摘要:

The synthetic α 1,2 and α 1,3 disaccharide methyl glycosides of 4-amino-4,6-dideoxy-D-mannose1–6that constitute key structural elements ofBrucellaA and M antigens were analyzed by1H and13C nuclear magnetic resonance spectroscopy. A detailed conformational analysis was performed for theN-acetylated derivatives1and4as well as for the amino derivatives2and5. Potential energy calculations using the GESA program established the global minima for the disaccharides1,2,4, and5, and mapped the energy surface as a function of the glycosidic torsion anglesand ψ. Ensemble averaged nuclear Overhauser enhancements, weighted according to the Boltzmann distribution function, were derived for each of the four disaccharides. This procedure improved the fit between experimentally and theoretically derived nOe values, when compared to interpretations based on a single conformer model. TheBrucellaA and M antigens are homopolymers of 4-formamido-4,6-dideoxy-D-mannose and, as.N-formyl groups (saccharides3,6, and8) are essential for the binding of these antigens to their respective antibodies, amide conformation was analyzed by examination of the vicinal proton–proton and proton–carbon coupling constants of the model monosaccharide8. The conformational properties of the model compounds1–8were used to model theBrucellaA and M polysaccharide antigens.Keywords: conformational analysis, disaccharide conformation, nuclear Overhauser measurements, molecular modeling,Brucellaantigen.

ISSN:0008-4042    

DOI:10.1139/v90-154

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The self-interaction-corrected generalized exchange local-spin-density functional theory (LSD-GX-SIC) with Gopinathan, Whitehead, and Bogdanovic (GWB) Fermi-hole exchange parameters is used to calculate the ionization potentials of the multiply charged ions for the elements C, Al, Cl, Ar, Ca, Fe, and Br. The effect of the correlation correction on the ionization potentials is considered. The calculated ionization potentials are compared with Hartree–Fock, recent Xα, and experiment. The correlation correction is important in the calculations. The results in the LSD-GX-SIC theory with GWB Fermi-hole exchange parameters and the Vosko, Wilk, and Nusair correlation correction are in excellent agreement with experiment. The LSD-GX-SIC theory can predict unknown ionization potentials for the multiply charged ions with acceptable computational time, since there is no need to optimize the exchange parameters.Keywords: ionization potentials, multiply charged ions, density functional theory.

ISSN:0008-4042    

DOI:10.1139/v90-155

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Analyses of the1H nuclear magnetic resonance spectra of 3-methoxybenzaldehyde in CS2/C6D12and in acetone-d6solutions at 300 K yield the values of stereospecific long-range spin–spin coupling constants for the aldehydic and methyl protons. These coupling constants are used to deduce the conformational bias for both substituents. The conformers in which the carbonyl group liestransto the methoxy group have a populationPt, which is 0.63(2)Pcfor the CS2/C6D12solution,Pcbeing the population of the conformers in which the carbonyl group liescisto the methoxy substituent; in acetone-d6solution,Pt = 0.80(2)Pc. Both of these numbers are substantially larger than that deduced from nuclear Overhauser enhancements for a CDCl3solution. IfPtis the fractional population of the conformers in which the methoxy is orientedtransto the aldehyde substituent, thenPt = 0.53(1)Pcin CS2/C6D12andPt = 0.67(1)Pcin acetone-d6, both numbers again being rather larger than that deduced from polarization experiments. STO-3G and 4-21G MO computations of the conformer stabilities agree qualitatively with the relative stabilities of the four planar conformers. Extrapolation of the populational data in solution yields vapor phase estimates midway between the two theoretical estimates. Limits can be placed on the fractional population of each of the conformers in the vapor and in the solutions. The relative stabilities of the free conformers can be rationalized in terms of electrostatics and of preferences of the carbonyl and methoxy groups for certain orientations with respect to the ring C—C bonds of highest double bond character.Keywords: 3-methoxybenzaldehyde, NMR; 3-methoxybenzaldehyde, conformations for solutions and vapor; 3-methoxybenzaldehyde, MO calculations.

ISSN:0008-4042    

DOI:10.1139/v90-156

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Carboxyl groups inAcaciasaligna(syn.cyanophylla) gum were reduced by reacting the gum propionate with diborane. Acetolysis of the reduced gum afforded a mixture of oligosaccharide acetates. After de-O-acetylation, 3-O-(α-D-galactopyranosyl)-L-arabinose, 3-O-(β-D-galactopyranosyl)-D-galactose, and 4-O-(α-L-rhamnopyranosyl)-D-glucose were isolated from the mixture.Keywords: acetolysis,Acaciagums.

ISSN:0008-4042    

DOI:10.1139/v90-157

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Di-n-butylstannylene acetals of benzyl 4,6-O-benzylidene-α- and -β-D-glucopyranoside and galactopyranoside have been prepared and studied in solution by1H,13C, and119Sn nuclear magnetic resonance spectroscopy. The species present in solution have been identified from the119Sn nmr spectral data, by comparison of the13C nmr chemical shifts of the stannylene acetals and their precursor diols and also by analysis of the products of reactions performed without added nucleophiles. The orientations of the two substituents on the carbons in the pyranose ring attached to the carbons in the stannylene ring determine the structures adopted by the stannylene acetal in solution. If one substituent is axial and the other equatorial, the stannylene acetal exists as a single symmetrical dimer in which the two oxygen atoms in the two 1,3,2-dioxastannolane rings adjacent to the axial substituents are dicoordinate. A stannylene acetal with two adjacent equatorial substituents exists as a non-interconverting mixture of dimers; one with two adjacent axial substituents is present as a rapidly interconverting mixture of dimers, trimers, and tetramers. Benzoylation and benzylation of the latter two types of stannylene acetals have been performed and have been shown to be only slightly regioselective in contrast to the known highly regioselective reactions of the first type. Only when single dimers are present are regiospecific or highly regioselective reactions obtained. The causes of the variation in the species present and of the reaction regioselectivity for different stannylene acetals are discussed.Keywords: stannylene acetals, 1,3,2-dioxastannolanes,119Sn NMR spectroscopy, regioselective reactions, carbohydrates.

