Magnetic Resonance in Chemistry

Ribeiro, Anthony A.; Shen, Zhongqi; Wang, Ming; Zhang, Yongzheng; Zoretic, Phillip A.

doi: 10.1002/(SICI)1097-458X(199805)36:5<325::AID-OMR268>3.0.CO;2-2pmid: N/A

Long‐range (LR COSY, RELAY, TOCSY, HMBC) and neighbor (COSY, HMQC) 2D correlation NMR methods were used in tandem to derive complete 1H and 13C NMR assignments for synthetic d,l‐isospongiadiol and six spongian intermediates obtained from a novel free‐radical tandem cyclization process. The 13C NMR spectra of synthetic d,l‐isospongiadiol are identical with those reported previously for natural isospongiadiol, whereas the 1H NMR spectra reflect solvent differences. NOESY and difference NOE data demonstrate that synthetic d,l‐isospongiadiol and the spongian intermediates have the desired 19β‐methylene and 18α‐, 17β‐ and 20β‐methyl stereoselectivity at the stereogenic centers, and that rings A–C are in chair conformations. © 1998 John Wiley & Sons, Ltd.

de Kowalewski, Dora G.; Kowalewski, Valdemar J.; Contreras, Rubén H.; Diez, Ernesto; Esteban, Angel L.

doi: 10.1002/(SICI)1097-458X(199805)36:5<336::AID-OMR273>3.0.CO;2-Cpmid: N/A

The 13C NMR spectra of a series of alkyl formates which show the coexistence of s‐cis and s‐trans rotamers at room temperature were measured at 125 MHz. 13C chemical shifts and 1J(CH) couplings are compared for both types of rotamers. Their differences are rationalized in terms of different intramolecular interactions. The 17O NMR spectra of these compounds could be observed only for the most abundant rotamer, which in all cases was identified as the s‐cis rotamer. In the three members of this series with the shortest alkyl chains, a 2J(17O,1H) coupling constant of ca. 40 Hz was observed. In ethyl formate, 13C magnetic shielding constants were calculated using the LORG approach with ab initio optimized geometries in both types of rotamers for the carbon atom β to the dicoordinated oxygen atom. A comparison between the calculated and experimental values yields support for the rationalizations quoted above. © 1998 John Wiley & Sons, Ltd.

Licht, H. H.; Ritter, H.; Bircher, H. R.; Bigler, P.

doi: 10.1002/(SICI)1097-458X(199805)36:5<343::AID-OMR272>3.0.CO;2-Npmid: N/A

The tautomerism and isomerism of triazoles make the structure analysis of such compounds a difficult problem, and in the case of several nitrotriazoles a detailed structure assignment had been lacking up to now. Supported by selected compounds, a definite structure determination of ten nitrotriazoles was achieved by combined application of 15N NMR spectroscopy and increment calculations. This method also succeeded in cases where equilibria of tautomers were present. An evaluation of the shift data and coupling modes gave systematic references to structure and substituent influences. © 1998 John Wiley & Sons, Ltd.

Jirman, Josef; Nádvorník, Milan; Sopková, Jana; Popkov, Alexander

doi: 10.1002/(SICI)1097-458X(199805)36:5<351::AID-OMR252>3.0.CO;2-Upmid: N/A

Chiral synthons containing either 13C‐ or 15N‐labelled glycine were prepared. The 13C and 15N NMR spectra of the Ni(II) complex of the Schiff base of (S)‐2‐(N‐benzylprolyl)aminobenzophenone and 13C‐1‐, 13C‐2‐ or 15N‐labelled glycine were measured and assigned. The observed splitting of the carbon signals is due to long‐range J(13C,13C) and J(15N,13C) couplings. The mutual influence of nuclei is transferred via the central Ni(II) atom in square‐planar complexes. © 1998 John Wiley & Sons, Ltd.

