Structural revision of glabramycins B and C, antibiotics from the fungus Neosartorya glabra by DFT calculations of NMR chemical shifts and coupling constants

All possible mono-, di-, and tri- O-methyl, O-benzyl, O-benzoyl, and O-acetyl derivatives of methyl β-D-xylopyranoside were fully trimethylsilylated and their 1H and 13C NMR spectra measured in deuteriochloroform solutions. The spectra were analysed and the chemical shifts completely assigned on the basis of decoupling experiments. The proton-proton coupling constants vary only very little throughout the series. Conformer population analysis by the method of average coupling constants shows that trimethylsilylation increases the C1 conformer population and makes the series conformationally homogenous. Owing to this conformational homogeneity chemical shifts (both 1H and 13C) satisfy very well direct additivity rule. It is suggested that pertrimethylsilylation should be employed whenever deviations from chemical shift additivity are caused by conformational mobility of the investigated series of compounds and when bulky groups can stabilize one of the conformers.

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Carbon-13 nuclear magnetic resonance spectra of oxazoles

Canadian Journal of Chemistry ◽

10.1139/v79-517 ◽

1979 ◽

Vol 57 (23) ◽

pp. 3168-3170 ◽

Cited By ~ 36

Author(s):

Henk Hiemstra ◽

Hendrik A. Houwing ◽

Okko Possel ◽

Albert M. van Leusen

Keyword(s):

Nuclear Magnetic Resonance ◽

Magnetic Resonance ◽

Nmr Spectra ◽

Chemical Shifts ◽

Coupling Constants ◽

Proton Coupling ◽

Nuclear Magnetic Resonance Spectra ◽

C Nmr ◽

Magnetic Resonance Spectra ◽

Nuclear Magnetic

The 13C nmr spectra of oxazole and eight mono- and disubstituted derivatives have been analyzed with regard to the chemical shifts and the various carbon–proton coupling constants of the ring carbons. The data of the parent oxazole are compared with thiazole and 1-methylimidazole.

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1H NMR spectra of some chloro-alkenes

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc19812924 ◽

1981 ◽

Vol 46 (11) ◽

pp. 2924-2934

Author(s):

Seán Cawley ◽

Jan Schraml ◽

Petr Svoboda ◽

Robert Ponec ◽

Václav Chvalovský

Keyword(s):

Nmr Spectra ◽

Chemical Shifts ◽

Dipole Moments ◽

Coupling Constants ◽

The Other ◽

Dominant Factor ◽

Proton Proton ◽

Electric Field Effects ◽

H Nmr ◽

Proton Coupling

1H NMR spectra and electric dipole moments of a series of propene derivatives, (CH3)3-nCln.CCH=CH2 (n = 0-3), were measured and the spectra analyzed. Discrepancies in literature data on compounds with n = 0 and 3 are probably due to systematic spectrometer errors. Vinylic proton chemical shifts are found to vary linearly with n in the direction expected on the electronegativity ground (the variations with n occurs in the order HA > HB ≥ HC). Calculations show that even with the extreme reported values of C-C and C-Cl bond magnetic anisotropies, the shift variations with n cannot be accounted for by magnetic anisotropy effects of a freely rotating (CH3)3-nClnC group. Similar calculations rule out electric field effects as the dominant factor. On the other hand, the electron charge distribution, as obtained from CNDO/2 calculations, including d-orbitals and employing Del Bene-Jaffé parametrization, is linearly related to the vinylic proton shifts. This finding supports the earlier interpretation of NMR spectra of vinylsilanes in terms of back-bonding. The vinyl proton-proton coupling constants decrease with increasing n, the changes being larger in the constants which couple proton A (cis) to the other two.

