b). Synthetic methods involved reactions (i-iv): (i) azole with chlorophosphine (1–3, 8, 9) or [Formula: see text] (4a) in ethereal solution with triethylamine as hydrogen halide acceptor; (ii) trimethylsilylazole with phosphorus trichloride (5, 6); (iii) potassium pyrrolide with diphenylchlorophosphine (8); (iv) sulfur oxidation of 1a (4b). Nuclear magnetic resonance spectral assignments are discussed.
The Alkylation of Ambident Anions. IV. The Alkali Metal and Silver Salts of Formanilides
Authors: Stein Allan R. , Tan See-Hua
Published: May 2006

Systematic studies of the alkylation of a number of ambident or ambifunctional anions have been reported. For the vast majority of the anions examined reaction involved competition between carbon and a heteroatom and few competition between two heteroatoms. A study of the factors influencing the site of alkylation of the ambident anion of mono-N-substituted amides, where competition is between oxygen and nitrogen, has been undertaken. With the alkali metal and tetraalkylammonium salts of formanilide anions, the alkylation product is invariably the corresponding N-alkylformanilide. No detectable O-alkylation is observed as the solvent is varied from nonpolar through polar aprotic solvents, the alkylating agent through the various benzyl halides and para-substituted benzyl halides and the para-substituent on the formanilide from strongly electron donating (—OCH3) through strongly electron withdrawing (—NO2) groups. In contrast, with the silver formanilide salts, O-alkylation predominates but here the O:N alkylation ratio varies with the parameters examined. A rationale based on thermodynamic υs. kinetic control and reaction of dissociated υs. undissociated anion is presented to predict the site of reaction in these and other ambident systems.
Complex Fluoroanions in Solution. VI. Complexes of Methyl and Ethyl Fluorides with Group VB Pentafluorides
Authors: Brownstein S.
Published: May 2006

Methyl and ethyl fluorides form two unique complexes with tantalum and niobium pentafluorides. These complexes have been identified by their proton and fluorine resonance spectra and some evidence obtained for their structures.
A New Thiophene Synthesis
Authors: Mcintosh John M. , Khalil Hamdy
Published: May 2006

2,5-Dihydrothiophenes are dehydrogenated to thiophenes in high yield using chloranil in tert-butanol or pyridine. The presence of a fully substituted carbon atom blocks the reaction. The method allows a flexible regiospecific synthesis of thiophenes in high yield from readily available starting materials.
The Adsorption, Desorption, and Exchange Reactions of Oxygen, Hydrogen, and Water on Platinum Surfaces. II. Hydrogen Adsorption, Exchange, and Equilibration
Authors: Peng Y. K. , Dawson P. T.
Published: May 2006

The adsorption, desorption, exchange, and equilibration reactions of hydrogen and deuterium on a platinum filament have been investigated by thermal desorption mass spectrometry. A surface saturated with hydrogen at 120 °K has a coverage 4.2 × 1014 molecules cm−2 and gives desorption spectra with four distinct peaks: β1,(165 °K), β2(220 °K), β3(280 °K), and β4(350 °K). Apparent activation energies and pre-exponential factors were determined for the β2-, β3-, and β4-peaks. For both co-adsorption and sequential adsorption of H2 and D2 the mass 2, 3, and 4 desorption spectra have identical shapes and the gas desorbs at equilibrium throughout. It is concluded that hydrogen adsorbs dissociatively. Exchange and equilibration were studied at 120, 210, and 285 °K by determining the surface composition and isotope distribution after varying fractions of preadsorbed H had been replaced. Following exchange at 120 °K the desorption spectra show a higher D content and a lack of equilibrium in the desorbing gas at low temperature. In most other experiments the mass 2,3, and 4 desorption spectra had identical shapes and the gas desorbed at equilibrium. The results are interpreted by a model which requires that the polycrystalline platinum surface is intrinsically heterogeneous. It appears that different mechanisms are unnecessary to interpret the differences in kinetics observed for exchange and equilibration at low temperatures.
Déroulement Anormal d'une réaction de Michaelis-Becker: Caractère ambident de l'anion diéthylphosphonate (C2H5O)2PO−; stéréochimie de la réaction de Michaelis-Becker
Authors: Benezra Claude , Bravet Jean-Louis
Published: May 2006

The reaction of sodium diethylphosphonate NaOP(OC2H5)2 with the mesylates of cis and trans-4-t-butyl cyclohexanols has been studied in toluene, dioxane, and dimethylformamide (DMF). Beside the products expected from the reaction (elimination and P-alkylation products originating from initial O-attack have been isolated. This latter reaction competes with P-alkylation (normal Michaelis–Becker reaction) and is the only substitution reaction in DMF. Results are interpreted in terms of the HSAB (Hard and Soft Acids and Bases) theory and by preferential solvation. Since the substitution products are of inverted configuration, the SN2 mechanism seems to be well substantiated.
Photoreactions of Simple Halonaphthalenes in Solution
Authors: Ruzo Luis Octavio , Bunce Nigel J. , Safe S.
Published: May 2006

