A reassignment of the 13C-NMR spectrum of the alkaloid ajmaline through the use of two-dimensional nmr techniques

The reassignment of the ¹³C chemical nmr spectrum of the alkaloid, ajmaline, is reported. An earlier assignment based on analogy to other alkaloids and deductions from molecular orbital theory was found to be partially in error. The present study uses broadband decoupled (both dimensions) proton-carbon shift correlation and long-range proton-carbon chemical shift correlation techniques. These experiments, in conjunction with earlier work by others on the assignment of the proton spectra, suffice to now unequivocably assign the ¹³C nmr spectrum of the title compound.     

Two-dimensional relayed coherence transfer experiments as a means of subgrouping protonated carbon resonances of complex polynuclear heteroaromatics on the basis of vicinal proton-proton connectivities

Application of autocorrelated homonuclear (COSY) and proton-carbon chemical shift correlation two-dimensional nmr experiments to the problem of assigning the ¹H- and ¹³C-nmr spectra of phenanthro[4,3-a]dibenzothiophene fails despite their considerable utility in the assignment of the spectra of the smaller phenanthro[b]thiophenes. The failure of these techniques in the case of phenanthro[4,3-a]dibenzothiophene is predominantly due to the inability of the COSY spectrum to provide the means of subgrouping the proton resonances into their component spin subsystems. Two-dimensional relayed coherence transfer nmr experiments which first established coherence between vicinally coupled protons which is then transferred to the carbon spins which are ultimately observed circumvent these problems. The application of two-dimensional relayed coherence transfer to phenanthro[4,3-a]dibenzothiophene is described, the technique leading to the sub-grouping of all five of the proton spin subsystems and thus providing a means of beginning the total assignment.     

Assignment of the 1H- and 13C-NMR spectra of phenanthro[1,2-b]thiophene through the concerted application of two-dimensional NMR spectroscopic techniques

Two-dimensional nmr spectroscopy, by virtue of its second frequency domain which permits the segregation of spectral information along two frequency axes, considerably simplifies many assignment problems and facilitates others which may be impossible using conventional nmr methodology. A compound which falls into the latter category of assignment problem is phenanthro[1,2-b]thiophene. The assignment of the ¹H- and ¹³C-nmr spectra of phenanthro[1,2-b]thiophene are, however, reported through the concerted application of two-dimensional nmr techniques. Experiments utilized in making the assignments included: auto-correlated homonuclear (COSY) two-dimensional spectroscopy; heteronuclear two-dimensional J-resolved spectroscopy; proton-carbon chemical shift correlation two-dimensional spectroscopy; and two-dimensional ¹³C-¹³C double quantum coherence spectroscopy.     

Two-dimensional NMR studies of marine natural products. IV. Isolation of the cembranoid diterpene jeunicin from the mollusc planaxis sulcatus: Assignment of the proton and carbon NMR spectra by two-dimensional techniques

Numerous cembranoic diterpenes have been isolated from marine gorgonians and terrestrial plants. There have, however, been only a few reports of the isolation and identification of these interesting compounds from higher animal sources. The isolation of the oxo-bridged cembranoid diterpene jeunicin from the mollusc Planaxis sulcatus is reported. A comparative discussion of the utility of COSY, homonuclear RELAY (relayed COSY) and proton double quantum coherence techniques is presented, the latter successfully establishing the identities of all of the proton resonances. Acquisition of a two-dimensional proton-carbon chemical shift correlation spectrum provided the means of unequivocally establishing ¹³C resonance assignments.     

The study of longifolene by two-dimensional NMR spectroscopy

The complete assignment of the ¹³C NMR spectrum of longifolene was achieved from double quantum coherence measurements, while combined evaluation of a ¹H? ¹³C heteronuclear chemical shift correlation diagram and a homonuclear ¹H J-resolved diagram provided all proton chemical shifts. Conformational information on the seven-membered ring of the tricyclic sesquiterpene was obtained from proton chemical shift considerations.     

Total assignment of the 13C-NMR spectra of 5H-indolo[1,7-ab][1]benzazepine and 6,7-dihydro-5H-indolo[1,7-ab][1]benzazepine using two-dimensional proton-carbon chemical shift correlation experiments

¹³C-nmr chemical shift assignments for 5H-Indolo[1,7-ab][1]benzazepine and 6,7-Dihydro-5H-indolo-[1,7-ab][1]benzazepine, made on the basis of two-dimensional ¹H-¹³C chemical shift correlation experiments are reported.     

