Magnetic Resonance in Chemistry

Gharbi‐Benarous, Josyane; Delaforge, Marcel; Artaud, Isabelle; Girault, Jean‐Pierre

doi: 10.1002/mrc.1260281003pmid: N/A

A complete assignment of the 1H and 13C NMR spectra of roxithromycin in deuteriochloroform, methanol‐d4 and D2O was made using different two‐dimensional (2D) chemical shift correlation methods. The coupling constants observed by 1H NMR and the NOEs show that the predominant conformation of roxithromycin in CDCl3 solution is very similar to that in the crystalline state. However, variable‐temperature and variable‐solvent NMR experiments show that the major conformation of roxithromycin presents a conformational flexibility in the C‐2–C‐8 region and, particularly, in the C‐4–C‐5 bond of the molecule. The NOE data indicate that the 9‐[O‐(2,5‐dioxahexyl)oxime] chain is directed towards the 6‐OH, certainly involving an intramolecular hydrogen bond. This results in a globular structure for roxithromycin with less freedom for the macrolactone ring and the sugar units.

Amato, Maria E.; Irgolic, Kurt J.; Junk, Thomas; Pappalardo, Giuseppe C.; Perly, Bruno

doi: 10.1002/mrc.1260281004pmid: N/A

The 1H (500 MHz), 13C (300 MHz) and 31P (202 MHz) NMR spectra of rac‐2,3‐bis(palmitoyloxy)propyl(2‐trimethylarsonioethyl)phosphonate were recorded in CDCl3 (3 M) solution. Homonuclear 1H‐1H and heteronuclear 13C–1H chemical shift correlation experiments (COSY) were performed. All resonances were assigned. The shifts of the arsenic‐containing phosphonolipid were compared with the shifts of the N‐containing phospholipids. The most significant shift differences were found for the groups directly bonded to arsenic. Spectral evidence was indicative of free rotation about the CH2 CH2 bond in the arsenocholine fragment.

Hricovíni, Miloš; Tvaroška, Igor

doi: 10.1002/mrc.1260281005pmid: N/A

A study is presented of the dependence on conformation of one‐bond carbon‐proton coupling constants in three 1‐4‐linked disaccharides. Calculated 1J(CH) values are based on the FPT formulation in the semi‐empirical INDO MO method. The configuration at the anomeric carbon influences the 1J(CH) value, and the 1J(C‐1′, H‐1′) values are 20–30 Hz higher than the 1J(C‐4, H‐4) values. The conformational dependence of 1J(CH) on the dihedral angle about the CO bonds is satisfactorily expressed in the analytical form 1J(CH) = A cos 2ϕ + B cos ϕ. The constants A‐E are different for the α‐ and β‐anomers. Calculated average values 〈1J(C‐1′, H‐1′)〉 = 163.7 Hz and 〈1J(C‐4, H‐4)〉 = 146 Hz for methyl β‐xylobioside using PCILO‐calculated abundances of conformers agree well with the experimental values of 162.7 and 147.5 Hz, respectively.

Kalidhar, Suraj B.

doi: 10.1002/mrc.1260281006pmid: N/A

A comparison of the 1H NMR spectra of benzofuran and its dihydrofuran shows that the benzene proton ortho to the furan oxygen undergoes a larger change in chemical shift than those for the other protons. This observation is useful for locating the fusion sites of the furan ring in naturally occurring benzofuran derivatives.

Seidl, P. R.; Leal, K. Z.; Costa, V. E. U.; Poli, N. D.

doi: 10.1002/mrc.1260281007pmid: N/A

The 1H and 13C spectra of endo‐endo‐tetracyclo[6.1.13,6.02,7]dodec‐4‐en‐10‐ol have been interpreted using shift reagents and HETCOR experiments. Carbon chemical shifts are compared with tetracyclic analogs and bicyclic model compounds. Evidence for upfield δ and ε effects of 3–5 ppm is presented.

Sanderson, P. N.; Farrant, R. D.; Lindon, J. C.; Barraclough, P.

doi: 10.1002/mrc.1260281008pmid: N/A

1H and 13C NMR spectra are reported and assigned for imidazo[1,2‐a] pyrazine and for N‐1‐methyl‐ and N‐7‐methyl‐imidazo[1,2‐a] pyrazinium iodides. The effect of protonation on the chemical shifts of the parent molecule is demonstrated, and through the use of 13C and 15N NMR spectra the site of protonation is deduced to be at N‐1, in contrast to 2‐aryl substituted compounds synthesised as potential cardiotonic agents where protonation occurs at N‐7.

Screttas, Constantinos G.; Heropoulos, Georgios A.

doi: 10.1002/mrc.1260281009pmid: N/A

The molar paramagnetic solvent NMR shifts of galvinoxyl (1, G′) in toluene were measured at 34°C and found to be 2.21 and 2.39 ppm mol−1 for the aliphatic and the aromatic proton signals of toluene, respectively. The implications of the inequality of the two molar shifts are discussed. The paramagnetic solvent shift of G. in toluene in the presence of tertiary amines decays according to a first‐order rate law. The apparent first‐order rate constants were found to be linearly related to the initial concentration of the amine. The kobs vs. [R3N]0 regression lines in some cases exhibit a positive intercept, indicating the involvement of the solvent in the reaction. The logarithms of the second‐order rate constants are shown to be linear with respect to the ionization potentials of the amines. The reaction products of galvinoxyl with n‐Bu3N are hydrogalvinoxyl, GH and the enamine n‐Bu2NCH=CHEt whereas, with N/N‐dimethylaniline. the products are hydrogalvinoxyl and the hemiaminal ether of formaldehyde, derived from the combination of the radical PhN(Me)CH2. with G. through the oxygen of the latter. A mechanistic scheme is proposed involving a rate‐determining electron‐transfer step.

Dahn, Hans; Péchy, Peter; Van Toan, Vien

doi: 10.1002/mrc.1260281010pmid: N/A

The 17O NMR spectra for aryl‐substituted P,P‐diphenylbenzoylphosphanes (p‐YC6H4COPPh2, 1), trimethylbenzoylsilanes (p‐YC6H4COSiMe3, 2) and dialkyl benzoylphosphonates (p‐YC6H4COPO(OR)2, 3) were recorded. Compounds 1, 2 and 3 show a similar high substituent sensitivity (p+), indicating a strong electron demand of the carbonyl group; as resonance stabilization is not possible in 2 and 3 it is concluded that electron donation from P to carbonyl is equally negligible in 1. The carbonyl oxygen signals of 1, 3 and, in particular, 2 appear at low field. The 31P NMR shifts of 1 were also measured; they are less sensitive to the ring substituents.