Magnetic Resonance in Chemistry

Boyle, Grant A.; Govender, Thavendran; Kruger, Hendrik G.; Maguire, Glenn E. M.; Naicker, Tricia

doi: 10.1002/mrc.2279pmid: 18924121

The NMR elucidation of a novel ligand (S)‐pentacyclo‐undecane bis‐(4‐phenyloxazoline) and related pentacyclo‐undecane (PCU) derivatives is reported. Two‐dimensional NMR proved to be a powerful technique in overcoming the difficulties associated with the elucidation of these compounds when only one‐dimensional NMR data is utilized. A chiral substituent was introduced to both ‘arms’ of the PCU skeleton to produce derivatives 1–3. These derivatives display C1 symmetry with all thecage atoms being nonequivalent. Owing to overlapping of peaks in the 1H spectra, identification of these diastereomeric protons was very difficult. The 13C spectra gave rise to clear splitting of the nonequivalent carbons. This is unusual compared to similar PCU derivatives with chiral substituents as splitting of all the diastereomeric cage carbons has not yet been reported. Nuclear Overhauser enhancement spectroscopy (NOESY) correlations of derivatives 1–3 confirm the different conformations of the molecule in which the side ‘arms’ occupy different orientations with respect to cage moiety. Copyright © 2008 John Wiley & Sons, Ltd.

Finiguerra, Michela G.; Prudêncio, Miguel; Ubbink, Marcellus; Huber, Martina

doi: 10.1002/mrc.2290pmid: 18932181

Distance determination in disordered systems by a four‐pulse double electron–electron resonance method (DEER or PELDOR) is becoming increasingly popular because long distances (several nanometers) and their distributions can be measured. From the distance distributions eventual heterogeneities and dynamics can be deduced. To make full use of the method, typical distance distributions for structurally well‐defined systems are needed. Here, the structurally well‐characterized protein azurin is investigated by attaching two (1‐oxyl‐2,2,5,5‐tetramethylpyrroline‐3‐methyl) methanethiosulfonate spin labels (MTSL) by site‐directed mutagenesis. Mutations at the surface sites of the protein Q12, K27, and N42 are combined in the double mutants Q12C/K27C and K27C/N42C. A distance of 4.3 nm is found for Q12C/K27C and 4.6 nm for K27C/N42C. For Q12C/K27C the width of the distribution (0.24 nm) is smaller than for the K27C/N42C mutant (0.36 nm). The shapes of the distributions are close to Gaussian. These distance distributions agree well with those derived from a model to determine the maximally accessible conformational space of the spin‐label linker. Additionally, the expected distribution for the shorter distance variant Q12C/N42C was modeled. The width is larger than the calculated one for Q12C/K27C by 21%, revealing the effect of the different orientation and shorter distance. The widths and the shapes of the distributions are suited as a reference for two unperturbed MTSL labels at structurally well‐defined sites. Copyright © 2008 John Wiley & Sons, Ltd.

Fragaki, Georgia; Stefanaki, Irine; Dais, Photis; Mikros, Emmanuel

doi: 10.1002/mrc.2298pmid: 18924119

Phase‐sensitive nuclear Overhauser enhancement spectroscopy (NOESY) experiments, 3J couplings and computational molecular modeling (MM2* and MMFF force fields) were employed to examine the conformational properties of verrucarin A in chloroform solutions. The MMFF force field calculations resulted in a family of 12 low‐energy structures along with their populations, the latter being determined by the NMR analysis of molecular flexibility in solution(NAMFIS) deconvolution analysis. The concluded model was capable of reproducing successfully the experimental NOESY cross‐peak volumes and the proton‐coupling constants. Among the 12 conformers, the one which was similar to the structure of verrucarin A in the solid state was the predominant accounting for 75% of the total relative population, although other low‐energy conformations contributed to a lesser degree in order to explain the experimental data. Copyright © 2008 John Wiley & Sons, Ltd.

Sýkora, Jan; Blechta, Vratislav; Soukupová, Ludmila; Schraml, Jan

doi: 10.1002/mrc.2302pmid: 18781702

29Si13C spin–spin couplings over one, two, and three bonds as well as other NMR parameters (δ(29Si), δ(13C), δ(1H), 1J(13C1H), and 2J(29SiC1H)) were calculated and measured for a series of trimethylsilylated alcohols of the types Me3SiO(CH2)nCH3 and Me3SiOCH3−nRn(n = 03; R = Me, Ph, or Vi). The signs of the coupling constants determined for selected compounds can likely be extended to all such compounds, as supported by theoretical calculations. Similar to couplings between other pairs of nuclei, the 2‐bond and 3‐bond 29SiO13C couplings are of opposite signs (2J > 0 and 3J < 0), and their relative magnitudes depend on the extent of branching at the α‐carbon. Copyright © 2008 John Wiley & Sons, Ltd.

Roy, Abhijeet Deb; Jayalakshmi, K.; Dasgupta, Somnath; Roy, Raja; Mukhopadhyay, Balaram

doi: 10.1002/mrc.2321pmid: 18853391

A novel application of in situ 1H high‐resolution magic angle spinning (HR–MAS) NMR technique for real‐time monitoring of H2SO4‐silica promoted formation of 2, 2‐disubstituted quinozolin‐4(3H)‐ones is reported. The detailed NMR spectroscopic data led to elucidation of the mechanism, reaction optimization, kinetics and quantitative analysis of the product accurately and efficiently. The translation of the optimized parameters obtained by 1H HR–MAS NMR in the wet laboratory provided similar results. It is proposed that 1H HR‐MAS has a potential utility for optimization of various organic transformations in solid supported catalyzed reactions. Copyright © 2008 John Wiley & Sons, Ltd.

