摘要:

The1H and13C NMR spectra of the cardenolide ouabain and its aglycon ouabagenin have been completely assigned by two-dimensional NMR techniques, including phase-sensitive COSY and carbon–proton correlation (HETCOR, HMQC, and COLOC) spectra. The major conformer of these two compounds in solution is all-chair as determined from proton–proton coupling constants and is similar to that in the crystal lattice as previously determined by X-ray diffraction. The conformations of the A and D rings of ouabain in water are somewhat different than in DMSO/CDCl3(2:1). At lower temperatures (−20 °C) signals from two conformers in slow exchange were readily observed in the13C spectra, with an approximate ratio of 1:7. Molecular mechanics and dynamics calculations indicate that the conformational process responsible for this involves a chair/twist-boat interconversion of the A ring, with the all-chair conformer highly preferred.Keywords: ouabain, conformational analysis, 2-D NMR, molecular mechanics, molecular dynamics.

ISSN:0008-4042    

DOI:10.1139/v90-195

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Several syntheses of special isoxazoles bearing a long alkyl or alkenyl side chain in the α position of the nitrogen are presented. A very convenient route in the case of an alkyl side chain is the classical 1,3-dipolar cycloaddition of a nitrile-oxide to a vinyl acetate. Although it is a rather long approach, the reaction of hydroxylamine with enone-epoxides represents a new and unequivocal method compatible with alkenyl side chains. The isomerisation of the easily synthesized isoxazoles bearing such substitutents α to the oxygen, by the reaction of their isoxazolium salts with hydroxylamine, is a fast and general method. Another very convenient access to "α-Nsubstituted" isoxazoles is the coupling of magnesium compounds with 3-bromomethyl 5-methyl isoxazole.Keywords: isoxazoles, substitution in the α position of nitrogen.

ISSN:0008-4042    

DOI:10.1139/v90-196

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Tetrakis(p-methylsulphonylphenyl)tin(IV) and tetrakis(p-methylsulphinylphenyl)tin(IV)n-hydrate have been prepared and their spectra (ir 1350–400 cm−1; nmr,1H,13C,119Sn) and X-ray crystal structures are reported. The first compound is monoclinic, space groupC2/c,Z = 4, witha = 21.589(6),b = 6.207(3),c = 22.861(11) Å, β = 93.80(3)° (22 °C); the structure was solved by the direct method and refined by full-matrix least squares calculations toR = 0.043 for 2755 observed reflections. It has 2 molecular symmetry with the methyl group and one oxygen atom completely disordered in both CH3S(O2) groups in the asymmetric unit. The second compound is tetragonal, space groupP42/n,Z = 2, witha = b = 15.408(6),c = 6.379(2) Å (−100 °C); the structure was solved by the Patterson method and refined by full-matrix least squares calculations toR = 0.060 for 1209 observed reflections. It hasmolecular symmetry with the whole asymmetric unit disordered. Water molecules occupy positions on parallel 42axes but molecular packing requirements prevent all sites having 100% occupancy givingn ~ 1 for the hydrate.Keywords: Tetra-aryltins, crystal structures, sulphone, sulphoxide, hydrogen-bonding.

ISSN:0008-4042    

DOI:10.1139/v90-197

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Coherent deposits of oriented, crystalline MxM′4−xV6O16+δ, with M, M′ = K, Rb, Cs, or NH4, 0.0 ≤ x ≤4.0 and 0.00 ≤ δ ≤ 0.13, have been prepared electrochemically on various conducting substrates from metavanadate solutions containing ammonium or alkali metal cations, singly or in combination. Chemical analysis and X-ray diffraction show that the deposits consist of a continuous series of mixed crystals based on the isomorphic end members M4V6O16and M′4V6O16.The deposits can be oxidized electrochemically, with removal of M and M′ atoms, and the products can be used as cathodes for insertion of lithium, with the largest capacity achieved about 1.0 mol% of total vanadium. Cycling of the electrodes improves capacity, with insertion of lithium reaching 25 mol% of total vanadium. This gain in capacity results from further removal of alkali metal or ammonium ions from the electrode during the oxidation step of the cycling process.The results extend the range of composition of homogeneous phases in the system (NH4)2O–K2O–V2O4–V2O5.Keywords: alkali metal hexavanadates, electrochemical preparation.On a préparé des dépôts cohérents qui contiennent des cristaux orientés de MxM′4−xV6O16+δ, avec M, M′ = K, Rb, Cs ou NH4, 0,0 ≤ x ≤4,0 et 0,00 ≤ δ ≤ 0,13, par la méthode de voltammétrie cyclique sur des substrats divers et dans des solutions de metavanadates qui contiennent des cations des métaux alcalins ou d'ammonium seuls ou des combinations de tels ions. L'analyse chimique et la diffraction des rayons X ont démontré que les dépôts comprennent une série continue de cristaux mixtes basée sur les membres terminaux M4V6O16et M′4V6O16.