Witanowski, Michal; Biedrzycka, Zenobia; Grela, Karol

doi: 10.1002/(SICI)1097-458X(199805)36:5<356::AID-OMR278>3.0.CO;2-Wpmid: N/A

One‐bond 13C–13C coupling constants across a single bond are reported for a number of nitroalkanes including cases where two or three nitro groups are attached to the same carbon atom. The nitro group effect on the 1J(CC) across the adjacent bond becomes significant when more than one nitro moiety is present at the same carbon, and it can amount to an increase in the coupling of 7 Hz, i.e. by about 20% with respect to a typical value of 35 Hz which is characteristic of unsubstituted alkanes. The effect is comparable to that exerted by chlorine substituents, but is smaller than those induced by hydroxy or alkoxy oxygen atoms and by fluorine. The results for nitroalkanes seem to suggest that a major factor in substituent‐induced effects on 1J(CC)s across a single bond is the electronegativity of the atoms which are attached directly to the bond, but some effects induced by methyl substituents suggest a contribution from steric hindrance. Semi‐empirical calculations by the TNDO/2 method reproduced satisfactorily the general trends observed in substituent effects on 1J(CC)s, including those exerted by nitro groups. © 1998 John Wiley & Sons, Ltd.

Vlahov, Giovanna

doi: 10.1002/(SICI)1097-458X(199805)36:5<359::AID-OMR274>3.0.CO;2-Zpmid: N/A

High‐resolution 13C NMR spectroscopy of the carbonyl carbons of triglyceride acyl chains was used as a quantitative method for the regiospecific analysis of the triglyceride fraction of vegetable oils of different botanical origins. The carbonyl carbon resonances of saturated, oleyl and linoleyl chains were baseline resolved. Moreover, the chains attached at the 1,3‐position were shifted by 0.4 ppm at higher frequencies than those of the corresponding chains attached at the 2‐position. This quantitative NMR method was adopted after demonstrating that proton decoupling affected the carbonyl intensities to the same extent, no significant differences being found among the NOE factors of different acyl chains. The proton‐decoupled spectra were measured for quantitative purposes with full NOE enhancement. As a result, the spectrum signal‐to‐noise ratios were significantly improved in shorter experiment times. The 13C NMR method was used to determine the acyl chain composition of vegetable oil triglycerides, the composition of the two acyl chain pools which entered the glycerol 1,3‐ and 2‐positions and the specificity of each chain for the 2‐position. © 1998 John Wiley & Sons Ltd.

Ämmälahti, E.; Bardet, M.; Cadet, J.; Molko, D.

doi: 10.1002/(SICI)1097-458X(199805)36:5<363::AID-OMR280>3.0.CO;2-Qpmid: N/A

Structural assignment and conformational features of two diastereomeric oxidation products of 2′‐deoxycytidine, the 5S* and 5R* diastereomers of N1‐(2‐deoxy‐β‐D‐erythro‐pentofuranosyl)‐5‐hydroxyhydantoin were achieved by combining NMR studies and restrained molecular dynamics. Interproton distances were measured from NOESY and ROESY experiments. The absolute configuration of these two compounds was unambiguously determined and their conformation properties in solution were also studied. © 1998 John Wiley & Sons, Ltd.

Okogun, Joseph I.; Duddeck, Helmut; Habermehl, Gerhard; Krebs, Hans C.; Tóth, Gábor; Simon, András

doi: 10.1002/(SICI)1097-458X(199805)36:5<371::AID-OMR279>3.0.CO;2-5pmid: N/A

By means of 2D NMR experiments (COSY, HMQC, HMBC, ROESY), the constitution of the photoreaction product 3‐deoxo‐1α,3 α‐diacetoxy‐1,2,3,3,8,30‐hexahydro‐18βH‐andirobin (2), obtained by photolysis of 7‐deacetoxy‐7‐oxokhivorin (1) in methanol, was shown to be a ring B seco‐limonoid with the 14,15‐epoxide intact. The configuration of C‐8 is R. Ring C adopts a twist‐boat conformation. No conformational preferences for the rotation of ring A in 2 with respect to rings C/D and for the side‐chain C‐6/C‐7 could be detected. Nearly all 1H and all 13C resonances were assigned unambiguously. © 1998 John Wiley & Sons, Ltd.

Tóth, Gábor; Simon, András; Elgamal, M. Hani A.; Soliman, Hesham S. M.; Elmunajjed, Dalal T.; Horváth, Gyula; Duddeck, Helmut

doi: 10.1002/(SICI)1097-458X(199805)36:5<376::AID-OMR269>3.0.CO;2-Spmid: N/A

A novel triterpenoid saponin, Snatzkein E, was isolated from Arenaria filicaulis Boiss. It possesses a new aglycone and a glucose moiety carrying a sulfate group at C‐2′. Its structure and conformational behaviour were investigated by one‐ and two‐dimensional 1H and 13C NMR spectroscopy. © 1998 John Wiley & Sons, Ltd.