The photolysis products and reaction quantum yields from a number of simple halonaphthalenes have been determined. Fluoronaphthalenes react from a singlet state to yield substitution products in nucleophilic solvents. Chloronaphthalenes and 1-bromonaphthalene afford mainly radical products (naphthalene and binaphthyls) through a triplet state, though some photonucleophilic substitution occurs with chloronaphthalene. Attempts at quenching lead to abnormal behavior, in that enhanced quantum yields for reaction are found with several potential triplet quenchers. Electron transfer processes are proposed to rationalize the observed products.
Neutral Solvolysis of Acyl Carbon and the Temperature Dependence of the Kinetic Solvent Isotope Effect
Authors: Rossall B. , Robertson R. E.
Published: May 2006

The temperature dependence of the rate of hydrolysis of benzoic, phthalic, and succinic anhydrides have been determined in H2O and D2O under "neutral" conditions. Corresponding data have been obtained for methyl trifluoroacetate. While both series supposedly react by the same BAc2 mechanism, remarkable differences are made obvious by this investigation. Possible sources of such differences are proposed.
The Role of Hemicellulose in the Delignification of Wood
Authors: Kerr A. J. , Goring D. A. I.
Published: May 2006

Hemicellulose was selectively removed from white birch by treatment with cold alkali. Untreated and alkali-treated samples were pulped to various stages of delignification by the acid chlorite process. The acid chlorite reagent was shown to be selective for lignin removal from both samples during the first 60% of delignification.In alkali-treated white birch, the median pore width increased considerably as acid chlorite delignification proceeded. In contrast, in untreated white birch, this parameter remained relatively constant during the first 60% of delignification.Alkali-treated white birch delignified at a much faster rate than untreated white birch. In addition, there was a topochemical preference for removal of lignin from the secondary wall of the fibers in the hemicellulose-deficient wood. Apparently, prior removal of hemicellulose caused larger pores to be produced in the fiber walls during pulping, thereby facilitating a more rapid removal of lignin.
Information on the Proton Affinity and Protolysis of Propane From Measurement of the Ion Cluster Equilibrium: C2H5+ + CH4 = C3H9+
Authors: Hiraoka Kenzo , Kebarle Paul
Published: May 2006

The clustering equilibrium C2H5+ + CH4 = C3H9+ [1] was measured between −60 and −110 °C in 0.5–8 Torr methane. A van't Hoff plot of K1 leads to ΔH10 = −6.6 kcal/mol, ΔS10 = −23.4 e.u.(std. st. 1 atm), and ΔG10 (300 K) ≈ +0.4 kcal/mol. The C3H9+formed by [1] is probably the most stable structure of protonated propane which corresponds to a three center bond of the proton with the electrons of a C—C bond. This structure is also similar to an ethyl ion forming a three center bond with C—H in CH4i.e. the product of reaction 1. With this assumption ΔHf(C3H9+) = 194.5 kcal/mol and p.a.(C3H8) = 147 kcal/mol are obtained. The pathways for the decomposition of C3H9+ (protolysis of propane) are examined. The reaction to C3H7+ + H2 is shown to involve a 9 kcal/mol energy barrier compared to the decomposition to CH4 and C2H5+. The latter is favored at low temperature and low excitation of C3H9+.
The Photoelectron Spectrum of Methyl Tri-t-butylcyclobutadienecarboxylate
Authors: Brown R. S. , Masamune S.
Published: May 2006

The photoelectron spectrum of the title compound has been determined and found to contain a very low ionization potential. An interpretation of the spectrum and comparison with other cyclobutadienes are presented. The effect of the substituents on the ring and hence on the ionization potential of π2 is discussed.
Sorption of the Inert Gases (Ar, Kr, and Xe) in Type A Zeolites
Authors: Derrah Russell Ian , Ruthven Douglas Morris
Published: May 2006

Theoretical potential profiles for Ar, Kr, and Xe occluded in an idealized 5A zeolite are calculated and used to evaluate the Henry's law equilibrium constants for these species. Calculations were made using the three common formulae to estimate the dispersion constants (Kirkwood–Mueller, Slater–Kirkwood, and London) and the theoretical Henry constants are compared with the values obtained directly from experimental equilibrium isotherms. For both Kr and Xe the Slater–Kirkwood formula gives the best agreement with the experimental values while, for argon, the London formula is best. At higher concentrations the isotherms for Kr can be well represented by a simple Langmuir expression with eleven sites per cavity while for Xe there appears to be a transition from localized sorption at low temperatures to a mobile adsorbed phase at higher temperatures. These conclusions are qualitatively consistent with the results of the theoretical potential calculations.
Synthesis And Fungicidal Activity of N,S- Substituted α-Thio-β-aminocrotonanilides
Authors: Pierce James Benjamin , Fanning Robert James , Davis Robert Allan
Published: May 2006

A number of N,S-substituted α -thio- β-aminocrotonanilides have been synthesized. Their fungicidal activity on a number of organisms has been studied and in general it has been found that it is inferior to that of the 1,4-oxathiins to which these compounds bear some structural similarities.
The Preparation and Properties of Some 2,1-Benzisoxazole and 2,1-Benzisothiazole Derivatives
Authors: Chauhan Mohinder S. , McKinnon David M.
Published: May 2006

