摘要:

The transannular Diels–Alder strategy has been applied to three 13-membered macrocyclic trienes whose characteristics are: acis-transdiene and acismethoxymethyl enol ether as the diénophile. The successful trials led to three 6.6.5 tricyles having atrans-syn-cisgeometry exactly as in the B.C.D. rings of the 14β-hydroxysteroids.Keywords: transannular, Diels–Alder, 14β-hydroxysteroids, 13-membered rings.

ISSN:0008-4042    

DOI:10.1139/v96-017

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Complexes of the type [Pt(R-CN)Cl3]−were synthesized and studied by1H,13C, and195Pt NMR spectroscopies. The2J(195Pt-13C) coupling constants are about 240 Hz. The signals due to the cyano carbon atoms of the coordinated ligands were observed at higher fields than those of the free ligands. The shielding on the cyano carbon increases as the alkyl chain lengthens and also when the branching increases. The195Pt signals of all the complexes were observed around −2000 ppm. The crystal structures of (NMe4)[Pt(C3H7-CN)Cl3] (1) and (NMe4)[Pt(p-HO-C6H4-CN)Cl3] (2) were determined. Compound1is monoclinic,P21/c,a = 8.384(5),b = 15.336(19),c = 11.759(9) Å, β = 99.52(6)°,Z = 4,R = 0.054, andwR = 0.051. Crystal2is tetragonal witha = 16.222(6),c = 12.052(5) Å,Z = 8,R = 0.059, andwR = 0.044. The Pt—CL bondtransto the nitrile ligand is shorter than normal (2.276(3) Å for1and 2.264(7) Å for2) while the two other bonds are normal (2.293(4), 2.287(3) Å for1and 2.320(7), 2.275(8) Å for2). The Pt—N bonds are 1.97(1) Å (1) and 1.92(2) Å (2) and the segments Pt-N≡C-C are linear. Disubstituted compounds were also synthesized and studied by multinuclear NMR. The195Pt signals of thecisisomers were observed at lower fields than those of thetransisomers (Δ ≈ 65 ppm), while the13C signals of the cyano carbons of thetransisomers were observed at lower fields than those of theciscompounds (Δ ≈ 0.6 ppm). Theciscomplexes isomerize to thetranscompounds upon heating. The crystal structures ofcis-(3) andtrans-Pt(C2H5-CN)2Cl2(4) and also ofcis-Pt(p-HO-C6H4-CN)2Cl2(5) were determined. Crystal3is monoclinic,P21/c,a = 7.506(5),b = 9.539(5),c = 14.823(7) Å, β = 92.31(4)°,Z = 4,R = 0.050, andwR = 0.042. Thetransisomer4is monoclinic with the Pt atom on an inversion centre,P21/c,a = 5.149(4),b = 9.394(8),c = 10.944(10) Å, β = 97.84(7)°,Z = 4,R = 0.017 andwR = 0.020. Finally, compound5is triclinic,P-1,a = 7.464(3),b = 10.712(6),c = 12.291(5) Å, α = 75.63(4)°, β = 75.63(4)°, γ = 80.32(4)°,Z = 2,R = 0.045, andwR = 0.056. The Pt—Cl bond distances for thecisisomers are 2.269(5), 2.270(4) Å for3and 2.274(2), 2.279(3) Å for5while they are 2.289(3) Å for thetransisomer (4). The Pt—N bonds are 1.962(14), 1.988(11) Å (3), 1.972(7), 1.976(7) Å (5) and 1.969(5) Å for4.Keywords: platinum, nitrile, NMR, isomerization, crystal struct

ISSN:0008-4042    

DOI:10.1139/v96-018

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

In a continuing study on the optimization of the electrolyte medium for high-energy lithium batteries, volumes, heat capacities, and specific conductivities of LiClO4and LiBr were measured in mixtures of γ-butyrolactone (BUTY) and 1,2-dimethoxyethane (DME) and of propylene carbonate (PC) and BUTY. These results are compared with those of the electrolytes in the pure solvents. Phase diagrams are also reported when appropriate. The effect of addition of water to these binary and ternary systems was investigated with the same techniques. The mixtures DME–BUTY, PC–DME, DME–H2O, and BUTY–H2O are typical of mixtures of aprotic solvents and mixtures of aprotic solvents and water. The electrolytes at high concentrations in aprotic solvents of low dielectric constants are largely associated. The medium still conducts electrolytically since the ion pairs are in a state that resembles to a large extent that of a molten salt. With some systems at high concentration, stable solvates persist in the solution medium, as evidenced mostly by heat capacities, and are in equilibrium with either the excess solvent or unsolvated molten salts. In mixed solvents, the properties of electrolytes can largely be predicted from the binary systems and by the coexistence of these solvates. The properties of water in DME, BUTY, or mixtures of the two solvents are modified significantly in the presence of LiBr but only slightly with LiClO4. These specific interactions, which affect the heat capacities much more than the volumes and which are especially large with the system LiBr–DME, could be responsible for the decrease in reactivity of water with lithium metal in an aprotic medium in the presence of certain electrolytes.Keywords: LiClO4, LiBr, γ-butyrolactone, dimethoxyethane, propylene carbonate, lithium battery, aprotic solvent, water, association, solvates, solid–liquid phase diagrams, volumes, heat capacities, specific conductivities.

