Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260230601pmid: N/A

Agrawal, P. K.; Thakur, R. S.

doi: 10.1002/mrc.1260230602pmid: N/A

13C NMR shielding data for about 300 compounds are tabulated, and critical spectral features are discussed as a guide to the use of the technique in structure elucidation and stereochemical allocations of lignan, neolignan and their derivatives. Unanalysed reported data have been analysed and, wherever necessary, 13C signal assignments have been revised on the basis of more recent evidence and, to obtain consistency throughout the series.

Nieminen, Antti O. K.; Rautio, Marjatta; Rahkamaa, Erkki

doi: 10.1002/mrc.1260230603pmid: N/A

The 15N chemical shifts and the one‐, two‐ and three‐bond 15N‐1H spin‐spin coupling constants for ten unenriched barbituric acid derivatives have been measured at 10.14 MHz in dimethyl sulphoxide‐d6. The measured 15N chemical shifts are in three distinct ranges: −270 to −280 ppm for urea‐type nitrogens, approximately −264 ppm for amide‐type nitrogens and between −226 and −236 ppm for imide‐type nitrogens, in accord with the gradually increasing possibility for delocalization of the nitrogen lone‐pair electrons. The 2 ppm shielding effect of the side‐chain hydroxy group, together with the 5 ppm shielding effect of the N‐methylation on the ring nitrogens, allows the identification of the diastereoisomers of 5‐allyl‐5‐(2‐hydroxypropyl)‐1‐methyl‐2,4,6(1H,3H,5H)‐pyrimidinetrione. The three‐bond coupling constant of 1.5 ± 0.2 Hz found in five compounds is a unique example of pure trans‐type spin‐spin coupling over an NCONH path. This study shows that particular attention must be paid to sample preparation and measuring conditions in order to achieve structurally significant nitrogen chemical shift and coupling information.

Häkkinen, A.‐M; Ruostesuo, P.

doi: 10.1002/mrc.1260230604pmid: N/A

15N, 17O and 33S NMR chemical shifts were determined for some aliphatic and aromatic sulphonamides, sulphinamides, sulphenamides and related sulphones and sulphoxides. The 17O and 33S NMR chemical shifts change only slightly for the sulphonyl compounds. In the sulphinyl compounds, on the other hand, the presence of nitrogen causes a noticeable shift to higher frequencies in the 17O resonance. The differences between the 17O chemical shifts of sulphinyl and sulphonyl compounds are more noticeable than those between sulphinamides and sulphoxides. The 15N NMR chemical shifts of sulphon‐, sulphin‐ and sulphen‐amides reflect well the effect of the environments of both nitrogen and the adjacent sulphur atom. The correlations between 15N, 17O and 33S NMR chemical shifts and the structures of sulphur amides and related sulphones and sulphoxides are discussed. The chemical shifts of the 13C nuclei are also presented.

Paz‐Sandoval, M. A.; Santiesteban, F.; Contreras, R.

doi: 10.1002/mrc.1260230605pmid: N/A

Carbon‐13 NMR chemical shifts in various oxazolidines derived from pseudoephedrine and their corresponding N‐borane adducts and oxazolidinium salts are examined. The observed shifts can be rationalized on the basis of steric factors, owing to the similarity between the borane adducts and the oxazolidinium salts, i.e., BH3 and CH3 appear to have the same substituent effects.

Nikki, Kunio; Nakagawa, Naoya

doi: 10.1002/mrc.1260230606pmid: N/A

The aromatic solvent‐induced shifts (ASIS) of various polar solutes induced by polar and non‐polar aromatic solvents were determined. The protons that produce a large ASIS in non‐polar solvents show a remarkable depression of the ASIS in highly polar aromatic solvents. The observed ASIS in polar aromatic solvents are in good agreement with the calculated ASIS based on the electrostatic interaction between solute and solvent molecules.

Chupakhin, O. N.; Charushin, V. N.; Chernyshev, A. I.; Esipov, S. E.

doi: 10.1002/mrc.1260230607pmid: N/A

The structure of N‐methylpyrido[2,3‐b]pyrazinium salts has been examined by 1H and 13C NMR. 6‐Dimethylamino‐ and 6‐morpholino‐pyrido[2,3‐b]pyrazines were shown to undergo quaternization by methyl iodide at N‐4 of the pyrazine ring, whereas the pyridine ring N‐5 was the N‐alkylation site in the reaction of unsubstituted pyrido[2,3‐b]pyrazine with methyl iodide under the same conditions. The effects of quaternization on the 1H and 13C chemical shifts and the nJ(CH) values are discussed.

Guilleme, Joaquin; Diez, Ernesto; Bermejo, Francisco J.

doi: 10.1002/mrc.1260230608pmid: N/A

The aryl carbon chemical shifts of methylbenzoic acids in CDCl3 and methylbenzoate anions in aqueous NaOH solutions have been assigned. The shifts have been interpreted by means of a model based on the additivity of substituents which is similar to that previously developed for the analysis of the methyl chemical shifts. Corrective parameters included to take into account the interactions of adjacent substituents are found to be related to both the twist angle, θ, of the carboxyl or carboxylate substituents and the conformation of the methyl groups. The derived values of the twist angle are in closer agreement with x‐ray data than those estimated from UV spectra or from Cl‐PPP computations.

Chu, Yen‐Ho; Wang, Kung‐Tsung

doi: 10.1002/mrc.1260230609pmid: N/A

The 1H NMR chemical shifts and spin‐spin coupling constants of phenylalanylphenylalanine (Phe‐Phe) dipeptide diastereomers have been completely analysed at 100 MHz. Assignment of the observed signals to individual residue protons is proposed, based on characteristic shifts and standard double resonance experiments. The rotamer populations have also been determined. The spectra of the aromatic regions of the dipeptidyl diastereomers, L‐Phe‐L‐Phe and L‐Phe‐D‐Phe, are very different at pH 1.95 in D2O but similar at pH 9.15 in CD3CND2O (34:66). These results are correlated with the retention behaviour on a C18 reversed‐phase HPLC column.

Uff, Barrie C.; Mallard, Angela S.; Davis, John A.; Henson, Ray

doi: 10.1002/mrc.1260230610pmid: N/A

The NMR spectra of eighteen 7α‐ and 7β‐substituted 6,14‐bridged thebaine derivatives are reported and the steric and other effects of the 7‐substituent on the C‐5‐H atoms examined. Although the H‐5 proton shift enabled compounds epimeric at C‐7 to be distinguished in the examples studied (values falling between δ 4.75‐4.40 ppm in the 7α series and δ 5.21‐4.97 ppm in the 7β series), the C‐5 shift values proved less useful owing to overlapping ranges.