19 F chemical shifts, coupling constants and conformational preferences in monosubstituted perfluoroparacyclophanesGhiviriga, Ion; Zhang, Lianhao; Martinez, Henry; Contreras, Rubén H.; Tormena, Cláudio F.; Nodin, Laura; Dolbier, William R.
doi: 10.1002/mrc.2713pmid: 21246624
In the process of studying the chemistry of perfluoro(2.2)paracyclophanes (PFPCs), a novel class of compounds, it became necessary to identify some disubstituted products. To achieve this goal, we characterize in this work some monosubstituted PFPCs, identifying their 19F19F coupling patterns, and establishing a methodology for the assignment of their 19F chemical shifts. The pattern of coupling constants indicates a skewed geometry in which the upper deck moves towards or away from the substituent, depending on the substituent electron‐donor character and size. Quantum chemical calculations, performed at the HF/6‐311 + G(d,p)//B3LYP/EPR‐III level of theory, confirmed the conformations inferred from coupling constants and reproduced well the values of the couplings. Transmission mechanisms for the FC term of four‐ and five‐bond 19F19F couplings are discussed in detail. Understanding the conformational preferences of PFPCs and how they are reflected by the coupling constants facilitates the assignment of 19F chemical shifts in monosubstituted PFPCs and the identification of the disubstituted products. Copyright © 2011 John Wiley & Sons, Ltd.
Low‐power composite CPMG HSQMBC experiment for accurate measurement of long‐range heteronuclear coupling constantsBoros, Sándor; Kövér, Katalin E.
doi: 10.1002/mrc.2717pmid: 21246626
A modified version of CPMG‐HSQMBC pulse scheme is presented for the measurement of long‐range heteronuclear coupling constants. The method implements adiabatic inversion and refocusing pulses on the heteronucleus. Low‐power composite 180° XY‐16 CPMG pulse train is applied on both proton and X nuclei during the evolution of long‐range couplings to eliminate phase distortions due to co‐evolution of homonuclear proton–proton couplings. The pulse sequence yields pure absorption antiphase multiplets allowing precise and direct measurement of the nJXH coupling constants regardless from the size of the proton‐proton couplings. The applicability of the method is demonstrated using strychnine as a model compound. The selective 1D version of the method is also presented. Copyright © 2011 John Wiley & Sons, Ltd.
2D correlation spectra edited by the sign of relative coupling constantBlechta, Vratislav; Schraml, Jan
doi: 10.1002/mrc.2720pmid: 21274902
Modifications (CSEc and CSEh) of recently published SQSQc and SQSQh pulse sequences are proposed and tested on detection of small (∼2 Hz) signed silicon–carbon coupling constants. The new sequences increase signal intensity by simplifying the spectra. The signals are about four times stronger than in SQSQc or SQSQh spectra, achieving the sensitivity of E.COSY‐type experiment. The information about sign and magnitude of the coupling is preserved. CSEc and CSEh spectra for two silicon compounds are presented and compared. The two new sequences allow editing of heteronuclear correlation spectra according to the sign of the selected heteronuclear coupling constants. Copyright © 2011 John Wiley & Sons, Ltd.
NMR analysis of conformationally dependent n J C, H and n J C, C in the trisaccharide α‐L‐Rha p ‐(1 → 2)(α‐L‐Rha p ‐(1 → 3))‐α‐L‐Rha p ‐OMe and a site‐specifically labeled isotopologue thereofJonsson, K. Hanna M.; Pendrill, Robert; Widmalm, Göran
doi: 10.1002/mrc.2723pmid: 21274903
An array of NMR spectroscopy experiments have been carried out to obtain conformationally dependent 1H,13C‐ and 13C,13C‐spin–spin coupling constants in the trisaccharide α‐L‐Rhap‐(1 → 2)(α‐L‐Rhap‐(1 → 3))‐α‐L‐Rhap‐OMe. The trisaccharide was synthesized with 13C site‐specific labeling at C2′ and C2″, i.e. in the rhamnosyl groups in order to alleviate 1H spectral overlap. This facilitated the measurement of a key trans‐glycosidic proton–proton cross‐relaxation rate using 1D 1H,1H‐T‐ROESY experiments as well as a 3JC, H coupling employing 1D 1H,13C‐long‐range experiments, devoid of potential interference from additional J coupling. By means of both the natural abundance compound and the 13C‐labeled sample 2D 1H,13C‐J‐HMBC and 1H,13C‐HSQC‐HECADE NMR experiments, total line‐shape analysis of 1H NMR spectra and 1D 13C NMR experiments were employed to extract 3JC, H, 2JC, H, 3JC, C, and 1JC, C coupling constants. The 13C site‐specific labeling facilitates straightforward determination of nJC, C as the splitting of the 13C natural abundance resonances. This study resulted in eight conformationally dependent coupling constants for the trisaccharide and illustrates the use of 13C site‐specific labeling as a valuable approach that extends the 1D and 2D NMR methods in current use to attain both hetero‐ and homonuclear spin–spin coupling constants that subsequently can be utilized for conformational analysis. Copyright © 2011 John Wiley & Sons, Ltd.