ISSN:0008-4042    

DOI:10.1139/v96-019

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Three antibiotic peptides, LP237-F8 (1), F5 (2), and F7 (3), were isolated from the liquid culture of the fungusTolypocladiumgeodes. Chemical shift assignments of the1H and13C NMR resonances and sequencing of these metabolites were achieved by extensive high-field 2D NMR spectroscopy. The N-terminal of peptides1and2is protected with an octanoyl (Oc) fatty acid unit, whereas that of peptide3is protected with a decanoyl (Dec) unit. The C-terminal of all three peptides is protected with the amino alcohol leucinol (Lol). All three metabolites contain the common amino acids Ala, Phe or Tyr, Pro, and Gln, as well as the unusual amino acid α-aminoisobutyric acid (Aib). In addition, peptides1and2contain the amino acid α-amino-α-ethyl-n-pentanoic acid (α-ethylnorvaline, EtNor), which has not been previously reported as a constituent of a natural product. Metabolites1,2, and3are new members of the class of natural products known as peptaibols.Keywords:Tolypocladiumgeodes, peptaibols, leucinol, α-amino-α-ethyl-n-pentanoic acid.

ISSN:0008-4042    

DOI:10.1139/v96-020

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The resolved1H and13C chemical shifts for the lignin model trimer acetoguaiacyl-dehydro-diisoeugenol methyl ether have been completely assigned using one- and two-dimensional NMR techniques. Identification of spin families and assignments within the spin families were made using routine one-dimensional experiments for chemical shifts and multiplicities, and COSY and heteronuclear correlation experiments to determine connectivities. NOESY, HOESY, and long-range heteronuclear correlation experiments were then used to determine the juxtaposition of the assigned groups and to provide a confirmation of assignments. Steady state NOE difference and time-dependent NOESY and HOESY experiments were then used to provide experimental measures of the trimer conformation. Semiempirical molecular orbital methods were used to calculate minimum energy structures and energy barriers for bond rotations within the molecule. We have found that for the trimer-size molecule the NOE experiments provide a less detailed picture of the molecular conformation than the molecular orbital results. A comparison of NOE and MO results with crystal structures from the literature indicates the extent to which side-chain and methoxyl torsion angles may be estimated using each method. The correlation between NOE and MO predictions for interatomic distances apparently is related to the narrow minima for β-O-4 torsion angles for which the different results confirm each other. Broad minima or low-energy barriers for torsion angle rotations may yield minimized structures by computational methods that are not experimentally justified.Keywords: semiempirical molecular orbital calculation, 2D NMR, conformation, lignin model, nuclear Overhauser effect.

ISSN:0008-4042    

DOI:10.1139/v96-021

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

This paper analyzes the nuclear magnetic shielding tensors underlying the chemical shift in NMR spectroscopy in terms of the field generated at the nucleus by the currentJ(1)(r) induced by an external magnetic field. The magnetic field at nucleusresulting from an element of the induced current density at a distanceis proportional towhich defines the shielding densityThe magnetic shielding of a nucleus is fundamentally an atomic property, a feature brought to the fore by using the theory of atoms in molecules and the integration ofover the individual atomic basins relates the shielding tensorto a sum of atomic contributions. The shielding of nucleus ** is primarily determined by the flow of current within the basin of atom, a contribution that varies from the approximate diamagnetic limit, given by the atomic Lamb value for the atom in the molecule, to values that are greatly reduced by the presence of paramagnetic current flows associated with particular bonding effects. Whether the contribution of a neighbouring atom is shielding or deshielding is readily understood by relating the form of the current flow within its basin to the magnetization density.. A study of the currents induced in benzene shows that the extent to which a proton, bonded to a ring of atoms, is deshielded by the field exerted by its bonded neighbour provides a direct diagnostic test for a ring current and an accurate relative measure of its strength. The theory of atoms in molecules isolates transferable atomic properties and because of this ability one finds, in addition to the anticipated result that a given functional group contributes identical amounts to the isotropic shieldingof a nucleus external to it through a series of molecules, the more remarkable result that the whole of the variation incan have its origin in the basin of atom, the contribution from external groups remaining constant. For example, the external contribution tofor a carbon nucleus in a normal hydrocarbon is independent of chain length and position ofwithin the chain, the methyl group in ethane contributing the same shielding to a methyl carbon as does the butyl group in pentane. This constancy in external contributions to the shielding is also found for N, O and F nuclei in substituted, saturated hydrocarbons.Keywords: NMR, magnetic shielding, current density, magnetic shielding density.