A number of 2,1-benzisoxazole and 2,1-benzisothiazole compounds containing alkoxy or amino substituents in the 3-position and carboxy substituents in the 7-position, and potentially capable of valency tautomerism have been synthesized by reduction or oxidation, respectively, of suitable 2-nitro- or 2-aminoisophthalic acid derivatives. Thionation of 2-aminoisophthalic esters gave benzothiazaphosphorine derivatives. A temperature dependent n.m.r. study of the benzisoxazole and benzisothiazole derivatives indicates that in only one case, for methyl 3-methoxy-2,1-benzisothiazole-7-thionocarboxylate is there evidence for tautomerism at 200°.
13C Nuclear Magnetic Resonance Studies. Part 48. 13C Spectra of a Variety of Bicyclic Ketones
Authors: Grover S. H. , Marr D. H. , Stothers J. B. , Tan C. T.
Published: May 2006

The 13C n.m.r. spectra of a series of 37 bicyclic ketones have been recorded to examine the variations in 13C shieldings with structure and with methyl substitution. Several examples of six skeletal types were included: bicyclo[4.4.0]decanones, bicyclo[4.1.0]heptanones, bicyclo-[3.1.0]hexanones, bicyclo[3.2.2]nonanones, bicyclo[3.2.1]- and -[3.3.0]octanones as well as bicyclo[2.1.1]hexan-2-one, nopinone (6,6-dimethylbicyclo[3.1.1]heptan-2-one), and two methyl derivatives of the latter. The observed trends associated with methyl substitution follow well-defined patterns and offer further support for the application of 13C shieldings as aids for stereochemical assignments especially through the well known γ effects. The variation in the shieldings of the carbonyl carbons in these systems suggested that bond eclipsing interactions have an important influence on this parameter. With this notion, the heretofore puzzling variations observed for the cycloalkanones are readily interprétable in terms of their favored conformations.
Studies on the Synthesis of Novel Carbohydrates with Sulfur in the Ring. Part III. The Synthesis of Derivatives of 2,6-Dithio-hex-4-en-2-ulopyranosidononitriles
Authors: Vyas Dolatrai M. , Hay George W.
Published: May 2006

The cis-hydroxylation of 3,6-dihydro-3-methoxy-2-methylthio-2H-thiopyran-2-carbonitrile and of 3-exo-cyano-3-methylthio-2-thiabicyclo[2.2.1]hept-5-ene by treatment with OsO4–pyridine afforded sulfur analogs of two new types of carbohydrates. The Diels–Alder reaction of trans,trans,-l,4-diacetoxy-l,3-butadiene with methyl cyanodithioformate gave a one-step synthesis of methyl 3,6-di-O-acetyl-4,5-dideoxy-2,6-dithio-α-DL-threo-hex-4-en-2-ulopyranosidononitrile.
Crystal Structure of and Electron Paramagnetic Resonance of Mn2+ in Cd2(NH4)2(SO4)3
Authors: Ng Hok Nam , Calvo Crispin
Published: May 2006

The room temperature structure of Cd2(NH4)2(SO4)3 (CAS), isotypic with the mineral langbeinite K2Mg2(SO4)3 has been refined by full-matrix least squares methods. The structure is cubic with a = 10.362(7) Å and space group P213. There are two inequivalent sites for the cation Cd2+ and e.p.r. spectra of doped Mn2+ show that Cd2+ was substituted in both sites with preferential occupancy by Mn2+ in a 1:3 ratio. Refinement of X-ray data on a mixed crystal of the stoichiometry Cd0.84Mn1.16(NH4)2(SO4)3 shows that site I contains 82% Mn2+ while site II has 38%. The zero-field splitting parameter D was calculated for Mn2+ in two sites using a point-charge model and confirms that Mn2+ substitutes into site I with greater preference. Although a monoclinic ferroelectric phase of CAS has been suggested below 90 K, no change in space group symmetry could be detected at 80 K. The proton second moment, calculated by the van Vleck's formula, and compared with the experimental values, suggests a tunneling motion of the protons among the four vertices of the ammonium tetrahedron.
Photoisomerization of Bicyclo[3.1.1]heptan-2-ones in Methanol: Chemical Transformations of 'Verbanones'
Authors: Fallis Alex G.
Published: May 2006

The products (predominantly unsaturated aldehydes) from photolysis in methanol of a series of 6,6-dimethylbicyclo[3.1.1]heptan-2-ones are reported. These results are rationalized on the basis of stereochemical interactions and the apparent preference for cleavage to occur adjacent to the less highly substituted α-carbon but more stable alkyl radical. It is concluded that the photochemical behavior of α-cyclobutyl ketones displays a closer parallel to the α-cyclopropyl ketones than previously recognized and that cleavage will occur preferentially on the side of the carbonyl group remote from the four-membered ring whenever possible. The synthetic steps involved in the preparation of the ketones from 'pinene' precursors are described.
Synthetic Studies in the Veratrum Alkaloid Series. I. Introduction and the Total Synthesis of Appropriate C-Nor-D-homo Steroid Derivatives
Authors: Kutney James P. , By Arnold , Cable John , Gladstone William A. F. , Inaba Tadanobu , Leong Sim Y. , Roller Peter , Torupka Edward J. , Warnock William D. C.
Published: May 2006