Structure elucidation and complete NMR spectral assignment of an unusual aromatic monacolin analog from Monascus purpureus ‐fermented riceLiu, Ming‐Tao; Li, Jin‐Jie; Shang, Xiao‐Ya; Li, Shuai; Li, Ling‐Ling; Luan, Na; Jin, Zong‐Lian
doi: 10.1002/mrc.2714pmid: 21322007
One unusual aromatic monacolin analog, monacophenyl, was isolated from the ethanolic extract of Monascus purpureus‐fermented rice. Its structure was completely and unambiguously assigned by one‐ and two‐dimensional NMR techniques (1H NMR, 13C NMR, HSQC, HMBC and NOESY) and high‐resolution ESI‐MS spectrometry. Copyright © 2011 John Wiley & Sons, Ltd.
Conformation of the tridimensional structure of 1,2,3,4,6‐pentagalloyl‐β‐D‐glucopyranose (PGG) by 1 H NMR, NOESY and theoretical study and membrane interaction in a simulated phospholipid bilayer: a first insightBeretta, Giangiacomo; Artali, Roberto; Caneva, Enrico; Maffei Facino, Roberto
doi: 10.1002/mrc.2718pmid: 21322008
1,2,3,4,6‐Penta‐O‐galloyl‐β‐D‐glucopyranose (PGG) is a polyphenolic compound found in substantial amounts in a number of medicinal herbs. We report (i) its conformational analysis by solution NMR and molecular dynamics calculation and (ii) theoretical study of its interaction with a model membrane bilayer. The galloyl groups B and E appear to play important roles in the interaction with the phospholipid bilayer. Copyright © 2011 John Wiley & Sons, Ltd.
NMR property calculations and experimental study of the 1,6‐epoxycarvone and α‐epoxypinene: a comparison of modelsQueiroz, Luiz H. K.; Lacerda, Valdemar; dos Santos, Reginaldo B.; Greco, Sandro J.; Cunha Neto, Álvaro; de Castro, Eustaquio V. R.
doi: 10.1002/mrc.2722pmid: 21322010
This work aims at using theoretical calculations of shielding tensors (σ) through different methods (gauge‐independent atomic orbital (GIAO), continuous set of gauge transformations (CSGT) and individual gauges for atoms in molecules (IGAIM)) and spin‐spin coupling constants J using GIAO method to compare these methods and to corroborate the data obtained with the assignment of all of 1H and 13C NMR signals and the relative stereochemistry of the 1,6‐epoxycarvone and the α‐epoxypinene. All the 1H and 13C NMR signals were assigned unequivocally. The stereochemistry for the epoxides is trans and the B3LYP theory level with CSGT and IGAIM methods is the best choice to evaluate theoretical chemical shifts for compounds studied. Copyright © 2011 John Wiley & Sons, Ltd.
Determination of substitution sites in monosubstituted five‐membered aromatic heterocyclesSchraml, Jan; Kubec, Roman; Kučerová, Petra
doi: 10.1002/mrc.2725pmid: 21322011
Similar magnitudes of proton–proton couplings across three, four, and five bonds and proton–carbon couplings across two and three bonds combined with difficult to predict substituent effects make the results of an indiscriminate use of routine (COSY, HSQC, HMBC, etc.) techniques for substitution site determination in C‐monosubstituted five‐membered heteroaromatics suspect. As demonstrated on two examples of natural products, the use of 1,1‐ADEQUATE leads to unambiguous substitution site determination lending thus further support to suggested inclusion of 1,1‐ADEQUATE data into computer‐assisted structure elucidation (CASE) protocols. Copyright © 2011 John Wiley & Sons, Ltd.