Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260311201pmid: N/A

Katsura, Tatsuo; Ueno, Katsuhiko; Furusawa, Kiyotaka

doi: 10.1002/mrc.1260311202pmid: N/A

The solution conformations of nine 3′,5′‐O‐(di‐tert‐butylsilanediyl) nucleosides, new sila analogues of cyclic nucleotides, were studied by proton NMR spectroscopy. The furanose rings of all the compounds studied are forced to take on the C‐3′‐endo conformation by the 3′,5′‐cyclization. The conformations at the glycosidic bond of the adenosine and deoxyadenosine derivatives were determined as mixtures of syn and anti conformations by the DESERT method; the fractions of anti conformation for these derivatives were 0.58 and 0.41, respectively.

Chimichi, Stefano; Cosimelli, Barbara; Bambagiotti‐Alberti, Massimo; Coran, Silvia A.; Vincieri, Franco F.

doi: 10.1002/mrc.1260311203pmid: N/A

The concerted applications of a variety of one‐ and two‐dimensional NMR techniques (COSY, NOESY, HETCOR, COLOC, etc.) to the structural elucidation of the natural 3,4‐bis[(3,4‐dimethoxyphenyl)methyl]dihydro‐(3R‐trans)‐2(3H)‐furanone, isolated from Oenanthe aquatica fruits, in CDCl3 solution are reported. The magnitude of the 1H,1H coupling constants obtained from the simulated spectrum together with a series of homonuclear NOE difference spectra were used to determine the full stereochemistry of the system.

Cho, Bongsup P.

doi: 10.1002/mrc.1260311204pmid: N/A

13C NMR spectra of two C‐8‐oxo purine nucleosides, 8‐oxoguanosine and 8‐oxoadenosine, were recorded as a function of pH. The pH dependence of 13C chemical shifts and 13C‐1H coupling constants associated with the purine carbon resonances was analyzed to probe the sites and extent of ionization and protonation of these oxidatively damaged nucleic acid adducts. In the course of this study, it was found that the C‐2′ and C‐3′ resonances of the ribofuranosyl sugar ring can be conveniently assigned on the basis of their unique pH‐independent 1H‐coupling spectral patterns in aqueous solution.

Radeglia, Reiner; Poleschner, Helmut; Haufe, GÜNter

doi: 10.1002/mrc.1260311205pmid: N/A

The four ZE isomers of cyclododeca‐1,5,9‐triene were examined by 13C NMR spectroscopy. The vicinal 1H1H coupling constants of the olefinic protons were used to determine the stereochemistry at the double bonds. For the symmetrically substituted CHCH fragments (chemically equivalent H atoms) this parameter was obtained from the 13C NMR multiplets which are observed in 1H continuous‐wave off‐resonance 13C NMR experiments. The precise assignment of the 1H and 13C chemical shifts, in particular for the Z,Z,E and Z,E,E isomers, was possible on the basis of off‐resonance measurements in combination with 13C/1H heteronuclear correlated and 1H/1H‐COSY‐45 2D experiments.

Van Zyl, Pieter W.; Steynberg, Jan P.; Brandt, Edward V.; Ferreira, Daneel

doi: 10.1002/mrc.1260311206pmid: N/A

A unique range of free phenolic 4‐arylflavan‐3‐ols consisting of four sets (3′,4′,5,7‐tetrahydroxyflavan‐3‐ol or its 5‐deoxy analogue coupled to phloroglucinol or resorcinol), each composed of three diastereomers (2,3‐trans‐3,4‐trans, 2,3‐trans‐3,4‐cis and 2,3‐cis‐3,4‐trans) were synthesized to assess their spectroscopic properties. 1H and 13C NMR and circular dichroism data are related to selected structural and stereochemical features with a view to modelling natural phenolic oligoflavanoids.

Groombridge, Christopher J.; Coker, Eric N.; Rees, Lovat V. C.

doi: 10.1002/mrc.1260311207pmid: N/A

Thallium‐205 NMR is reported for several zeolite samples (A, X and Y; hydrated and dehydrated) with thallium(I) exchanged for sodium. Detection at high field (7.05 T) clarifies the spectrum appearance, and it is seen that chemical shift anisotropy (CSA) dominates some of the band shapes. The large CSAs dictate that the spectrum is recorded piecewise by a frequency‐stepped spin‐echo integral (FUSSI) method. Magic‐angle sample spinning has been observed to give only a very minor line‐narrowing effect. Threefold‐coordinated zeolite A Tl+ sites adjacent to six‐membered rings [Tl(1) and Tl(3)] are both assigned to a resonance which has large CSA (δiso 1183, δδ—1842), while the remaining zeolite A site [Tl(2)] gives an isotropic line shape (δiso 300). Dehydrated zeolite X gives an isotropic peak (δiso 854) which is assigned to the double six‐ring Tl(I), and a broader anisotropic resonance which is tentatively attributed to supercage Tl(III). Data for dehydrated zeolite Y indicate that this has a disordered cation distribution.

Kövér, Katalin E.; Jiao, Ding; Fang, Sunan; Hruby, Victor J.

doi: 10.1002/mrc.1260311208pmid: N/A

The rotamer populations around the C‐α—C‐β bond of amino acid residues with one β‐proton can be calculated from homonuclear and heteronuclear vicinal coupling constants. The measurement of long‐range heteronuclear coupling constants, however, suffers from the inherent low sensitivity of the heteronuclear multiple bond experiments. In this paper it is demonstrated that z‐filtered homonuclear and heteronuclear relay spectroscopy provides a sensitive and accurate means for the evaluation of conformationally important vicinal coupling constants. Side‐chain conformations of the four stereoisomers of β‐methylphenylalanine residues in synthetic octapeptide analogues of CCK‐8 were derived from the measured coupling constants. This information is not easily available from conformational analysis based on simple steric considerations.

Moloney, Gerard P.; Craik, David J.; Iskander, Magdy N.

doi: 10.1002/mrc.1260311209pmid: N/A

1H NMR spectroscopy was used to determine the conformations of the aminoglycoside antibiotic N‐demethylclindamycin and two of its cyclic derivatives. The conformational features of these systems were determined by consideration of vicinal coupling constants and, in some cases, nuclear Overhauser enhancement (NOE) effects. 1H and 13C chemical shifts are reported and compared with previous results for the related antibiotic lincomycin. The stability of the two cyclized derivatives in aqueous solution was examined. Both cyclizations involved formation of a 4‐imidazolidinone ring. It was found that the ring system based on cyclization with formaldehyde was stable in aqueous solution, whereas that based on benzaldehyde was not.

Harris, R. K.; Jones, M. S.; Kenwright, A. M.

doi: 10.1002/mrc.1260311210pmid: N/A

The results of the direct analysis of the ABX spin system present in two symmetrical diphosphonates containing a single vinylic 13C nucleus are presented. This allows a comparison to be made with the two‐dimensional methods used by Beckmann et al. Significant differences are found in the results obtained: the need to recognize the existence of isotopic chemical shifts is emphasized.