摘要:

The Diels–Alder reactions ofN-phenylmaleimide and dimethyl acetylenedicarboxylate with benzene oxide (1,3,5-cyclohexatriene 1,2-oxide,3) and its more substituted derivatives 1,2-dimethyl-1,3,5-cyclohexatriene 1,2-oxide (7) and 10-oxatricyclo[4.3.1.0]deca-2,4-diene (11) in a kinetic manner gave exclusively products of additionantito the plane-nonsymmetrical oxygen. The structures of the adducts were determined unequivocally by nuclear Overhauser enhancements in their1H nuclear magnetic resonance spectra and by X-ray crystallographic methods. The π-facial stereoselectivity was rationalized in terms of unfavorable orbital interactions, steric hindrance between the dienophile and thesynface of benzene oxide, and σ-donation by the oxygen.Keywords: cycloaddition, Diels–Alder,syn-anti, π-facial stereoselectivity, benzene oxide.

ISSN:0008-4042    

DOI:10.1139/v91-199

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The stability constants of binary complexes of 1-canavanine (Can) and 1-arginine (Arg) with Ni2+and Zn2+have been calculated at 25 °C and at ionic strength μ = 0.1 (NaNO3). This technique is completed with spectrophotometry and circular dichroïsm measurements in the ultraviolet and visible ranges. Thus we determined the molar absorbances and the molar circular dichroïsm of ligands and complexes with Ni2+and Zn2+. Concerning the nickel(II), these measurements display three complexes, NiL, NiL2, and NiL3. 1-Canavanine, which generally has a tridentate ligand behaviour, becomes a bidentate ligand in the Ni(Can)3complex. With regard to zinc(II), this study only displays the ZnL and ZnL2species.Keywords: 1-arginine, 1-canavanine, circular dichroïsm, nickel binary coordination, zinc binary coordination.

ISSN:0008-4042    

DOI:10.1139/v91-200

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Isoxestovanin A (6), xestovanin B (7), xestovanin C (8), dehydroxestovanin A (9), epidehydroxestovanin A (10), dehydroxestovanin C (11), and secodehydroxestovanin A (12), seven new triterpenoid glycosides, have been isolated from the Northeastern Pacific spongeXestospongiavanilla. The structures of the metabolites were determined by a combination of spectroscopic analysis and chemical degradation. The aglycone of6has the new isoxestovanane triterpenoid carbon skeleton. Compounds7,8, and11are the firstX.vanillatriterpenoid glycosides known to contain three monosaccharide subunits.Keywords: triterpenoid, glycoside, sponge, Xestospongia.

ISSN:0008-4042    

DOI:10.1139/v91-201

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Carbon–carbon bond cleavage of the radical cations of 1-buteneand 4,4-dimethyl-1-pentenewill generate the allyl and alkyl radical and carbocation fragments. Alternative bonding arrangements between the allyl and methyl moieties inand between the allyl andtert-butyl moieties inpossible metastable intermediates or transition states preceding complete separation of the fragments, have been investigated byabinitiomolecular orbital calculations. Structures were fully optimized at the UHF/6-31G* or UHF/STO-3G levels, and some of the calculations onwere expanded with single point MP2/6-31G*//UHF/6-31G* computations. The C4H8+radical cation, having a structure similar to that of 1-butene, is more stable than the separated fragments: 183 kj mol−1lower in energy than the sum of the energies of the allyl cation and the methyl radical, and 385 kJ mol−1lower than the sum of the energies of an allyl radical and a methyl cation, at the MP2/6-31G* level. The corresponding values at the UHF/STO-3G level are 276 and 415 kj mol−1, respectively. There is less bonding interaction between the allyl andtert-butyl moieties inThe summation of the energies of the allyl radical andtert-butyl cation is 123 kj mol−1lower than the summation of the energies of the allyl cation andtert-butyl radical, and 115 kJ mol−1higher in energy than the bonded radical cationat the UHF/STO-3G level. These calculated values are compared with thermochemical data and with experimental results on the cleavage of these, and related, radical cations.Keywords: radical cation, cleavage,abinitiocalculations, electron transfer, photochemistry.

ISSN:0008-4042    

DOI:10.1139/v91-202

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The analysis of the hydration of formaldehyde in the presence of a hydrated proton was performed. Molecular quantum chemistry methods were used to explore the potential energy hypersurface. Calculations were performed at RHF-4-31G level. The structures were obtained through the use of first and second derivatives of the potential energy hypersurface thereby guaranteeing the nature of the extremum. The incidence of one or two water molecules on the reaction is discussed. It has been found that a low energy reaction path exists when the hydration process is catalyzed by a proton and two water molecules. The reaction mechanism rests on successive addition/elimination processes and reveals the respective roles of the oxygen lone pairs and of the C=O double bond upon the mechanism.Keywords: ab initio calculations, carbonyl reactivity, acid catalysis, hydration, reaction mechanism.