ISSN:0008-4042    

DOI:10.1139/v90-158

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Preparations of a 7-phosphanorbornene, from the reaction of 1,2,5-triphenylphosphole-1-oxide with inhibited acrylonitrile in benzene, were repeated under various conditions to determine if one, or more than one, stereoisomer was formed. Adduct formation failed to occur when the reaction was attempted in ethanol, or when the precursor 1,2,5-triphenylphosphole was substituted for the phosphole oxide. Elemental analysis and spectroscopic measurements confirmed that a 1:1 adduct had been obtained, and an X-ray structure determination of the ethanol complex established that this was 5-cyano-1,4,7-triphenyl-7-phosphabicyclo[2.2.1]hept-2-ene-7-oxide,3a. Details of the1H,13C, and31P NMR assignments, of potential value in the structural assignment of other similar adducts, are related to previous results. The results of a brief mass spectrometric analysis of the changes in the thermal fragmentation pattern of the adduct with temperature are discussed in relation to the synthetic reaction.Keywords: phosphole adduct, phosphanorbornene, phosphabicycloheptene, acrylonitrile.

ISSN:0008-4042    

DOI:10.1139/v90-159

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The core oligosaccharides obtained from the lipopolysaccharides ofNeisseriameningitidisserotypes 1 and 6 were isolated by mild acid hydrolysis, and their structures elucidated by combined chemical, biochemical, and physical techniques. Use was made of 500 MHz1H nuclear magnetic resonance measurements in both one-dimensional and two-dimensional modes as well as of nuclear Overhauser enhancement experiments. To assist in the structural assignments, the purified oligosaccharides were also degraded by chemical and enzymatic procedures to smaller fragments. The L1 oligosaccharide is a biantennary partiallyO-acetylated heptasaccharide in which the larger antenna terminates in a α-D-Galp1 → 4β-D-Galp1 → 4β-D-Glcpunit, whereas the smallest disaccharide antenna carries phosphorylethanolamine groups atO-3 of its heptosyl moiety. The L6 oligosaccharide is also a biantennary partiallyO-acetylated heptasaccharide but, unlike the L1 core, is terminated by a β-D-GlcpNAcl → 3β-D-Galp1 → 4β-D-Glcpunit in its larger antenna. The heptosyl moiety in the small disaccharide antenna bears a phosphorylethanolamine group atO-7, unlikeO-3 for the L1 determinant.Keywords: lipopolysaccharides,Neisseriameningitidis, oligosaccharide epitopes, structure, NMR spectroscopy.

ISSN:0008-4042    

DOI:10.1139/v90-160

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

An unambiguous synthesis of the (6R,7S)-diastereoisomer of asperlin fromD-glucose involving a tandem epoxide formation/intramolecular Wadsworth–Emmons–Horner olefination has established the absolute configuration of the oxirane moiety in natural asperlin as (6S,7R).Keywords: asperlin, synthesis; Wadsworth–Emmons–Horner olefination; epoxide formation.

ISSN:0008-4042    

DOI:10.1139/v90-161

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

Small quantities of pyrolysates were obtained from micro-scale off-line pyrolysis of the neutral polysaccharides: cellulose, amylose, laminaran, nigeran, and a mannan from ivory nut. The pyrolytic fragments were analyzed by desorption chemical ionization – mass spectrometry, trimethylsilyl derivatization – capillary gas chromatography (– ammonia chemical ionization mass spectrometry) and benzoyl derivatization – high pressure liquid chromatography. Intact 1,6-anhydrohexosemonosaccharide, and -disaccharide fragments were identified as major products. By using off-line analytical pyrolysis information on saccharide composition can be obtained from the formation of anhydromonosaccharide products while linkage information is obtainable from distinctive anhydrodisaccharide products.Keywords: pyrolysis, polysaccharides, 1,6-anhydrooligosaccharides, desorption chemical ionization, mass spectrometry, gas chromatography, high pressure liquid chromatography.

ISSN:0008-4042    

DOI:10.1139/v90-162

出版商:NRC Research Press    

年代:1990

数据来源: NRC

摘要:

The structure of Mezzettiaside 3, a highly acylated trisaccharide obtained from the stem bark ofMezzettialeptopoda, was determined by a combination of double-quantum-filtered {1H,1H} correlation spectroscopy, two-dimensional homonuclear Hartmann–Hahn, and1H-detected {1H,13C} one-bond and multiple-bond shift correlation nuclear magnetic resonance experiments, in conjunction with fast-atom-bombardment mass spectrometry. Mezzettiaside 3 was found to be the α-(n-octyl)glycoside of 3,4-di-O-acetyl-L-rhamnopyranosyl-α(1 → 3)-2,4-di-O-acetyl-L-rhamnopyranosyl-α(1 → 3)-4-O-hexanoyl-L-rhamnopyranose. The heteronuclear multiple-bond correlation NMR technique proved to be invaluable in establishing the location of theO-acyl groups in this complex carbohydrate.Keywords: oligosaccharide, HMBC, HMQC, HOHAHA, FAB-MS.

ISSN:0008-4042    

DOI:10.1139/v90-163

出版商:NRC Research Press    

年代:1990

数据来源: NRC