An introduction to the synthetic approach leading to the total synthesis of various Veratrum alkaloids is presented. The total synthesis of a series of C-nor-D-homo steroid derivatives is described. These substances are important intermediates in the laboratory synthesis of these natural products.
The 125Te Mössbauer Absorption Spectra of Tellurium Complexes with Sulfur-Containing Ligands
Authors: Cheyne Robert Michael , Jones Colin Henry Wilson , Husebye Steinar
Published: May 2006

The 125Te Mössbauer spectra of a number of Te(II) and Te(IV) complexes with sulfur-containing ligands have been measured. The Te(II) compounds exhibit large quadrupole splittings of 12.4 to 15.2 mm s−1 and isomer shifts (125I/Cu) of + 0.33 to + 0.91 mm s−1. These data are interpreted as evidence for the incorporation of some 5s character in the bonding scheme for several of the compounds, consistent with the interpretation of X-ray crystal structure data. For the Te(IV) compounds 6, 7, and 8 coordinate molecules have been studied containing tellurium in distorted octahedral, pentagonal bipyramidal, and dodecahedral coordination.
High Resolution 13C Spectrum and Long-Range 13C,H Couplings in 2-Pyrone
Authors: Chalmers A. A. , Pachler K. G. R.
Published: May 2006

The natural abundance 13C n.m.r. spectrum of 2-pyrone has been analyzed by the 13C–{1H} SPI technique. The magnitudes and signs of all long-range 13C,H couplings have been obtained and are compared with those of related compounds.
The Two-step Synthesis and Crystal and Molecular Structure of Tetracyclo[6.4.0.02,11.04,9]dodec-6-ene-3,10-dione
Authors: Ngan Yuen-Mui , Rettig Steven J. , Scheffer John R. , Trotter James
Published: May 2006

The photochemistry of the trans-1,3,5-hexatriene-p-benzoquinone Diels–Alder adduct has been investigated. Irradiation of this material in its long wavelength n → π* absorption band leads to a single photoproduct in high yield. A single crystal X-ray structure determination shows this product to be tetracyclo[6.4.0.02,11.04,9]dodec-6-ene-3,10-dione, the result of a novel regiospecific intramolecular 2 + 2 photoaddition between the side chain vinyl group and the enedione double bond. Crystals of tetracyclo[6.4.0.02,11.04,9]dodec-6-ene-3,10-dione are monoclinic, a = 7.5290(7), b = 19.072(1), c = 6.7237(4) Å, β = 107.918(6) °, Z = 4, space group P21/n. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to a final R of 0.041 for 1603 reflections with I ≥ 3σ(I). The structure consists of well-separated molecular units. Bond distances are: C=O, 1.216 and 1.219(2), C=C, 1.318(2), mean C(sp3)—C(sp2), 1.508, mean C(sp3)—C(sp3), 1.560 in the four-membered ring and 1.544 elsewhere, mean C(sp3)—H, 0.99, and mean C(sp2)—H, 0.95 Å. Bond distances between nonhy drogen atoms have been corrected for thermal motion. A possible rationalization for the observed photoprocess is discussed in terms of conformational and radical stability effects.
Conformations of Bridged Diphenyls. VII. Crystal Structure of 2-(4′-Carbomethoxy-2′-nitrophenoxy)-1,3,5-trimethylbenzene
Authors: van der Heijden Simon P. N. , Griffith Elizabeth A. H. , Chandler W. David , Robertson Beverly E.
Published: May 2006

The structure of 2-(4′-carbomethoxy-2′-nitrophenoxy)-1,3,5-trimethylbenzene, C17H17NO5, has been determined by X-ray diffraction. The crystals are monoclinic, a = 8.352(4), b = 17.892(10), c = 10.966(6) Å, β = 100.59(3) °, P21/c with Z = 4. The intensities of 3877 independent reflections were measured on a four circle diffractometer; 2272 of these reflections were used to obtain a final least-squares residual of 0.053 and traditional R factor of 0.054. The general conformation is similar to that found earlier for differently substituted diphenyl ethers. Comparison of this structure with those previously determined indicates that distortions in ring angles are primarily a result of electronic rather than steric effects.
Dielectric Models for Porous Adsorbent – Adsorbate Systems
Authors: Dignam M. J. , Rao B.
Published: May 2006

Equations are derived for the dielectric constant of porous adsorbent – adsorbate systems based on several geometrical models and taking account of both adsorbate molecular orientation and the special geometrical relationship which exists between adsorbed and substrate phases. Isotropic distributions of coated spheres, rods, disks, and their cavity counterparts are treated. Although the equations are based on the above idealized physical models, unlike many previous derivations, no additional intuitive postulates are involved. Furthermore, the equations are correct in the various limiting cases in which two of the three phases are absent, again in contrast with some earlier equations. Some implications of the equations are discussed, particularly as they apply to molecular orientation. The question of a break in the curve of composite dielectric constant vs. volume adsorbed at monolayer coverage is also investigated.
The Equilibrium and the Determination of D00 (H—CN)
Authors: Betowski Don , Mackay Gervase , Payzant John , Bohme Diethard
Published: May 2006