Multinuclear two-dimensional NMR: Assignments of natural abundance polypeptide 13C, 1H and 15N chemical shifts and demonstration of isomer interconversion

The polypeptide carbobenzoxy-glycyl-L -prolyl-L -leucyl-L -alanyl-L -proline (0.2 M in DMSO-d₆) was investigated using ¹³C, ¹H and ¹⁵N NMR in natural abundance at 4.7 tesla. The existence of cis–trans-Gly-Pro and -Ala-Pro bonds permits up to four isomers, and all four were observed (in a 60:30:7:3 ratio). ¹³C shifts of the proline β-CH₂ resonances are consistent only with the 60% form being trans–trans. The 30% form is either trans–cis or cis–trans (order as above) and was tentatively assigned as cis-trans on the basis of relaxation behavior. Refocused INEPT studies aided the ¹³C assignments. The ¹⁵N data were obtained using both NOE and INEPT excitation, with signals evident for the three major isomers. The spectra were analysed by starting from the ¹³C data, which were assigned based on known regularities in peptide spectra. A ¹³C? ¹H heteronuclear two-dimensional chemical shift correlation experiment allowed direct assignment of proton shifts for major and minor isomers. The NH proton shifts were assigned by running a homonuclear two-dimensional chemical shift correlation experiment and noting the correlation with the previously assigned α-CH protons. The ¹⁵N resonances were then assigned from a ¹⁵N? ¹H heteronuclear two-dimensional chemical shift correlation experiment, relating the ¹⁵N signals directly to the NH proton resonances. Isomer interconversion between the two major isomers was demonstrated by performing a magnetization transfer homonuclear 2D experiment. Off-diagonal intensity was noted relating the major and minor isomer alanine NH proton, as well as for the major and minor isomer leucine NH protons.     

13C NMR investigation of tautomeric and acid-base equilibria of pyrazolone T and three analogs

Pyrazolone T and three derivatives have been characterized by ¹³C and, in part, ¹⁵N nmr at several pH values. The ¹³C chemical shifts have been assigned at, or near, the equivalence points and pK_a values of these four compounds. Closely situatued quaternary carbon signals were assigned by means of a heteronuclear chemical shift correlation (FLOCK) experiment which is sensitive to, and was optimized for, 3-bond C-H couplings. The ¹³C chemical shift data indicate the existence of both tautomeric and acid-base equilibria and demonstrate that the four congeners exist in surprisingly different forms at certain common pH values.     

1-Methylphenanthro[3,4-b]thiophene: Determination of the tertiary structure in solution and in the crystalline state by NMR spectroscopy and X-ray diffraction

1-Methylphenanthro[3,4-b]thiophene was prepared by the photocyclization of 1-(4′-methyl-2′-thienyl)-2-(2″-naphthyl)ethene. The ¹H and ¹³C-nmr spectra were assigned using two-dimensional ¹H/¹³C heteronuclear chemical shift correlation and relayed coherence transfer (RELAY) experiments. From nuclear Overhauser difference spectra, the H11-C1 methyl-H intramolecular distance was determined to be 2.10 Å. The molecule crystallized from chloroform in the monoclinic system, space group P2₁/c. A total of 3536 unique reflections were measured and the structure was solved by direct methods and refined to a final R = 0.049. The molecule is helical with both chiral forms observed in the crystal. The H11-C1 methyl-H distance in the crystal was 2.12(3)Å in excellent agreement with the distance measured in solution by NOE techniques.     

Rapid and novel discrimination and quantification of oleanolic and ursolic acids in complex plant extracts using two-dimensional nuclear magnetic resonance spectroscopy—Comparison with HPLC methods

A novel strategy for NMR analysis of mixtures of oleanolic and ursolic acids that occur in natural products is described. These important phytochemicals have similar structure and their discrimination and quantification is rather difficult. We report herein the combined use of proton-carbon heteronuclear single-quantum coherence (¹H-¹³C HSQC) and proton-carbon heteronuclear multiple-bond correlation (¹H-¹³C HMBC) NMR spectroscopy, in the identification and quantitation of oleanolic acid (OA) and ursolic acid (UA)in plant extracts of the Lamiaceae and Oleaceae family. The combination of ¹H-¹³C HSQC and ¹H-¹³C HMBC techniques allows the connection of the proton and carbon-13 spins across the molecular backbone resulting in the identification and, thus, discrimination of oleanolic and ursolic acid without resorting to physicochemical separation of the components. The quantitative results provided by 2D ¹H-¹³C HSQC NMR data were obtained within a short period of time (∼14 min) and are in excellent agreement with those obtained by HPLC, which support the efficiency of the suggested methodology.     