ISSN:0008-4042    

DOI:10.1139/v91-203

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The solubilities of tris(2,4-pentanedionato)cobalt(III) and tris(3,5-heptanedionato)cobalt(III) in water, heptane, and 1,2-ethanediol were determined over the temperature range 5–50 °C and from these data the thermodynamic quantities of solution at 25 °C were estimated. The free energy, enthalpy, and entropyof transfer of these chelates from heptane to some solvents were calculated from the corresponding thermodynamic quantities of solution. Whenandwere separately plotted against, the data of transfer from heptane to water deviated markedly from a correlation obtained for the data of transfer to the solvents other than water. This finding indicates that these chelates are subject to hydrophobic hydration in their aqueous solutions. The solubility of tris(glycinato)cobalt(III) in water was also determined over the temperature range 5–60 °C and its temperature dependence of the solubility is compared with that for the cobalt(III) chelates of the β-diketones.Keywords: tris cobalt(III) chelates of β-diketones and glycine, temperature dependence of solubility, thermodynamic parameters of solution, thermodynamic parameters of transfer, hydrophobic hydration.

ISSN:0008-4042    

DOI:10.1139/v91-204

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The transport of gaseous In(I)Br in an argon carrier gas was studied in the temperature range 680 to 935 K by the modified entrainment method. Combination of the entrainment results with a literature equation for the total vapour pressure above the liquid gave the following expressions for the temperature dependence of the vapour pressures of the monomer (In2Br2) and dimer (In2Br2) respectively:lnT − 0.001164 ± 0.00021)T, (680 −950 K) and(680–810 K);p0 = 105 Pa. The dimer concentration isca. 14% at 680 K and decreases toca. 4% at 810 K.Keywords: high temperature thermodynamics, indium(I) bromide, vaporization thermodynamics.

ISSN:0008-4042    

DOI:10.1139/v91-205

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

α-Hydroperoxyalkyl diazenes (Me2C(OOH)N=NR,1, R = CH2CF3, CH2CH2OMe, CH(Me)2, CMe3, CH2Ph, Ph, CH2CH2OPh, andc-C3H5CD2) decompose in benzene, at 50 °C or less, by a mechanism involving free radical (R•) intermediates. The radicals were trapped with 1-methyl-4-nitroso-3,5-diphenylpyrazole,2, to afford spin adducts (nitroxyls) that were observed by ESR spectroscopy. When the solvent was ethyl vinyl ether, radicals from1(R = CH2CH2OPh) were trapped by the solvent and the adduct radicals so formed were spin trapped by2. These observations support free radical mechanisms for thermolysis of1and for the hydroxyalkylations that occur when1are decomposed in solutions containing enol ethers or other unsaturated substrates. The ring-opening of cyclopropylmethyl radicals (cpm) to 3-butenyl radicals was used to estimate the rate constant for radical trapping by2. For cpm the rate constant is given by logkcpm = (10.7 ± 0.4) − (3.9 ± 0.5)/θ where θ = 2.3RT kcal mol−1. At 25 °C, the spin trapping rate constant has the value 6.9 × 107 M−1s−1.Keywords: hydroperoxyalkyl diazenes; radicals, spin trapping; spin trapping, rate constant.

ISSN:0008-4042    

DOI:10.1139/v91-206

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Two novel lignans, kadsulignan C (4) and D (5), were isolated from the root bark ofKadsuralongipedunculata. Their structures, including absolute configurations, were elucidated by 2D1H–13C long-range COSY experiments and chemical transformation to the known compound10.Keywords: isolation and structure, absolute configuration, kadsulignan C, kadsulignan D, lignans,Kadsuralongipedunculata.

ISSN:0008-4042    

DOI:10.1139/v91-207

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

In the conformations that correspond to the C6-X1-C2-X3-C4 moieties of 1,3-dioxane, 1,3-dithiane, and cyclohexane in their chair conformations, the model systems CH2(OH)2, CH2(SH)2, andgauche-butane contain "axial" and "equatorial" hydrogens. The vibrational frequencies of the diastereomeric monodeuterated CHD derivatives of these model compounds have been computedabinitioat the 6-31G*//6-31G* level of theory, scaled by 0.9, and used to calculate the conformational deuterium isotope effects in these systems, as well as the stretching and bending components of these isotope effects. The results are in very good agreement with the experimental observations and with some of the interpretations given by Anet for the six-membered rings. Using an equation suggested by McKean, the scaled isolated stretching frequencies have been used to predict the bond dissociation energies of the diastereotopic hydrogens of these molecules. The results suggest that the preference for radical abstraction of the axial C2 hydrogen in a 1,3-dioxane will be diminished in cyclohexane, and reversed in a 1,3-dithiane.Keywords: stereochemistry, isotope effects, bond dissociation energies.

ISSN:0008-4042    

DOI:10.1139/v91-208

出版商:NRC Research Press    

年代:1991

数据来源: NRC