Magnetic Resonance in Chemistry

Agrawal, Pawan K.; Bush, C. Allen; Qureshi, Nilofer; Takayama, Kuni

doi: 10.1002/(SICI)1097-458X(199801)36:1<1::AID-OMR164>3.0.CO;2-Jpmid: N/A

The hexamethyl derivative of lipopolysaccharide (LPS), obtained by diazomethane methylation of LPS extracted from a deep mutant of Escherichia coli D31m4, followed by high‐performance liquid chromatographic purification, was subjected to homo‐ and heteronuclear two‐dimensional NMR spectroscopy (DQF‐COSY, HOHAHA, NOESY, HMQC and HMBC) to achieve 1H and 13C NMR assignments, particularly of the sugar backbone. These studies confirmed two each of glucosamine (GlcN), 3‐deoxy‐D‐manno‐octulosonic acid (Kdo), phosphate, β‐hydroxymyristate and β‐acyloxymyristate and the structure of the tetrasaccharide as α‐KdoP‐(2→4)‐α‐KdoP‐(2→6)‐β‐D‐GlcPN‐(1→6)α‐D‐GlcPN, bisphosphorylated at the 1 and 4′ positions. © 1998 John Wiley & Sons Ltd.

Stuhlmann, F.; Domschke, G.; Bartl, A.; Omelka, L.

doi: 10.1002/(SICI)1097-458X(199801)36:1<8::AID-OMR190>3.0.CO;2-7pmid: N/A

Voltammetric studies on mesoionic 1,2,3‐oxa‐ and ‐thiadiazoles confirmed that the former may be reduced irreversibly at potentials between ‐1.48 and ‐2.12 V vs. Fc/Fc+ in acetonitrile at a scan rate of 200 mV s‐1, whereas the latter are reduced reversibly under the same conditions. The corresponding anion radicals were examined by ESR spectroscopy. Cationic alkylation products showed different electrochemical behaviour depending on the character of the heterocyclic ring. 1,2,3‐Thiadiazolium ions were reduced reversibly. All coupling constants of the resulting neutral radicals were fully assigned. 1,2,3‐Oxadiazolium ions were all reduced irreversibly. © 1997 John Wiley & Sons, Ltd.

Staško, A.; Erentová, K.; Rapta, P.; Nuyken, O.; Voit, B.

doi: 10.1002/(SICI)1097-458X(199801)36:1<13::AID-OMR200>3.0.CO;2-5pmid: N/A

In photochemically, electrochemically and thermally initiated decomposition, compounds containing the azo group (1RN=N2R) usually form reactive, short‐lived radicals 1R• and 2R•, which corresponds to synchronous splitting. As examples, aliphatic azo compounds, 1‐aryl‐2‐alkylazo compounds, azophosphonates, azosulfonates, azosulfones, azosulfides, triazenes, pentazadienes, hexazadienes, acyl‐ and diacylazo compounds, diazonium salts and cyclic azo compounds have been studied. 1R• and 2R• were characterized by means of the EPR spin trap technique. These reactive radicals may abstract hydrogen from the solvent or any neighborhood functions, or dimerize (1R—2R). Some azo compounds trap 1R• and 2R• and thus form the corresponding hydrazyl radicals. In cathodic reduction, prior to the initiation of decomposition, corresponding anion radicals centered on the azo group were observed which have considerable stability if 1R and 2R are conjugated structures. © 1998 John Wiley & Sons, Ltd.

McDonnell, Patricia A.; Gauthier, A. Diane; Ferro, Michael P.

doi: 10.1002/(SICI)1097-458X(199801)36:1<35::AID-OMR208>3.0.CO;2-Apmid: N/A

The 1H/15N HMBC experiment at 15N natural abundance using pulsed field gradients is a useful tool for chemical shift assignment and structure elucidation. This experiment assisted in the identification of regioisomers that could not be distinguished with conventional 1H/1H nuclear Overhauser or 1H/13C correlation NMR experiments. The natural abundance 15N gradient‐enhanced HMBC experiment was easily implemented, provided high‐quality spectra and readily distinguished nitrogen‐containing regioisomers. This experiment can aid in structure elucidation when traditional 1H and 13C experiments fail to provide unique structural assignment. © 1998 John Wiley & Sons, Ltd.

Wrackmeyer, Bernd; Kehr, Gerald; Maisel, Heidi E.; Zhou, Hong

doi: 10.1002/(SICI)1097-458X(199801)36:1<39::AID-OMR209>3.0.CO;2-Wpmid: N/A

N‐Triorganostannyl (R3Sn)‐substituted pyrroles (1) and indoles (2) (R=Me (a), Et (b), tBu (c)), N‐trimethylstannyl‐carbazole (3), N‐trimethylstannyl‐2,5‐dimethylpyrrole (1d), the corresponding silicon and lead derivatives (1d(Si) and 1d(Pb)) and N‐trimethylstannyl‐2‐methylindole (2d) were prepared and studied by multinuclear magnetic resonance (1H, 13C, 15N, 29Si, 119Sn and 207Pb NMR). The absolute signs of numerous coupling constants nJ(119Sn,13C) and 1J(119Sn,15N) (<0) were determined by appropriate 2D heteronuclear shift correlations of the type 13C/1H and 15N/1H. Precise measurement of the coupling constants 1J(119Sn,15N) from 119Sn NMR spectra by using Hahn‐echo extended (HEED) polarization transfer pulse sequences enabled one to obtain isotope‐induced chemical shifts 1Δ14/15N(119Sn) at natural abundance of 15N. The 1Δ14/15N(119Sn) data become more negative with increased branching of the substituent R at the tin atom and with stronger steric interactions. The latter effect is also evident from δ119Sn values and changes in the magnitude of the coupling constants 1J(119Sn,13C) or 1J(119Sn,15N). © 1998 John Wiley & Sons, Ltd.