Grycová, Lenka; Hulová, Dagmar; Maier, Lukáš; Standara, Stanislav; Nečas, Marek; Lemière, Filip; Kareš, Radovan; Dostál, Jiří; Marek, Radek

doi: 10.1002/mrc.2325pmid: 18781707

Adducts of the quaternary protoberberine alkaloids (QPA) berberine, palmatine, and coptisine were prepared with nucleophiles derived from pyrrole, pyrazole, imidazole, and 1,2,4‐triazole. The products, 8‐substituted 7,8‐dihydroprotoberberines, were identified by mass spectrometry and 1D and 2D NMR spectroscopy, including 1H15N shift correlations at natural abundance. In addition, two adducts of QPA with chloroform and methanethiolate were characterized by using NMR data. Single‐crystal X‐ray structures of 8‐pyrrolyl‐7,8‐dihydroberberine, 8‐pyrazolyl‐7,8‐dihydroberberine, and 8‐imidazolyl‐7,8‐dihydroberberine are also presented. Copyright © 2008 John Wiley & Sons, Ltd.

Aski, Sahar Nikkhou; Takacs, Zoltan; Kowalewski, Jozef

doi: 10.1002/mrc.2327pmid: 18800339

Complexation equilibria and kinetics of exchange of chloroform and dichloromethane molecules between the cavity of cryptophane‐E and bulk solution were investigated using NMR methods. Using one‐dimensional magnetization transfer (1D‐EXSY type sequence), chemical exchange rates were measured in different temperature ranges, limited by the equilibrium constant values of the complexes and the boiling points of the guest substances. From the kinetic data, activation energies were calculated using the Arrhenius equation. From the temperature‐dependence of the association constant data, the enthalpy and entropy of complexation were estimated and compared with values for similar complexes of other cryptophanes. Copyright © 2008 John Wiley & Sons, Ltd.

Potmischil, Francisc; Marinescu, Maria; Nicolescu, Alina; Deleanu, Călin; Hillebrand, Mihaela

doi: 10.1002/mrc.2335pmid: 18844244

The 15N NMR chemical shifts of 1,2,3,4,5,6,7,8‐octahydroacridine, 12 of its 9‐substituted derivatives, and of the corresponding N‐oxides were measured and examined in terms of the 9‐substituent effects and the effects of N‐oxidation. For the 9‐substituent effects, good linear correlations were found with the Taft and Swain‐Lupton substituent constants, for both octahydroacridines and their N‐oxides. The 15N chemical shifts of both octahydroacridines and their N‐oxides also correlate well, linearly with the 13C chemical shifts of the para‐carbons in analogously substituted benzene derivatives. Within the studied compounds, the magnitudes of the N‐oxidation effects range from − 16.4 to − 27.4 ppm (shielding), and also correlate linearly with the Taft and Swain–Lupton substituent constants, as well as with the bond orders of the N+O− bonds in the corresponding N‐oxides. Furthermore, a very good linear correlation is found between the 15N chemical shifts of octahydroacridines and those of the corresponding N‐oxides. From the 15N chemical shifts data, the Taft and Swain‐Lupton substituent constants for the diacetylamino group (NAc2) were evaluated in the present paper, as follows: σR = 0.07 and σI = 0.15; ${{\cal R}} = 0.08$ and ${{\cal F}} = 0.20$. Copyright © 2008 John Wiley & Sons, Ltd.

Hao, Gao; Qing‐Hua, Zhang; Miao‐Miao, Jiang; Jin‐Shan, Tang; Cheng‐Du, Miao; Kui, Hong; Michio, Namikoshi; Nai‐Li, Wang; Xin‐Sheng, Yao

doi: 10.1002/mrc.2338pmid: 18846583

A series of polyketide‐originated metabolites (1–5) were isolated from a marine sponge‐derived fungus Mycelia sterilia. Of these, 1–3 were new compounds. Their structures were elucidated by spectroscopic methods as (4R*, 5S*, 6S*, 8S*, 13R*)‐1‐(2,8‐dihydroxy‐1,2,6‐trimethyl‐1,2,6,7,8,8a‐hexahydro‐naphthalen‐1‐yl)‐3‐methoxy‐propan‐1‐one (1), 4,8‐dihydroxy‐7‐(2‐hydroxy‐ethyl)‐6‐methoxy‐3,4‐dihydro‐2H‐naphthalen‐1‐one (2) and 1‐methyl‐naphthalene‐2,6‐dicarboxylic acid (3). In 1, the proton–proton long‐range coupling phenomenon claimed attention and was discussed. Copyright © 2008 John Wiley & Sons, Ltd.

Mikhova, B.; Stamboliyska, B.; Koch, A.; Duddeck, H.; Kleinpeter, E.

doi: 10.1002/mrc.2333pmid: 18828152

Six groups of diastereomeric 2,4‐disubstituted adamantanes were studied with DFT–GIAO–NBO (natural orbital analysis) methods. The calculated 13C chemical shifts reproduce well the experimental data. It was found that among all diastereomers, those bearing substituents in δ‐syn‐axial positions showed the largest overall deshielding, i.e. the sum of all 13C chemical shifts (Σδ(13C)) was the greatest and also had the highest delocalization contribution to the molecular energy evaluated with NBO. The higher delocalization energy is proposed to be the origin of the deshielding δ‐syn‐axial effect. Copyright © 2008 John Wiley & Sons, Ltd.