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Studies on Prototropic Tautomerism in Neutral and Monoanionic Forms of Pyrimidines by Nuclear Magnetic Resonance Spectroscopy

Zeitschrift für Naturforschung C ◽

10.1515/znc-1977-11-1202 ◽

1977 ◽

Vol 32 (11-12) ◽

pp. 894-900 ◽

Cited By ~ 26

Author(s):

Ryszard Stolarski ◽

Mieczyslaw Remin ◽

David Shugar

Keyword(s):

Quantitative Estimation ◽

Chemical Shifts ◽

Coupling Constants ◽

Optical Methods ◽

Resonance Spectroscopy ◽

Prototropic Tautomerism ◽

Proton Proton ◽

Proton Coupling ◽

Vicinal Proton ◽

Nmr Methods

Abstract NMR methods have been applied to evaluation of prototropic tautomerism, N(1)H ⇌ N (3) H, in several selected pyrimidines, viz. the neutral forms of Isocytosine and 2-alkylthiopyrimidone-4, and the monoanionic forms of uracil, 5-fluorouracil and 4-thiouracil. The predominant tautomeric species of the neutral forms could be estimated only qualitatively from 1H chemical shifts. For the monoanionic forms this procedure was not applicable, for reasons which are discussed in detail. For the monoanionic form of uracil, 13C chemical shifts of C(5) provided a suitable criterion for quantitative estimation of the populations of the two known tautomeric species. However, the potential scope of this procedure appears somewhat limited. By contrast, the values of the vicinal proton-proton coupling constants, J (5,6), provided both necessary and adequate criteria for quantitative evaluation of the tautomer populations for all the neutral and monoanionic forms. The results were in satisfactory agreement with those obtained by optical spectroscopic methods. In some instances the results obtained in this way may be more reliable than those derived from optical methods. The range of applicability, and utility, of NMR methods to studies on protropic tautomerism in pyrimidines are critically assessed.

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Solution Conformation of Benzimidazole Nucleosides with the Aid of Model Analogues

Zeitschrift für Naturforschung C ◽

10.1515/znc-1981-1-223 ◽

1981 ◽

Vol 36 (1-2) ◽

pp. 126-134 ◽

Cited By ~ 5

Author(s):

Zygmunt Kazimierczuk ◽

Ryszard Stolarski ◽

Lech Dudycz ◽

David Shugar

Keyword(s):

Chemical Shifts ◽

Coupling Constants ◽

Glycosidic Bond ◽

Benzimidazole Ring ◽

Proton Proton ◽

H Nmr ◽

Proton Coupling ◽

Vicinal Proton ◽

Sugar Ring ◽

Derivatives Of

Abstract The 4,6-dibromo-and 5,7-dibromo-derivatives of 1-β-ᴅ-ribofuranosylbenzimidazole, and the 4,6-dibromo-derivative of 1-α-ᴅ-arabinofuranosylbenzimidazole, have been synthesized by condensation of the trimethylsilylated derivative of 4,6-dibromobenzimidazole with the appropriate protected sugar in the presence of SnCl4. Application of the fusion method led to the β-ribofuranoside of 2-methyl-5,6-dichlorobenzimidazole. Two additional analogues of the biologically active ribofuranoside of 5,6-dichlorobenzimidazole, viz. the 2′,3′-O-isopropylidene and the 5′-chloro-5′-deoxy derivatives have been synthesized. Analyses of the 1H NMR spectra of the foregoing, and of previously synthesized 5(6)-mono-halogeno and 5,6-dihalogeno derivatives, were employed to determine solution conformations of these nucleosides. Conformations of the sugar rings and exocyclic groups were evaluated from vicinal proton-proton coupling constants, as for purine nucleosides. The conformation of the benzimidazole ring about the glycosidic bond was determined from the chemical shifts of the sugar protons, principally H(2′), with the aid of model analogues in fixed syn and anti conformations; and, independently, from the chemical shifts of the benzimidazole ring protons, principally H(4) and H(7). The halogenated benzimidazole ribofuranosides exhibit conformations of the sugar ring similar to those for purine β-nucleosides, but differ from the latter in their preference for the conformation syn about the glycosidic bond.

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Conformational analysis and complete assignment of the proton and carbon NMR spectra of ouabain and ouabagenin

Canadian Journal of Chemistry ◽

10.1139/v90-195 ◽

1990 ◽

Vol 68 (8) ◽

pp. 1263-1270 ◽

Cited By ~ 15

Author(s):

Deane D. McIntyre ◽

Markus W. Germann ◽

Hans J. Vogel

Keyword(s):

Conformational Analysis ◽

Molecular Mechanics ◽

Nmr Spectra ◽

Coupling Constants ◽

X Ray Diffraction ◽

Proton Proton ◽

Proton Coupling ◽

Nmr Techniques ◽

Phase Sensitive ◽

C Nmr

The 1H and 13C 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 the 13C 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.