Chachaty, Claude; Mathieu, Corinne; Mercier, Anne; Tordo, Paul

doi: 10.1002/(SICI)1097-458X(199801)36:1<46::AID-OMR210>3.0.CO;2-Tpmid: N/A

The magnetic and dynamic properties of several β‐phosphorylated nitroxide radicals in glycerol solution were studied by X‐band ESR spectroscopy in the 180–380 K range. The parameters of interest were obtained from the least‐squares fit of experimental spectra. These radicals display a large, weakly anisotropic 31P hyperfine coupling varying from 3 to 6 mT according to the orientation of the C(2)—P bond in the molecular frame and nearly independent of the temperature. Some of these radicals, with a five‐membered cyclic structure, undergo an exchange between two conformers due to ring puckering, whose kinetics have been determined from the evolution of spectral shapes between 230 and 280 K. This motion, which modulates the 31P hyperfine coupling and leaves that of nitrogen unchanged, is hindered by substitution in position 4 of the ring of a proton by a phenyl group. All these radicals are found to undergo an anisotropic tumbling motion with correlation times decreasing from 100 to 0.1 ns between 280 and 380 K. © 1998 John Wiley & Sons, Ltd.

Schraml, Jan; Kvíčalová, Magdalena; Blechta, Vratislav; Řeřicha, Roman; Rozenski, Jef; Herdewijn, Piet

doi: 10.1002/(SICI)1097-458X(199801)36:1<55::AID-OMR211>3.0.CO;2-Spmid: N/A

Trimethylsilyl and tert‐butyldimethylsilyl derivatives of naturally occurring purines and pyrimidines and also other closely related model compounds were prepared and their 29Si NMR spectra measured. Only the chemical shifts of the Si—NH— moiety could be assigned experimentally (i.e. exactly); the chemical shifts of Si—O— and Si—N— fragments could be assigned only on the basis of chemical shift correlations. The Si—N— lines are surprisingly narrow in all derivatives of nucleic bases studied. The lines are narrow because of the ‘self decoupling’ of fast relaxing 14N nuclei. The values of 29Si, 15N coupling constants could not be reliably determined (by the HEED–INEPT method), nor could the Si—N lines be completely assigned in these compounds. Measurement of 29Si NMR spectra is shown to be a sensitive and fast method for checking completeness of silylation of nucleobases or their analogues. Assignment difficulties limit other applications. © 1998 John Wiley & Sons, Ltd.

Etzel, W. A.; Gau, W.; Krämer, W.; Stelzer, U.; Weissmüller, J.

doi: 10.1002/(SICI)1097-458X(199801)36:1<64::AID-OMR217>3.0.CO;2-Cpmid: N/A

Spiroxamine is a new powdery mildew fungicide in cereals consisting of four biologically active isomers (two diastereomers, four enantiomers). The four isomers were separated by preparative high‐performance liquid chromatography (HPLC) on a chiral stationary phase. At this stage it was not possible to assign their stereochemistry. Using stereoselective synthesis starting with the corresponding chirally pure glycerol derivates, the configuration at the asymmetric center, could be fixed. The resulting diastereomers were separated by preparative HPLC. Using COSY, HSQC and NOESY NMR spectroscopy it was possible to assign the configuration of the amino residue relative to the cyclohexyl ring. The 600 MHz 1H NMR spectra permitted a complete assignment of all proton signals. The stereochemical assignment is based on NOEs observed in the NOESY spectrum. © 1998 John Wiley & Sons, Ltd.

Mphahlele, Malose J.; Kaye, Perry T.

doi: 10.1002/(SICI)1097-458X(199801)36:1<69::AID-OMR219>3.0.CO;2-Spmid: N/A

13C NMR spectra of 1,4‐ and 1,5‐benzoheterazepinones and their tetrazolo derivatives, prepared by the Schmidt rearrangement of flavanone analogues, are reported. The effects of the C‐ring heteroatoms on the 13C NMR chemical shifts of the ring‐expanded products were found to be of diagnostic value in distinguishing the isomeric products from one another and from the corresponding precursors. © 1998 John Wiley & Sons, Ltd.

Maślankiewicz, Andrzej; Pluta, Krystian; Wyszomirski, Mirosław; Gogoll, Adolf; Maślankiewicz, Maria J.

doi: 10.1002/(SICI)1097-458X(199801)36:1<73::AID-OMR220>3.0.CO;2-Ypmid: N/A

The proton and carbon chemical shifts and the coupling constants nJ(H,H) and nJ(C,H) of thioquinanthrene and isothioquinanthrene were completely assigned from COSY, HETCOR and INEPT studies. © 1998 John Wiley & Sons, Ltd.