ISSN:0008-4042    

DOI:10.1139/v96-022

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The crystal structures at −20 °C ofcis-2,2′-bipyridinium(1+) (BPTB,P21/n,a = 9.249(3),b = 14.093(7),c = 20.285(3) Å, β = 92.86(2)°,Z = 4) and 1,10-phenanthrolinium(1+) (PTB,P21/c,a = 11.194(2),b = 13.837(3),c = 18.303(3) Å, β = 107.82(1)°,Z = 4) tetraphenylborates have been determined. Inasmuch as 1,10-phenanthroline is an aromatically bridgedcis-2,2′-bipyridine, monoprotonation results, in both systems, in the formation of an intra-cation N—H … N′ hydrogen bond, the geometric and spectroscopic properties of which we have investigated. The cation skeleton in PTB is planar to 0.03(2) Å; in BPTB the dihedral angle between the two cation ring planes is 5.2°. In the pale yellow PTB there are significant π–π stacking interactions that persist into solution. The effect of protonation on the geometry of the 2,2′-bipyridine and 1,10-phenanthroline systems is examined in considerable detail and compared with the corresponding effects in the paraquat(2+) and similar cations. On both geometric and spectroscopic (infrared spectra between 10 and 295 K) evidence, the N—H … N′ hydrogen-bonding interaction is stronger in BPTB; in PTB this interaction is among the weakest reported in crystals, the ν(NH) stretching frequency at 10 K being as high as 3279 cm−1. A detailed comparison of the geometries of the intra-cation N—H … N′ bonds in BPTB and PTB with those in classical and modified proton-sponge cations has led to the formulation of criteria useful in predicting the occurrence of proton-sponge-like properties.Keywords: bipyridinium ions, hydrogen bonding, phenanthrolinium ions, proton sponges, tetraphenylborates.

ISSN:0008-4042    

DOI:10.1139/v96-023

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Irradiation of 2-methylbenzoyl cyanide (3a) in acetonitrile solution results in the formation of its dimer, which upon loss of HCN gives rise to the cycloadduct7a. The dimerization also proceeds efficiently with derivatives of3agiving adducts7band7c. When 2-methylaroyl cyanides are photolyzed in the presence of a more reactive acyl cyanide the mixed adducts8a–eare obtained in excellent yields. The cycloadducts7a–cand8a–ereact with carbon and nitrogen nucleophiles by a tandem addition–cyclization sequence furnishing substituted naphthols (10aand10b) and isoquinolones (11a–d), respectively. Isocoumarins12aand12bwere prepared from the adducts8aand8e by treatment with potassiumtert-butoxide in THF.Keywords: naphthols, isoquinolones, isocoumarins, synthesis of; acyl cyanides; hetero Diels–Alder.

ISSN:0008-4042    

DOI:10.1139/v96-024

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Treatment of tosylhydrazones of benzoin, benzoin acetate, and benzoin benzoate with alkali under protic and aprotic conditions yielded diphenyl acetylene together with desoxybenzoin. An increase in leaving aptitude of the adjacent group enhanced the formation of diphenyl acetylene. By treatment with LiAlH4and with NaBH4, the tosylhydrazones gave stilbenes in good yields. Selective formation ofcis- ortrans-stilbene was observed in some cases.Keywords: tosylhydrazone, benzoin derivatives, decomposition, metal complex hydrides.

ISSN:0008-4042    

DOI:10.1139/v96-025

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Dimethyldioxirane (DMD) is a cyclic peroxide made by oxidizing acetone with peroxymonosulfate (PMS) in water buffered at pH 7.5 using sodium bicarbonate. It has been shown that DMD generation can be achieved in situ within a pulp suspension allowing very selective TCF bleaching of kraft pulp. This process involves simultaneous generation of DMD, reaction of PMS and DMD with residual lignin, and spontaneous decomposition of both oxidants. The first part of this work is a kinetics study of the decomposition of PMS and DMD as a function of pH under conditions similar to those for in situ bleaching. The effect of chelation as well as the effect of transition metal ions on decomposition rate was also investigated. DMD is very sensitive to pH and its half-life is very short under alkaline conditions. The presence of any transition metal ion that can be involved in a one-electron transfer is detrimental to DMD stability. However, fast reaction of DMD with pulp almost counteracts the effect of metal ions.Keywords: bleaching, peroxymonosulphate, dimethyldioxirane, metal ions, kinetics, decomposition, kraft pulp.

ISSN:0008-4042    

DOI:10.1139/v96-026

出版商:NRC Research Press    

年代:1996

数据来源: NRC