摘要:

The diastereoselective intramolecular Diels–Alder reactions of the furan diene (IMDAF), in which the side arm comprises four carbon atoms and is substituted by a methyl group adjacent to, or one carbon removed from, the furan ring, are described. Adducts are formed in moderate to excellent yields when treated with either Florisil in methylene chloride at room temperature or methylaluminum dichloride in methylene chloride at −78 °C. Florisil is most effective for substrates containing unsubstituted dienophiles. An equimolar quantity (1.1 equivalents) of methylaluminum dichloride is most effective for precursors having methacrolein-type dienophiles, while a catalytic quantity (0.1 equivalent) is effective for crotonaldehyde-type dienophiles. Only the adducts with the side arm orientatedsynwith respect to the oxygen bridge are detected and isolated. The major diastereomer is the one in which the methyl group (initially on the side arm) is situated equatorially on the newly formed six-membered ring. An application of the diastereoselective IMDAF reaction to the synthesis of (±)-1,4-epoxycadinane is described, beginning with 2-methylfuran.

ISSN:0008-4042    

DOI:10.1139/v93-081

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The conformations of pyrrole derivatives (1,2,3a,b,4a,b,5,6, and7) have been elucidated from theoretical investigations, dipole moment measurements, use of stereospecific coupling through five bonds, as well as Overhauser experiments (6).

ISSN:0008-4042    

DOI:10.1139/v93-082

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Cu(II) – Schiff base complexes containing anthranilic acid and aldoses (sugars) as part of the base were prepared and characterized by microanalytical, thermogravimetric, magnetic, and spectroscopic data. The complexes are four-coordinate, anhydrous, and ML2type. The electron paramagnetic resonance spectral lines exhibited rhombic distortion from axial symmetry in the square-planar Cu(II) complexes withThe complexes were found to be active against kaolin paw oedema and standard strains ofEscherichiacoli,Staphylococcusaureus, andPseudomonasaeruginosa.

ISSN:0008-4042    

DOI:10.1139/v93-083

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

In the course of a preliminary investigation on the relationships between chemical structure of ryanoids and their affinity to the ryanodine (1) binding site, we have synthesized 3-epiryanodine (13) from anhydroryanodol (4), a key intermediate in our total synthesis of (+)-ryanodol (2).

ISSN:0008-4042    

DOI:10.1139/v93-084

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The1H nuclear magnetic resonance spectral parameters are reported for triphenylphosphine as solutions in CS2/C6D12and acetone-d6at 300 K. The internal rotational potential opposing torsion about the P—C bond is computed by AMI and STO-3G MO methods. The computed potentials are used to calculate the average shielding of theparaprotons caused by the diamagnetic anisotropies of the neighbouring phenyl groups. The results are used to estimate the degree of electron delocalization from the lone pair on phosphorus. The extent of delocalization depends on the internal motions and comparisons are made with phenylphosphine. The maximum possible screening of theparaprotons in phenylphosphine is calculated as 0.19 ppm for a conformation in which the lone pair on phosphorus is oriented perpendicular to the aromatic plane. The intramolecular rotational potentials then yield 0.029 ppm as the shielding of theparaprotons in triphenylphosphine due to electron delocalization, just as found for the CS2/C6D12solution after correction for diamagnetic anisotropy fields.

ISSN:0008-4042    

DOI:10.1139/v93-085

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The complete structure of the complex cell-wall C-polysaccharide fromStreptococcuspneumoniaeR36A was elucidated using a combination of high-resolution NMR spectroscopy and theoretical calculations. The C-polysaccharide is composed of the following repeating unit β-D-Glcp1 → 3-α-D-AATp1 → 4-α-D-GalpNAc1 → 3-β-D-GalpNAc1 → 1-ribitol-5-phosphate where AAT is 2-acetamido-4-amino-2,4,6-trideoxygalactopyranoside. The C-polysaccharide contains two phosphorylcholine substituents per repeating unit, which are situated on O-6 of each of the two 2-acetamido-2-deoxy-galactopyranose residues. The repeating units are linked through a diphosphate ester from ribitol to O-6 of the β-D-glucopyranosyl residue.