Analysis of the Electronic Structure of the Special Pair of a Bacterial Photosynthetic Reaction Center by 13C Photochemically Induced Dynamic Nuclear Polarization Magic‐Angle Spinning NMR Using a Double‐Quantum Axis

The origin of the functional symmetry break in bacterial photosynthesis challenges since several decades. Although structurally very similar, the two branches of cofactors in the reaction center (RC ) protein complex act very differently. Upon photochemical excitation, an electron is transported along one branch, while the other remains inactive. Photochemically induced dynamic nuclear polarization (photo‐CIDNP ) magic‐angle spinning (MAS ) ¹³C NMR revealed that the two bacteriochlorophyll cofactors forming the “Special Pair” donor dimer are already well distinguished in the electronic ground state. These previous studies are relying solely on ¹³C‐¹³C correlation experiments as radio‐frequency‐driven recoupling (RFDR ) and dipolar‐assisted rotational resonance (DARR ). Obviously, the chemical‐shift assignment is difficult in a dimer of tetrapyrrole macrocycles, having eight pyrrole rings of similar chemical shifts. To overcome this problem, an INADEQUATE type of experiment using a POST C7 symmetry‐based approach is applied to selectively isotope‐labeled bacterial RC of Rhodobacter (R.) sphaeroides wild type (WT ). We, therefore, were able to distinguish unresolved sites of the macromolecular dimer. The obtained chemical‐shift pattern is in‐line with a concentric assembly of negative charge within the common center of the Special Pair supermolecule in the electronic ground state.     

13C‐15N Connectivity networks via unsymmetrical indirect covariance processing of 1H‐13C HSQC and 1H‐15N IMPEACH spectra

Long‐range ¹H‐¹⁵N heteronuclear shift correlation methods at natural abundance to facilitate the elucidation of small molecule structures have assumed a role of growing importance over the past decade. Recently, there has also been a high level of interest in the exploration of indirect covariance NMR methods. From two coherence transfer experiments, A→B and A→C, it is possible to indirectly determine B?C. We have shown that unsymmetrical indirect covariance methods can be employed to indirectly determine several types of hyphenated 2D NMR data from higher sensitivity experiments. Examples include the calculation of hyphenated 2D NMR spectra such as 2D GHSQC‐COSY and GHSQC‐NOESY from the discrete component 2D NMR experiments. We now wish to report the further extension of unsymmetrical indirect covariance NMR methods for the combination of ¹H‐¹³C GHSQC and ¹H‐¹⁵N longrange (GHMBC, IMPEACH‐MBC, CIGAR‐HMBC, etc.) heteronuclear chemical shift correlation spectra to establish ¹³C‐¹⁵N correlation pathways.     

Long range heteronuclear chemical shift correlation with one bond modulation decoupling: Phenanthro[1,2-b]thiophene

Long range heteronuclear chemical shift correlation has become an extremely useful technique for complex spectral assignment and structure elucidation. The sensitivity is higher than that afforded by ¹³C-¹³C double quantum INADEQUATE experiment. The long range heteronuclear correlation provides the means of establishing connectivities across heteroatoms. Early attempts to use long range heteronuclear chemical shift correlation by simply optimizing the delays for long range couplings met with varied success. Long range coupling responses to protonated carbon resonances can be modulated by the one bond coupling of the protonated carbon. These modulations can lead to precipitous valleys in the response curve and a corresponding loss of connectivity information. A newly reported pulse sequence employing a BIRD pulse midway through the Δ₂ refocusing delay “decouples” one bond modulation effects substantially increasing the likelihood of observing the desired connectivities. The application of this technique is reported for phenanthro[1,2-b]thiophene. Unequivocal assignment of the title compound is reported using heteronuclear chemical shift correlation and long range correlation with modulation decoupling. The assignment took a total of < 15 hours of spectrometer time.     

Autocorrelated 13c-13c double quantum coherence two-dimensional nmr spectroscopy: Utilization of a modified version of the technique as an adjunct in the total assignment of the 1h- and 13c-nmr spectra of the mutagen phenanthro[3,4-b]thiophene

Development of successively higher field nmr spectrometers has facilitated the study of increasingly more complex molecules, although smaller molecules such as phenanthro[3,4-b]thiophene still offer very substantial assignment problems because of the highly congested nature of their ¹H- and ¹³C-nmr spectra. Assignments of such spectra, if they are to be unequivocal, frequently require the utilization of two-dimensional nmr spectroscopic techniques. Total assignments of the ¹H- and ¹³C-nmr spectra of phenanthro[3,4-b]thiophene are reported. Assignments were based on a conventional high resolution 500 MHz ¹H-nmr spectrum, autocorrelated two-dimensional ¹H-nmr spectra (COSY), two-dimensional ¹H-¹³C chemical shift correlation spectra and a modified version of autocorrelated ¹³C-¹³C double quantum coherence two-dimensional nmr spectroscopy. From NOE measurements, a separation of 1.99 Å between H1 and H11 was computed, suggesting that phenanthro[3,4-b]thiophene has a pronounced helical conformation in solution.     

