摘要:

Eight aporphinoids and six morphinans have been isolated from aerial parts ofStephaniazippelianaMiq. (Menispermaceae): four of them are new: (+)-erromangine2, (+)-tannagine3, (+)-zippeline5, and (+)-zippelianine6. The previously isolated alkaloids (+)-stephodeline1, (+)-isostephodeline4, and compounds2and3are isomeric. The structure of (+)-isostephodeline4has been revised.Keywords:Stephaniazippeliana, morphinans, alkaloids.

ISSN:0008-4042    

DOI:10.1139/v89-191

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

The geometries and the energies of the reactants, transition state, and products for the gas phase reaction: Cl− + CH3Br = ClCH3 + Br−, were obtained fromabinitiocalculations using a closed shell SCF method with a MINI basis set developed by Huzinagaetal. The energy changes predicted by the calculations are found in good agreement with the experimental data. The energies and geometries of the reactants and the transition state for the gas phase reactions: Cl− + RBr = ClR + Br−, where R = C2H5and iso-C3H7, were also obtained. The resulting activation energies follow the same trend as the experimental data: Me < Et < iso-Pr; however, the predicted increase of activation energy is considerably larger. The energies and geometries for the reactants, transition state, and products of the gas phase ion-dihydrate reaction: Cl−(H2O)2 + CH3Br → H2O(ClCH3Br)−H2O → Br−(H2O)2 + CH3Cl were obtained as well. These data provide an interesting comparison with experimental results in aqueous solution. The reaction coordinate of the ion-dihydrate reaction is very much closer to that for aqueous solution than to that for the gas phase.Keywords: nucleophilic substitution reactions, ion–molecule reactions, activation energy.

ISSN:0008-4042    

DOI:10.1139/v89-192

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

The standard absolute potentials of hydrogen, Ag–AgX (X = Cl, Br, and I) and M/M+(M = Li, Na, K, Rb, and Cs) electrodes in nine different acetone + water solvents containing up to 80 wt. % acetone were determined from the emf data at 25 °C of the cells: glass electrode/HCl (m), solvent/AgCl–Ag and glass electrode (M)/MX (m), solvent/AgX–Ag. The standard Gibbs free energies of a transfer of halogen acids and alkali metal halides as well as their constituent individual ions from water to the respective solvents were computed. The observed increases invalues of all ions with increasing acetone content of the solvent and their relative order in each solvent were interpreted and discussed. A comparison of the present results with those obtained earlier in the dimethyl sulphoxide (DMSO) + water solvents shows the different nature of the two dipolar aprotic solvents, acetone and DMSO, in their aqueous mixtures.Keywords: acetone + water solvents, electrode potentials, emf measurements, individual ions, transfer free energies.

ISSN:0008-4042    

DOI:10.1139/v89-193

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

We have used iodometry to study the enolization of acetone in water catalyzed by a series of general acids, comprised of hydrochloric acid, methanesulfonic acid, 24 aliphatic monocarboxylic acids, nine aromatic monocarboxylic acids, eight aliphatic dicarboxylic acids, and 20 monoanions of dicarboxylic acids. The logk–pKprofile for unbuffered solutions of strong and moderately strong acids shows a maximum near pk ≈ 0. The Brønsted α value for a set of eight aliphatic monocarboxylic acids in which effects of bulk, charge, and polarizability are at a minimum is 0.56. Steric effects, probably augmented by polarizability effects in some cases, cause positive deviations from the Brønsted line drawn with respect to these standard acids. Anionic carboxylic acids are also more reactive than would be predicted from their equilibrium acid strengths, whereas cationic acids tend to be less reactive. Using D2O as solvent has only a small effect on the rate of carboxylic acid catalysis. Using acetone-d6gives values ofkH/kDin the range of 7.0–8.0 at 25 °C, values consistent with proton or deuteron being transferred between two bases of comparable strength, the carboxylate anion and the enol form of acetone.Keywords: general acid catalysis, enolization, Brønsted relation, steric effects, deuterium isotope effect

ISSN:0008-4042    

DOI:10.1139/v89-194

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

The1H nuclear magnetic resonance spectra and their analyses are reported for benzyltrimethyl X (X = C, Si, Ge, Sn, Pb) and for the 2,6-dichloro derivatives where X = Si, Ge, Sn, Pb. The steric barrier to rotation about the Csp2—Csp3bond for X = C, based on the long-range proton–proton coupling constant over six bonds,6J(H,CH2), must be greater than 20 kJ/mol. Unexpectedly,6J(H,CH2) implies that the barriers for X = Si, Ge, Sn, Pb have relatively minor steric components. If these barriers are dominated by hyperconjugative interactions of the C—X bond with the aromatic π system, then they are probably mainly twofold in nature. Furthermore, their magnitude is likely to be near 9.5 kJ/mol for X = Si, Ge, Sn and may be as high as 12 kJ/mol for X = Pb.Keywords: NMR, benzyl X(CH3)3, X = C, Si, Ge, Sn, Pb; NMR, conformations; NMR, hyperconjugation.

