摘要:

Seven derivatives of methyl 5-phenyl-3,4-dihydro-2H-pyrrole-2-carboxylates1were synthetized. These compounds are precursors of pentagonal cyclic azomethine ylids, the two sides of which are diastereotopic. The 1,3-dipolar species react withN-methyl andN-phenylmaleimides diastereospecifically. The approach of the reactant species occurs from the less hindered side of the 1,3-dipole and inendoto lead the thermodynamically stableexocycloadduct. Moreover, oxidation of the compounds1gives the corresponding substituted pyrroles.

ISSN:0008-4042    

DOI:10.1139/v94-178

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The reaction of diphenylnitrilimine with some arylidenebenzothiazinone dioxides leads to a mixture of two products that were isolated by chromatography. Spectroscopic data (infrared, nuclear magnetic resonance) and a radiocrystallographic study illustrate a competition between the two dipolarophilic sites in dipolar cycloaddition.

ISSN:0008-4042    

DOI:10.1139/v94-179

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The isomeric purity of products from certain group-selective reactions can be significantly amplified when reactions can occur sequentially. The theoretical basis for a strategy that exploits reactions with modest enantiotopic group selectivity for asymmetric synthesis is described. The relationships between conversion, yield, and isomeric purity for such a process are calculated using a simple kinetic model. A simple method for selecting candidate group-selective reactions from known face-selective reactions is presented. Application of the strategy is illustrated with the reduction ofD-glucose andD-galactose derived dialdehydes withB-isopinocampheyl-9-borabicyclo[3.3.1]nonane (Alpine-Borane®).

ISSN:0008-4042    

DOI:10.1139/v94-180

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The synthesis of β-mannopyranosides by intramolecular aglycon delivery is shown to proceed with complete stereoselectivity in six separate cases. This strategy has been successfully applied to the synthesis of several disaccharides, including octyl 3,6-di-O-benzyl-4-O-(3,4,6-tri-O-benzyl-β-D-mannopyranosyl)-2-deoxy-2-phthalimido-β-D-glucopyranoside, a precursor of the naturally occurring β-D-Man-(1→4)-β-D-GlcNAc linkage, present in all N-linked glycoproteins. Exclusive formation of the β-mannosidic linkage has been confirmed in all six cases, since independently synthesized α-linked mannopyranoside standards were shown to be absent from the reaction products. The intramolecular stereocontrolled reaction proceeds even in the presence of competing methanol. The extension of this strategy to the synthesis of the core pentasaccharide of N-linked glycoproteins has revealed limitations to the methodology, especially when a block synthesis approach is investigated.

ISSN:0008-4042    

DOI:10.1139/v94-181

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The cell-surface lipopolysaccharide produced byMoraxellacatarrhalisserotype A is composed of a hydrophobic lipid A moiety and an oligosaccharide, but lacks high-molecular-weight O-polysaccharide chains. The oligosaccharide component is composed ofD-glucose,D-galactose,D-glucosamine, and 3-deoxy-D-manno-octulosonic acid. The carbohydrate backbone was obtained from the lipopolysaccharide by employing a reaction sequence involving deacylation, dephosphorylation, and reduction of the reducingD-glucosamine terminus of the lipid A moiety. Structural analysis of the backbone oligosaccharide employed a combination of microanalytical methods and nuclear magnetic resonance spectroscopy. Homo- and hetero-nuclear chemical shift correlation techniques and nuclear Overhauser enhancement (NOE) experiments led to the unambiguous assignment of the1H andl3C resonances associated with each of the component glycosyl residues and established their sequence within the backbone oligosaccharide as shown,This lipopolysaccharide was found to consist of a highly branched,D-glucose-containing inner-core region which possesses a unique and unusual solution conformation. This was established by measurement of transglycosidic NOEs and three-bond1H−13C coupling constants, in conjunction with molecular modeling. AD-galactose-containing disaccharide identified as a terminal group of the lipopolysaccharide was structurally identical to antigenic epitopes expressed by certain mammalian epithelial cells and may be related to the virulence potential of this human pathogen.

ISSN:0008-4042    

DOI:10.1139/v94-182

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

A series of tetrakis(4-hydroxyphenyl)pentanes and tetrakis(4-hydroxyphenyl)-p-xylenes having relatively high molecular weights (550−810) were prepared and evaluated as antioxidants for tetralin at 60 °C by means of an oxygen-absorption method. Introduction of alkyl groups to theorthopositions of the phenolic hydroxyl group was shown to improve antioxidant activities for both series of compounds.

ISSN:0008-4042    

DOI:10.1139/v94-183

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The crystal structures for four α-chloro derivatives2–5of the titled ketone1have been determined, confirming their earlier assignments of stereochemistry. Variations in the conformations of2–5suggested some flexibility in the central seven-membered ring. Molecular mechanics calculations of the changes in energy as a function of the internal torsional angles at the carbonyl of these chloro derivatives indicated the ring flexibility to be comparable to that of cyclohexanone. In the three compounds containing an axial chlorine the distance between the chlorine atom and two of the carbon atoms of the more remote benzene ring was 3.2 ± 0.1 Å, suggesting the value for the thickness of a benzene ring may be significantly smaller than the accepted value when it is interacting with a single atom.

ISSN:0008-4042    

DOI:10.1139/v94-184

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Densities and volumetric heat capacities have been measured for aqueous solutions ofL-valine,L-leucine, andL-isoleucine at 288.15, 298.15, 313.15, and 328.15 K. These data have been used to calculate apparent molar volumes,and apparent molar heat capacities,which in turn have been used to obtain standard state volumes,and standard state heat capacities,for each aqueous amino acid system. Helgeson, Kirkham, and Flowers equations, for neutral organics in water, have been used to model the calculated standard state volumes and heat capacities of the amino acids as a function of temperature at constant pressure. The results of our fitting procedures may be used to predict the behaviour ofandfor the selected amino acid systems outside of the temperature range utilised in this investigation.

ISSN:0008-4042    

DOI:10.1139/v94-185

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

FT-IR spectroscopy was used to study the conformation of the porcine neuropeptide galanin, fragments 1−16 of the porcine and human peptides and the antagonists M15 and M35. All peptides were shown to be structureless in aqueous solution. Upon addition to SDS micelles, only porcine galanin and the fragment consisting of amino acids 1−16 showed any evidence of interaction, adopting a helical structure. No interaction could be demonstrated with zwitterionic lipids for any peptide except M15 which formed a thermally unstable helical conformation which unfolded promoting aggregation at around 45 °C.Additional studies on rat galanin in various solvent systems were made by using circular dichroism spectroscopy. The results obtained support the observations made by FT-IR spectroscopy.

ISSN:0008-4042    

DOI:10.1139/v94-186

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Based on the analysis of the crystal structure of the Ca2+salt of ionomycin and the chemical and physical data on ionomycin, a number of ionomycin analogues have been synthesized to study the structural features affecting the Ca2+binding and transport. Compounds2,3, and4were synthesized to study the effect of additional intramolecular oxygen coordination sites on Ca2+transport. Compounds5a–5dwere prepared to study the effect of lipid solubility on Ca2+binding and transport. Compounds6a–6cwere prepared to study the effect of the distance between the β-diketone and the carboxyl group on Ca2+transport. A general synthetic route to these compounds has been developed. The key reactions in this route are the consecutive regioselective alkylations of the dianion of 2,4-pentanedione with the appropriate bromides.

ISSN:0008-4042    

DOI:10.1139/v94-187

出版商:NRC Research Press    

年代:1994

数据来源: NRC