The rate constants and equilibrium constant for the proton transfer reaction [Formula: see text] have been measured at 296 ± 2 K using the flowing afterglow technique: kforward = (2.9 ± 0.6) × 10−9 cm3molecule−1s−1, kreverse = (1.8 ± 0.4) × 10−10 cm3 molecule1 s−1, and K = 16 ± 2. The measured value of K corresponds to a standard free energy change, ΔG296°, of −1.6 ± 0.1 kcal mol−1 which provides values for the standard enthalpy change, ΔH298°= −1.0 ± 0.2 kcal mol−1, the bond dissociation energy, D00(H—CN) = 124 ± 2 kcal mol−1, and the proton affinity, p.a.(CN−) = 350 ± 1 kcal mol−1.
The Ultrasonic Degradation of Thymine
Authors: Mead Eric L. , Sutherland Ronald G. , Verrall Ronald E.
Published: May 2006

Sonolysis at 450 kHz degrades an aqueous solution of thymine into at least six products of which four, 5-hydroxymethyluracil, cis- and trans-5,6-dihydroxy-5,6-dihydrothymine, and urea have been identified and a fifth tentatively assigned as N-formyl-N′-pyruvylurea. A mechanism of sonolytic degradation is proposed.
Volumes and Heat Capacities of Cyclic Ethers and Amines in Water and of some Electrolytes in these Mixed Aqueous Solvents
Authors: Klyohara Osamu , Perron Gérald , Desnoyers Jacques E.
Published: May 2006

The densities and volumetric specific heats of p-dioxane, tetrahydropyran, morpholine, piperidine, and piperazine were measured in water at 25 °C with a flow densimeter and a flow microcalorimeter. The same properties were also determined for LiCl, NaCl, Me4NBr, and Bu4NBr at 0.1 m in dioxane–water, morpholine–water, and piperidine–water mixtures. The derived apparent molal volumes and heat capacities of the nonelectrolytes in water and the transfer functions of the electrolytes from water to the mixed solvent suggest that all the present cyclic amines and ethers are hydrophobic; the overall structural influence is very small with dioxane and large with piperidine.
The Infrared Spectrum Between 4000 and 200 cm−1 and the X-Ray Powder Diffraction Pattern of Hexamethylenetetramine Hexahydrate in Four Isotopic Forms at 100 K
Authors: Bertie John E. , Solinas Marco
Published: May 2006

The X-ray diffraction pattern of powdered hexamethylenetetramine hexahydrate (HMT-h12·6H2O ) at 120 K has been recorded and analyzed. The thermal contraction occurs primarily along the trigonal axis. The infrared spectra of polycrystallineHMT-h12·6H2O, HMT-h12·6D2O, HMT-d12·6H2O, and HMT-d12·6D2O at 95 K have been recorded. The most sensitive test for ice impurity is obtained from the OD stretching bands of HDO molecules in dilute solution in the hydrate, vOD(HDO), but ice impurity also affects the OH and OD stretching bands of H2O and D2O, vOH(H2O) and vOD(D2O). The absorption by HMT can be assigned to the single HMT molecule per diffraction unit cell. The symmetries of the vibrations under the C3v diffraction site symmetry cannot be determined from this data, so the absorption is assigned to the parent vibration of pure HMT.Three peaks arise from vOH(HDO) and vOD(HDO), at 2464 2369, and 2295 cm−1 for vOD(HDO). They can, with considerable uncertainty, be assigned to the three nonequivalent hydrogen bond lengths in the structure. An alternative assignment uses the hypothesis that the frequency of an O—D … O bond of given length is abnormally low when a three-coordinated oxygen atom is the base. A partial interpretation of the vOH(H2O) and vOD(D2O) bands is presented. It is quite clear that a large part of their breadth is due to Fermi resonance with the continuum of overtone and combination levels that lie close to the fundamental levels. Several fairly sharp features due to rotational vibrations of H2O and D2O, vR(H2O) and vR(D2O), provide the only evidence of the partial order in the positions of the hydrogen atoms. Three vR(HDO) frequencies have been detected, 860, 570, and 490 cm−1. Their ratios agree well with the ratios of the square roots of the reciprocal moments of inertia, but not with the ratios of frequencies calculated following Blue's arguments.
Stereoselective Total Synthesis of Copa and Ylango Sesquiterpenoids: (+)-Copacamphor, (+)-Copaborneol, (+)-Copaisoborneol, (−)-Ylangocamphor, (−)-Ylangoborneol, (−)-Ylangoisoborneol
Authors: Piers Edward , Britton Ronald W. , Geraghty M. Bert , Keziere Robert J. , Kido Fusao
Published: May 2006