ISSN:0008-4042    

DOI:10.1139/v90-198

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Spectra of caffeine in D2O solution (0.003 to 0.129 M) were studied by Fourier-transform infrared spectroscopy. Spectral changes in the carbonyl stretching region were interpreted in terms of the self-association equilibria resulting from the stacking of caffeine molecules. Absence of isosbestic points indicated the occurrence of more than two spectroscopically distinct caffeine species. A simple monomer–dimer–polymer model was employed, in which the successive equilibria are governed by two association constants,Kdfor the formation of dimers andKpfor the formation of trimers and larger polymers. The valueKd = 158.1 L mol−1was taken from a recent ultraviolet study by Izaetal. and the valueKp = 27 L mol−1was derived from the analysis of infrared band profiles sharpened by Fourier self-deconvolution. From these values ofKdandKp, the weight fractions of monomers, dimers, and polymers were calculated for every caffeine concentration and their individual molar absorptivity spectra were derived. In agreement with the earlier work of Maevsky and Sukhorukov, it was found that stacking association causes shifts of both C=O stretching fundamentals to higher wavenumbers. The monomer-dimer shifts are smaller than the dimer-polymer shifts. Analogous shifts were observed for the stacking self-association of 1,3-dimethyluracil in D2O. The shifts to higher wavenumbers are probably caused by the diminished hydrogen bonding of water to C=O groups of the stacked bases.Keywords: caffeine, self association, infrared spectroscopy, base stacking.

ISSN:0008-4042    

DOI:10.1139/v90-199

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The pioneering research on Lycopodium alkaloids carried out by R. H. F. Manske and L. Marion led to the characterization of a large number of alkaloids. Many of these alkaloids were designated by numbers preceded by an L (e.g., L1, which has since been shown to be dihydrolycopodine). While the identity of many of these alkaloids has now been established, several remain unreported. We have examined samples of alkaloids L15, L16, L17, L18, L22, L24, L25, L26, L28, L29, L31, and L35, obtained from Marion's collection, by GC–MS and GC–IR, and report herein the identity of these alkaloids and alkaloid mixtures. Most of the samples proved to be mixtures of known alkaloids. Acetylannofoline (5), a component of alkaloid L17, and 5,15-oxidolycopodane (9), a component of alkaloids L28 and L31, are identified for the first time as natural products. A complete listing of the L-numbered alkaloids is provided.Keywords: Lycopodium alkaloids, alkaloids, acetylannofoline, oxidolycopodane.

ISSN:0008-4042    

DOI:10.1139/v90-200

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

α-(Dialkylamino)-α-(pyrrol-2-yl)acetonitriles (3) are versatile intermediates that are readily converted into regiochemically pure 2-bromo-6-dialkylamino-1-azafulvenes and 5-bromo-2-acylpyrroles (acyl = CHO, COR). In addition,3(R1 = H),4(R1 = H), and the β-substituted compound9are transformed easily and under mild conditions, via the corresponding α-methylthio compounds (e.g.,7a), into the methyl pyrrole carboxylates8a,8b, and10, respectively.Keywords: α-(dialkylamino)-α-(pyrrol-2-yl)acetonitriles, 5-bromo-2-acylpyrroles, methyl pyrrole carboxylates.

ISSN:0008-4042    

DOI:10.1139/v90-201

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Structures for trivalent silyl anionsand, wherentakes values from 0 to 3, have been optimized at the HF/6-31 + +G* level and single point calculations made at the MP2/6-31 + +G* level (core included). SiH3−and ions containing one halogen invert by the vertex mechanism in which the lone-pair has π-symmetry, and the monosubstituted ions have high barriers (SiH2F−45.2 kcal/mol and SiH2Cl−44.0 kcal/mol). Further substitution by halogens results in a change to the edge inversion mechanism involving a T-shaped transition structure with the lone-pair coplanar with the ligands. Barriers (kcal/mol) at the MP2/6-31 + +G* level including ZPE are lower than for the monosubstituted ions and are SiHF2−35.0, SiF3−35.9, SiHCl2−28.4, and SiCl3−32.5. In SiLi3−edge inversion is preferred, but the surface is much flatter and the barrier is low (9.8 kcal/mol). Trivalent silyl radicals SiHnF(3−n), SiHnCl(3−n)and SiFnCl(3−n)(withnhaving values 0 to 3) have also been examined with the 6-31 + +G* basis set, with optimization at the UHF level and single point calculations at the UMP2 level. Radicals SiH3, SiH2F, SiH2Cl, and SiHCl2all invert by the vertex mechanism. Increased halogenation results in a change of mechanism and SiF3, SiCl3, SiF2Cl, and SiFCl2invert by the edge mechanism. For radical SiHF2the calculated barriers for the two mechanisms are almost identical with the higher level of theory slightly favouring edge inversion.Keywords: inversion mechanism, halogenated silyl radicals and anions.