ISSN:0008-4042    

DOI:10.1139/v93-086

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The inhibition effect of Mn2+on the rates of the Cr(VI) oxidations of 13 different organic reductants (RH) in aqueous perchloric acid has been studied. The reductants were formaldehyde, formic acid, maleic acid, and 10 alcohols (8 primary and 2 secondary). The second-order rate constants decrease with increasing [Mn2+] according to the lawParameterb′ is interpreted as the ratio between the rate constants for the Cr(IV)–Mn2+and Cr(IV)–RH reactions. Cr(IV) is reduced by Mn2+(and also by Ce3+) much faster than by organic reductants RH. The Cr(IV) oxidations of primary alcohols follow Taft's equation with a negative reaction constant (ρ* = −0.80 ± 0.06). The Cr(IV) present in aqueous perchloric acid is postulated as an oxo species (CrO2+) capable of reacting by one-electron transfer with both Mn2+and Ce3+, and by hydride-ion transfer with organic reductants. Relevance of these results to the general mechanism for the Cr(VI) oxidations of organic substrates is discussed.

ISSN:0008-4042    

DOI:10.1139/v93-087

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The geometrical parameters of acryloyl fluoride were optimized completely at the MP2/6-31G* computational level for 17 points on the internal rotation potential energy (IRPE) curve for rotation around the formal single carbon–carbon bond. The expansion coefficients of the reduced rotational constant functionF(φ) and the four, five, and six-term expansions of the IRPE function,were obtained from these data. The theoretical IRPE functions were then refined using only the experimental torsional transition frequencies in both thes-transands-ciswells. The IRPE functions obtained are compared with those in the literature, calculated at lower levels of theory in both the rigid and nonrigid rotation approximations. The best representation of the refined IRPE function is given by the six-term expansion withV1 = 71.7,V2 = 1944.8,V3 = 113.0,V4 = −122.8,V5 = −8.7, andV6 = 12.5 cm−1, respectively. From this IRPE function, one correctly predicts thes-transconformer to be more stable with ΔH0 = 168 cm−1. The barrier to rotation from thes-transto thes-cispositions, ΔH#, is 2048 cm−1at 88° from thes-transwell. The advantages of using the nonrigid rotation approximation, based on high-quality quantum mechanical calculations that include correlation effects, to construct the effective IRPE function for molecules are emphasized

ISSN:0008-4042    

DOI:10.1139/v93-088

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Hydrogen isotopic exchange between D2and H2O has been investigated on alumina prepared by sputtering at 803 K and at low pressures. The alumina is amorphous, consisting of aluminum with coordination numbers four, five, and six in the ratio 1:2:1 as determined by NMR. A kinetic model is proposed on the basis of the pressure dependence of the exchange rate and the heats of adsorption at adsorbed sites, evaluating by molecular orbital calculations. The adsorbed sites are assumed to be Lewis acid–base pairs composed of a coordinatively unsaturated aluminum and the adjacent unsaturated oxygen site. The site relating to the rate-determining step contains threefold aluminum, which is formed by unsaturation of fourfold aluminum at the surface. As a consequence, the exchange rate depends on the amount of fourfold aluminum when the alumina is heat treated and changes its phase.

ISSN:0008-4042    

DOI:10.1139/v93-089

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The hydrolysis of 5-nitro-2-(trifluoroacetylamino)benzoic acid (1) has been studied at pH 7 in water and in the presence of micelles of cetyltrimethylammonium bromide (ctab) and of copper-containing micelles formed from the reaction ofN,N,N′-trimethyl-N′-hexadecylethylenediamine and cupric chloride. It has been found that the hydrolysis of1is inhibited by micelles of ctab but strongly catalysed by the copper-containing micelle at this pH. At a higher pH where the hydroxide ion reaction becomes important the reaction is catalysed by micelles of ctab as well, but the catalysis is stronger by the copper-containing micelle. The effect of added sodium chloride on the rate of reaction is shown to be larger for reaction in the presence of ctab than for reaction in the presence of the copper micelles. Also reported are the effects of the buffer concentration on the rate of reaction at various pH for both micelles. It is concluded that the mechanism of reaction in the copper-containing micelle involves a metal-bound hydroxyl rather than a free hydroxide ion loosely associated with the cationic micelle surface. It is interesting that the catalysis of this reaction by the copper-containing micelle is large enough to allow amide hydrolysis at a reasonable rate at neutral pH at ambient temperature.

ISSN:0008-4042    

DOI:10.1139/v93-090

出版商:NRC Research Press    

年代:1993

数据来源: NRC