Efficient, stereoselective syntheses of the tricyclic sesquiterpenoids (+)-copacamphor (3), (+)-copaborneol (4), (+)-copaisoborneol (5), (−)-ylangocamphor (6), (−)-ylangoborneol (7), and (−)-ylangoisoborneol (8) are described. Conversion of the keto acetate 9 (previously synthesized from the dione 1) into the keto tosylate 17 was accomplished via an eight-step sequence. Intramolecular alkylation of 17 afforded, in high yield, (+)-copacamphor (3), which had previously been converted into the corresponding alcohols 4 and 5 by Kolbe-Haugwitz and Westfelt. Alkylation of the enolate anion of the bicyclic dione 2 with 2-bromopropane in hexamethylphosphoramide gave mainly the O-alkylation product 19. Conversion of 19 into the keto mesylate 29 was carried out in 5 synthetic steps. Intramolecular alkylation of 29 afforded (−)-ylangocamphor (6). Reduction of the latter with calcium in liquid ammonia gave (−)-ylangoborneol (7), while reduction with lithium aluminum hydride yielded (−)-ylangoisoborneol (8).
General-base-catalyzed Proton Exchange in Glycocyamidiniom Ions: A Comparison of Activation by Protonation and by Alkylation
Authors: Stewart Ross , Srinivasan R.
Published: May 2006

The rates of exchange of methylene protons in seven glycocyamidinium ions in formate and chloroacetate buffers were measured in D2O by means of proton magnetic resonance spectroscopy. When protonation and methylation of glycocyamidines occur at the same site the methyated products react faster than the protonated products by factors of 1.8–1.9. Replacing ring protons in glycocyamidinium ions by methyl groups causes small increases in the méthylène exchange rate whereas corresponding substitution at the exocyclic nitrogen causes small rate decreases. It is concluded that in this reaction a quaternary ion ZR+ can serve as a satisfactory model for the analogous conjugate acid ZH+.
Electrochemistry of Ruthenium Ammine Complexes
Authors: Elson C. M. , Itzkovitch I. J. , McKenney J. , Page John A.
Published: May 2006

The electrochemistry of ruthenium(III) penta- and tetraammine chloro complexes has been investigated. In each case, the ruthenium(III) species was found to undergo an initial one electron reduction step, followed by hydrolysis of the chloride(s) in the ruthenium(II) product. The rate constants for the substitution steps were evaluated by cyclic voltammetry. An aqueous 0.30 M methane sulfonate electrolyte of pH 2.0 was used at 25 °C.Values for the reversible "E1/2" (V υs. SCE) and first order rate constants for the (stepwise) replacement of the Cl− are: Ru(NH3)5Cl+, −0.282 V and 17 s−1; cis-Ru(NH3)4Cl2+, −0.328 V, 80 s−1, and 5 s−1; trans-Ru(NH3)4Cl2+, −0.412 V, 2 s−1, and 0.4 s−1.The electrochemistry of Ru(NH3)5SO22+was also investigated. In a 0.10 M CH3SO3H – 0.20 M CH3SO3Na electrolyte the ruthenium(II) species was found to undergo an initial one electron oxidation step with "E1/2" = +0.50 V, followed by a slow hydrolysis of the SO2 in the ruthenium(III) product with k = 2.4 × 10−2 s−1 at 25 °C.
The Crystal and Molecular Structures of Pyrazolyl-, 3-Methylpyrazolyl-, and Indazolylgallium Dimethyl Dimers
Authors: Rendle David F. , Storr Alan , Trotter James
Published: May 2006

Crystals of the pyrazolylgallium dimethyl dimer (1a) are monoclinic, a = 16.914(3), b = 21.747(6), c = 8.117(2) Å, β = 92.09(3)°, Z = 8, space group P21/c, those of the 3-methyl-pyrazolylgallium dimethyl dimer (1b) are also monoclinic, a = 13.473(6), b = 7.162(2), c = 18.468(4) Å, β = 113.78(2)°, Z = 4, space group P21/c, and those of the indazolylgallium dimethyl dimer (1c) are orthorhombic, a = 8.909(2), b = 27.463(3), c = 15.738(2) Å, Z = 8, space group Pbca. All three structures were solved by Patterson and Fourier methods and were refined by full-matrix least-squares methods to final R values of 0.065, 0.061, and 0.036 for 2542, 2011, and 1986 reflections with I ≥ 3σ(I) respectively. The three crystal structure analyses emphasize the critical role of steric interaction in determining the molecular geometry for these molecules. The results clearly demonstrate that changing deuterium for methyl groups on the gallium atoms and introducing substituents on the bridging 'pyrazolyl' ligands in the [D2Ga·N2C3H3]2 dimer cause a pronounced flattening of the central Ga—(N—N)2—Ga boat conformation and a noticeable lengthening of the Ga—N bonds. Average Ga—N distances are 1.996, 1.986,1.996 Å, and V-angles (the angle between the Ga2N2C3 moieties) are 131.5,139.5, and 147.8° in 1a, 1b, and 1c respectively.
The Preparation of Hexamethylcyclotriphosphazene
Authors: Oakley Richard T. , Paddock Norman L.
Published: May 2006

Dimethyltrichlorophosphorane reacts with methylamine hydrochloride to give an oil which decomposes on heating, yielding the quaternary chloride N3P3Me6·MeCl as the principal product. The controlled pyrolysis of this salt gives N3P3Me6 in high yield. A simple method for the separation of N3P3Me6 and N4P4Me8 is described, based on the different solubilities of their salts in acetonitrile.
The Isolation and Synthesis of E-24-Ethylidene-5α-lanost-8-en-3β-yl Acetate
Authors: Safe S. , Safe L. M.
Published: May 2006