ISSN:0008-4042    

DOI:10.1139/v90-202

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Soret coefficients of aqueous phosphoric acid and aqueous iodic acid are determined conductimetrically at 25 °C. As the molality of phosphoric acid drops from 0.05 toward 0.00 mol kg−1, the enthalpy of transport jumps from 2 to 15 kJ mol−1. For iodic acid, a stronger electrolyte, the corresponding increase is smaller, from 12 to 17 kJ mol−1. Equations are developed for the enthalpy of transport of 1:1 weak electrolytes. The equations are used to evaluate the reactive enthalpy of transport and the intrinsic enthalpies of transport of the ionized and molecular forms of phosphoric and iodic acids. Equating the enthalpies of transport of the acid molecules and the acid anions provides estimates of single-ion enthalpies of transport. Thermal diffusion measurements are reported for potassium dihydrogen phosphate and potassium iodate to help interpret the results.Keywords: thermal diffusion, enthalpy of transport, Soret coefficients, weak electrolytes, phosphoric acid, iodic acid.

ISSN:0008-4042    

DOI:10.1139/v90-203

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Tin metal reacts with 2,2′-dithiobis (pyridine) in refluxing toluene to give the pyridine-2-thiolate complex Sn(C5H4NS)4,1a. Multinuclear (77Se,119Sn) magnetic resonance shows that ligand redistribution occurs in1a:Sn(EPh)4(E = S or Se) mixtures to give Sn(EPh)4−x(C5H4NS)x. From the pattern of the119Sn chemical shifts of these series, compared with that of Sn(EPh)4−x(SR)x(R = simple alkyl or aryl group), the complexes are shown to be five-coordinate Sn(EPh)3(C5H4NS-N,S) whenx = 1 and six-coordinate Sn(EPh)4−x(C4H4NS-S)x−2(C5H4NS-N,S)2whenx = 2–4. Reaction of Sn(SPh)2and (C5H4NS)2yields pure Sn(SPh)2(C5H4NS)2. The13C NMR spectrum of Sn(C5H4NS-S)2(C5H4NS-N,S)2is an exchange average down to 178 K, probably as a result of fluxionality. The series Sn(EPh)4−x,(L′)x(E = Se, L′ =5-nitropyridine- or quinoline-2-thiolate,x = 0–4; E = S or Se, L′ = pyridine-2-thiolate-N-oxide,x = 0–2) can be preparedinsituby redox between Sn(EPh)4and the disulfide (L′)2. Treatment of1awith X2(X = Br or E- gives SnX4−x(C5H4NS-N,S)x(x = 2 or 3), which have been characterized by NMR, and provides a new synthesis of the known dihalo complexes.In a recrystallization of1afrom CH2Cl2:hexane Sn(C5H4NS)4•C5H5NS,2, was obtained, probably as a result of hydrolysis. The structure of2has been solved by single crystal X-ray diffraction techniques. The compound crystallizes in the orthorhombic space groupPbca(No. 61) with a unit cell of dimensionsa = 17.132(2) Å,b = 15.337(2) Å,c = 21.268(3) Å, andZ = 8. Full-matrix least-squares onFled to agreement factorsR = 0.0439 andRw = 0.0426 for 3145 independent reflections withFo > 2.5σ (Fo) and 206 variables. The molecule Sn(C5H4NS-S)2(C5H4NS-N,S)2has highly distorted octahedral coordination about the tin atom, with the monodentate ligands incispositions and the bidentate ligands attached with mutuallycisnitrogen atoms. The Sn—S distances are 2.463(2)–2.542(2) Å and the Sn—N distances are 2.332(2) and 2.324(2) Å. The N—Sn—S angles in the chelate rings are 64.2(2) and 65.1(1)°. The molecule of pyridine-2-thione in the lattice is hydrogen-bonded to the N atom of one of the monodentate ligands in the tin complex, with ahydrogen bond distance of 1.92(7) Å.Keywords: pyridine-2-thiolate, selenium-77 NMR, tin-119 NMR, tin(IV) complexes, X-ray analysis.

ISSN:0008-4042    

DOI:10.1139/v90-204

出版商:NRC Research Press    

年代:1990

数据来源: NRC