ISSN:0008-4042    

DOI:10.1139/v89-195

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Two new aromadendrane sesquiterpenes 4β,9β-dihydroxyaromadendrene and 4β-hydroxy-9β-acetoxyaromadendrene have been isolated fromSideritisvaroissp•cuatrecasasii, and their stereochemistry has been established by a series of 2D nuclear magnetic resonance experiments.Keywords:Sideritis,Labiatae, new aromadendrenes, sesquiterpenes, 2D NMR.

ISSN:0008-4042    

DOI:10.1139/v89-196

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

Attempts to prepare stoichiometric complexes of the type dicarboxylic acid•18C6•H2O succeeded only with oxalic, succinic, and glutaric acids. The crystal structure of the succinic acid complex was established by X-ray diffraction at 173 K, and was refined toRw = 0.038 for 3204 observed reflections. The stoichiometry of the complex is 1:1:1 (succinic acid•H2O•18C6). In the solid state there is a dimeric H-bonded association, through a center of symmetry, of two succinic acid molecules. Each diacid is also hydrogen bonded to a water molecule. The water molecules, in turn, form two hydrogen bonds each with the crown ether. The diacid–diacid, diacid–water, and the two water–18C6 hydrogen bonds are 2.657(2), 2.636(2), 2.819(2), and 2.910(2) Å long respectively. They are all nearly linear. The structural units are associated and form the centrosymmetric dimer (18C6–H2O–succinic acid)2. The conformation of the macrocyclic polyether differs significantly from that normally found. Instead of the usual (ttg)6conformation, that adopted in this complex is (ttgttgstg)2. The two s torsion angles have values of 90.1(3) and 86.9(3)°. In this complex the succinic acid itself is in thegaucheconformation.Keywords: macrocycle – succinic acid complex.

ISSN:0008-4042    

DOI:10.1139/v89-197

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

AnabinitioMO-SCF calculations have been performed with a minimal Gaussian lobe basis set for numerous Mg(II) 1,4,7,10-tetraazacyclododecane (cyclen) configurations. Mg(II) shows a strong interaction with all functional groups of cyclen. The strongest interaction occurs when magnesium points to the cavity and perpendicular to the ligand plane. However, in the complex formation processes, the ion does not prefer to move along this direction because of the repulsive interaction with the NH functional groups. The favourable path for entering the cavity of the ligand encloses an angle of 30° with the z axis and lies in the plane between the opposite NH groups.Keywords: cyclen, macrocyclic effect, complex formation, favourable path,abinitiocalculations.

ISSN:0008-4042    

DOI:10.1139/v89-198

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

A tricyclic diene, traversiadiene, isolated from cultures ofCercosporatraversianahas been shown to have the structure and stereochemistry of the previously postulated hydrocarbon intermediate on the biosynthetic pathway to traversianal (1). Detailed:1H and13C magnetic resonance studies, including homo- and heteronuclear correlation spectra, led to the gross structure, and the stereochemistry was established through a series of nuclear Overhauser effect difference spectra.Keywords: diterpene, traversiadiene,1H and13C magnetic resonance spectra.

ISSN:0008-4042    

DOI:10.1139/v89-199

出版商:NRC Research Press    

年代:1989

数据来源: NRC

摘要:

The kinetics and mechanism of the reaction between Cr(III) and 2,4-hexanedione in aqueous solution have been studied. Complexation occurs by coordination of both oxygen atoms of the ligand to the chromium(III) center, with concomitant loss of a proton yielding 1:1 complex of the type [Cr(CH3COCHCOC2H5)(H2O)4]2+. When the metal ion is in pseudo first-order excess the concentration dependence of the observed rate constants agrees with a mechanism involving reversible reaction between the enol tautomer of 2,4-hexanedione and the metallic species Cr(H2O)63+and Cr(H2O)5OH2+, with rate constants of 8.0 × 10−4and 3.5 × 10−2 dm3 mol−1 s−1, respectively.Keywords: chromium(III), β-diketones, β-diketonates, kinetics, complexation.

ISSN:0008-4042    

DOI:10.1139/v89-200

出版商:NRC Research Press    

年代:1989

数据来源: NRC