Extraction of the fungus Mucorrouxii gave small amounts of a 32 carbon triterpene (M+ 454). Spectroscopic studies suggested a lanosterol ring skeleton with a 24-ethylidene group introduced into the side chain. The structure was confirmed by synthesis and shown to be E-24-ethylidene-5α-lanost-8-en-3β-yl acetate.
Four New Sesquiterpene Analogs of Common Monoterpenes
Authors: Terhune Stuart J. , Hogg James W. , Bromstein A. Christine , Lawrence Brian M.
Published: May 2006

Four new sesquiterpenes: sesquithujene, cis-sesquisabinene hydrate, and 2-methyl-6-(trans-4′-methyl-4′-hydroxycyclohex-2′-enyl)-hept-2-ene (zingiberenol) from Zingiberofficinale and sesquisabinene from Pipernigrum are reported. Similarities in the i.r. and n.m.r. spectra between monoterpenes and their sesquiterpene analogs are used to assist in the structural elucidation.
The Structures of some Stress Metabolites from Solanummelongena
Authors: Stoessl A. , Stothers J. B. , Ward E.W. B.
Published: May 2006

The structures of the acyclic sesquiterpenes 1–5 and the bicyclic sesquiterpenes 6 and 7, all isolated from eggplant fruit inoculated with fungi, were elucidated with the aid of 1H and 13C n.m.r. studies. Trienone 1 was found to be identical to 9-oxonerolidol from camphor leaf oil while 2 and 3 are isomeric alcohols derived from 1. Compounds 4 and 5 are probably artifacts. The hydroxyaldehyde 6 is identical with the potato phytoalexin lubimin, originally represented by a structure shown here to be incorrect. Enone 7 is a novel eudesmane. The stereochemistries of these compounds are discussed.
Effect of Pressure on the Alkaline Hydrolysis of Ethyl Acetate in Acetone–Water Solutions. Parameters of Activation at Constant Volume
Authors: Tonnet M. L. , Whalley E.
Published: May 2006

The effect of pressures up to 3 kbar on the rate of the base-catalyzed hydrolysis of ethyl acetate in acetone–water mixtures has been measured. From these measurements and the enthalpy and entropy of activation at constant pressure measured by others, the internal energy and entropy of activation at constant volume have been obtained as a function of solvent composition. The constant-volume parameters of activation vary with composition in a simpler way than the constant-pressure parameters. It seems likely that theoretical discussions of the solvent effect are easier if they are based on the constant-volume parameters. At low acetone concentrations, the addition of acetone reduces the rate constant at low pressure, but increases it at 3 kbar.
Thermal Decomposition of EDTA, NTA, and Nitrilotrimethylenephosphonic Acid in Aqueous Solution
Authors: Martell A. E. , Motekaitis R. J. , Fried A. R. , Wilson J. S. , MacMillan D. T.
Published: May 2006

The decomposition of ethylenediaminetetraacetic acid (EDTA) in water solution at 200 and 260 °C, nitrilotriacetic acid (NTA) at 260 and 293 °C, and of nitrilotrimethylenephosphonic acid (NTPO) at 260 °C was studied by n.m.r. as a function of time at pH 9.5 and decomposition rates and products were determined. The primary (fast) decomposition reaction of EDTA involves the hydrolytic cleavage of the ethylenic C—N bond to produce the relatively stable pair: N-(2-hydroxyethyl)iminodiacetic acid and iminodiacetic acid. NTA does not cleave below 260 °C but decomposes at about 290 °C and above through a stepwise decarboxylation reaction. NTPO cleaves hydrolytically at 260 °C at two C—N bonds to produce aminomethylenephosphonic acid and 2 mol of hydroxymethylenephosphonic acid. The further breakdown of the primary products of EDTA at higher temperatures occurs by the loss of carbon dioxide producing the corresponding methylamines, concomitantly with the hydrolytic cleavage of the remaining CH2CH2—N bond giving ethylene glycol. The pseudo first order rate constant kobs for NTA decarboxylation at 293 °C and pH 9.3 is 0.19 ± 0.01 h−1. The value of kobs for EDTA hydrolysis at 200 °C is 1.4 ± 0.2 h−1.
The Mechanism for the Thermal Decomposition of E-Aldoxime Carbonates
Authors: Prokipcak J. M. , Forte P. A.
Published: May 2006

Pyrolysis of aldoxime carbonates at relatively low temperatures (100–120 °C) produced nitriles in excellent yields. These decompositions followed first-order kinetics and the decomposition rates showed little dependence on inductive effects or solvent polarity. Low entropy values along with the fact that the E and Z-isomers behaved quite differently led to the proposal of a cyclic transition state for the decompositions.
Secondary β-Hydrogen Isotope Effects in Ketal Formation Equilibria
Authors: Stoute Valerie A. , Winnik Mitchell A.
Published: May 2006