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Carbon-13 and nitrogen-14 NMR spectra of 1-(substituted phenyl)pyridinium salts

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc19802766 ◽

1980 ◽

Vol 45 (10) ◽

pp. 2766-2771 ◽

Cited By ~ 8

Author(s):

Antonín Lyčka

Keyword(s):

Heavy Water ◽

Nmr Spectra ◽

Chemical Shifts ◽

Coupling Constants ◽

Pyridinium Salts ◽

Nmr Signals ◽

C Nmr

The 13C and 14N NMR spectra of 1M solutions of 1-(substituted phenyl)pyridinium salts (4-CH3, 4-OCH3, H, 4-Cl, 4-Br, 4-I, 3-NO2, 4-NO2, 2,4-(NO2)2 (the 13C NMR only)) have been measured in heavy water at 30 °C. The 13C and 14N chemical shifts, the 1J(CH) coupling constants, some 3J(CH) coupling constants, and values of half-widths Δ 1/2 of the 14N NMR signals are given. The 13C chemical shifts of C(4) correlate with the σ0 constants (δC(4) = (1.79 ± 0.097) σ0 + (147.67 ± 0.041)), whereas no correlation of the nitrogen chemical shifts with the σ constants has been found. The half-widths Δ 1/2 correlate with the σ0 constants (Δ 1/2 = (76.2 ± 4.9) σ0 + (106.4 ± 2.2)) except for 1-phenylpyridinium chloride.

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21,22-Disubstituted 18α,19βH-Ursane Derivatives with Oxabicyclooctane System in Ring E

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc19930173 ◽

1993 ◽

Vol 58 (1) ◽

pp. 173-190 ◽

Cited By ~ 1

Author(s):

Eva Klinotová ◽

Jiří Klinot ◽

Václav Křeček ◽

Miloš Buděšínský ◽

Bohumil Máca

Keyword(s):

Spectral Data ◽

Spin Systems ◽

Coupling Constants ◽

Complete Analysis ◽

Nmr Spectrum ◽

Proton Proton ◽

H Nmr ◽

Proton Coupling ◽

C Nmr

Reaction of 3β-acetoxy-21,22-dioxo-18α,19βH-ursan-28,20β-olide (IIIa) and 20β,28-epoxy-21,22-dioxo-19α,19βH-ursan-3β-yl acetate (IIIb) with diazomethane afforded derivatives XII-XIV with spiroepoxide group in position 21 or 22, which were further converted into hydroxy derivatives XV and XVII. Ethylene ketals VIII-X were also prepared. In connection with the determination of position and configuration of the functional groups at C(21) and C(22), the 1H and 13C NMR spectral data of the prepared compounds are discussed. Complete analysis of two four-spin systems in the 1H NMR spectrum of bisethylenedioxy derivative Xb led to the proton-proton coupling constants from which the structure with two 1,4-dioxane rings condensed with ring E, and their conformation, was derived.

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Proton magnetic resonance study of the conformation of the pyrrolidine ring in some proline-thiohydantoins

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc19882503 ◽

1988 ◽

Vol 53 (11) ◽

pp. 2503-2510 ◽

Cited By ~ 3

Author(s):

Jef J. M. Sleeckx ◽

Marc J. O. Anteunis ◽

Frans A. M. Borremans

Keyword(s):

Present Model ◽

Coupling Constants ◽

Proton Proton ◽

Proton Coupling ◽

Proton Magnetic Resonance Study ◽

Vicinal Proton ◽

Different Temperatures ◽

Experimental Justification ◽

State Assumption ◽

Methyl Phenyl

The conformational behaviour of a series of N-substituted thiohydantoins of proline was studied by proton NMR. The pseudorotational parameters of the proline moiety were calculated from the ten vicinal proton-proton coupling constants assuming a two state equilibrium (N ⇋ S). The effect of the different substituents (methyl, phenyl, para-nitrophenyl) at the thiohydantoin nitrogen on the conformation of the pyrrolidine rings is discussed. The spectra of proline-N-methylthiohydantoin were recorded at different temperatures in octadeuterotoluene solution. The conformational analysis of these spectra showed that the ratio of the populations of the N and S forms changes considerably with temperature, while the conformational identity of both the N and S forms remains strictly preserved. These facts provide an additional experimental justification of the two-state assumption, at least so for the present model compounds.

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