An n.m.r. technique was used to measure KH/KD of ketal formation in methylene chloride solution for acetone and phenylacetone. For acetone, KH/KD = 0.95 per D for methyl ketal formation. Phenylacetone-1,1-d2, -3,3,3-d3, and -1,1,1,3,3-d5 were examined for both dimethyl and ethylene ketal formation. For both ketals, KH/KD = 0.91 per D, for each ketone. Liquid scintillation counting was used to examine KH/KT for ketal formation in methylene chloride. An extraordinarily large isotope effect was found for ethylene ketal formation in 1,3-diphenyl-acetone. Here KH/KT = 0.67 for one tritium atom. This isotope effect is too large to arise from hyperconjugative effects alone. It must have a substantial steric component.
The Molecular Structure of Tetramethylgermane Determined by Gas Phase Electron Diffraction
Authors: Hencher J. Lawrence , Mustoe Frank J.
Published: May 2006

The molecular structure of tetramethylgermane has been determined by vapor phase electron diffraction. The structural parameters, based on rg interatomic distances are r(GeC) = 1.945 ± 0.003 Å, r(CH) = 1.12 ± 0.02 Å, and [Formula: see text] Tetrahedral symmetry of the GeC4 skeleton was preserved in the analysis by including a shrinkage correction for the C … C interatomic distance. The quoted uncertainties were estimated to include both systematic and random errors.
Syntheses of Pentacyclic Morphinan Analogs
Authors: Douglas James L. , Meunier Jacques
Published: May 2006

The synthesis of a series of morphinans with an additional aromatic or alicyclic ring fused to the 5,6 positions or an alicyclic ring fused to the 16,17 positions is described. The synthetic sequence involved elaboration of a 1,2,3,4,5,6-hexahydrobenz[f]isoquinoline or a quinolizidine, Grewe cyclization to a morphinan, and modification of the functional groups of the morphinan. These morphinans showed poor to moderate analgesic and narcotic antagonist activity.
Splicing Factor Tra2-β1 Is Specifically Induced in Breast Cancer and Regulates Alternative Splicing of theCD44Gene
Authors: Watermann Dirk O. , Tang Yesheng , zur Hausen Axel , Jäger Markus , Stamm Stefan , Stickeler Elmar
Published: May 2006

The human CD44 gene undergoes extensive alternative splicing of multiple variable exons positioned in a cassette in the middle of the gene. Expression of alternative exons is often restricted to certain tissues and could be associated with tumor progression and metastasis of several human malignancies, including breast cancer. Exon v4 contains multiple copies of a C/A-rich exon enhancer sequence required for optimal inclusion of the exon and binding to the nucleic acid–binding proteins YB-1 and human Tra2-β1. Here, we show that hTra2-β1, a member of the extended family of serine/arginine-rich (SR) splicing factors, enhances the in vivo inclusion of CD44 exons v4 and v5. It increased inclusion of exons v4 and v5 and acted synergistically with YB-1. Activation required the C/A-rich enhancer within exon v4. Several other SR proteins had none or only a slight effect on CD44 exon inclusion. In contrast, SC35 inhibited exon usage and antagonized the effects of Tra2 or YB-1. In a matched pair analysis of human breast cancers and their corresponding nonpathologic tissue controls, we found a significant induction of Tra2-β1 in invasive breast cancer, both on the RNA and protein levels. Together with our functional data, these results suggest an important role for Tra2-β1 in breast cancer. Induction of this splicing factor might be responsible for splicing of CD44 isoforms associated with tumor progression and metastasis. (Cancer Res 2006; 66(9): 4774-80)
Epigenetic Inactivation of Betaig-h3 Gene in Human Cancer Cells
Authors: Shao Genze , Berenguer Jessica , Borczuk Alain C. , Powell Charles A. , Hei Tom K. , Zhao Yongliang
Published: May 2006

Gene silencing by CpG island methylation in the promoter region is one of the mechanisms by which tumor suppressor genes are inactivated in human cancers. It has been shown previously that Betaig-h3 gene, which encodes an extracellular matrix protein involved in cell adhesion and tumorigenesis, is down-regulated or silenced in a variety of human cancer cell lines. To unravel the underlying molecular mechanism(s) for this phenomenon, DNA methylation patterns of Betaig-h3 CpG island were examined in normal, immortalized, and cancer cell lines derived from lung, prostate, mammary, and kidney. A good correlation was observed between promoter hypermethylation and lost expression of Betaig-h3 gene, which was supported by the data that demethylation of promoter by 5-aza-2′-deoxycytidine reactivated Betaig-h3 and restored its expression in Betaig-h3-silenced tumor cell lines. This result was further substantiated by a luciferase reporter assay, showing the restoration of promoter activities and increased response to transforming growth factor-β treatment in Betaig-h3-negative 293T cells when transfected with unmethylated Betaig-h3 promoter. In contrast, activity of Betaig-h3 promoter was completely inactivated by in vitro methylation. Furthermore, CpG methylation of Betaig-h3 promoter was also shown in primary lung tumors that expressed decreased level of Betaig-h3 protein. These results suggest that promoter methylation plays a critical role in promoter silencing of the Betaig-h3 gene in human tumor cells. (Cancer Res 2006; 66(9): 4566-73)