NMR studies on 1,2‐dihydro‐2‐(4‐aminophenyl)‐4‐[4‐(4‐aminophenoxy)‐4‐phenyl]‐(2H)phthalazin‐1‐one

An unsymmetrical heterocyclic diamine, 1,2‐dihydro‐2‐(4‐aminophenyl)‐4‐[4‐(4‐aminophenoxy)‐4‐phenyl]‐(2H)phthalazin‐1‐one, was synthesized. Its ¹H and ¹³C NMR spectra were completely assigned by utilizing the two‐dimensional heteronuclear ¹³C–¹H multiple‐bond coherence (HMBC) spectroscopy, and heteronuclear ¹³C–¹H one‐bond correlation spectroscopy, homonuclear shift correlation spectroscopy (H,H‐COSY) and rotating frame Overhauser enhancement spectroscopy (ROESY). The structure of the compound was shown to be the phthalazinone rather than the phthalazine ether from cross peaks and chemical shifts of the protons. Copyright © 2002 John Wiley & Sons, Ltd.     

Saturated amine oxides: part 10. hydroacridines: part 32. linear 17O/13C and 13C/13C chemical shift correlations between saturated azaheterocyclic N‐methylamine N‐oxides,and the methiodides of their parent amines

Two kinds of good linear correlations were found between the chemical shifts of saturated six‐membered azaheterocyclic N‐methylamine N‐oxides and the chemical shifts of the methiodides of their parent amines. One of the correlations occurs between the ¹⁷O chemical shift of the N⁺―O^– oxygen in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9778). This correlation enables unambiguous configuration assignment of the N⁺―O^– bond, even if the experimentally observed ¹⁷O chemical shift of only one N‐epimer is available, provided the ¹³C chemical shifts of both N⁺―CH₃ groups in the methiodide are known and assigned; furthermore, it can be used also for the estimation of ¹⁷O chemical shifts of the N⁺―O^– oxygens in N‐epimeric pairs of N‐oxides, for which observed ¹⁷O data hardly become available. The second correlation is observed between the ¹³C chemical shift of the N⁺―CH₃ methyl group in the N‐oxides and the ¹³C chemical shift of the N⁺―CH₃ methyl group analogously situated in the appropriate methiodide (r = 0.9785). It can be used for safe configuration assignment of the N⁺―CH₃ group and, indirectly, also of the N⁺―O^– bond in an amine N‐oxide, even if no ¹⁷O NMR data, and the ¹³C chemical shift of only one N‐epimer is available. Copyright © 2015 John Wiley & Sons, Ltd.     

2H, 13C shift correlation as an analytical tool for the study of deuteriated carbon compounds

A chemical shift correlation experiment for ²H, ¹³C spin systems is described. Applications to the analysis of deuteriated carbon compounds are discussed and advantages over existing methods are pointed out.     

Exploiting natural abundance 13C–15N coupling as a method for identification of nitrogen heterocycles: practical use of the HCNMBC sequence

In this paper, we detail the results of ¹H–¹⁵N correlation data obtained via ¹³C–¹⁵N coupling at natural abundance on a number of classes of azoles including pyrazoles, imidazoles and triazoles. The experiment produces data that is highly complementary to direct ¹H–¹⁵N HMBC type correlations in that it can provide ¹⁵N chemical shift data for nitrogen that may not show up in the HMBC. This is particularly advantageous in the triazoles where ¹⁵N chemical shift can be diagnostic of regiochemistry. Because of the consistency in J_CN values among the azoles, the experiment produces distinctive correlation patterns that can be used for identification of regiochemistry. The experiment can also be used to directly measure ¹³C–¹⁵N coupling constants. Copyright © 2015 John Wiley & Sons, Ltd.     

Chemistry of the phenoxathiins and isosterically related heterocycles. XXXVII The synthesis and molecular structure of benzo[2, 3]naphtho[5, 6, 7-ij][1, 4]dithiepin and its 1-oxide

Benzo[2, 3]naphtho[5, 6, 7-ij][1, 4]dithiepin was prepared by the condensation of the disodium salt of 1, 2-dimercaptobenzene with 1-chloro-8-nitronaphthalene. The structure was established by nmr and mass spectroscopy. A small quantity of benzo[2, 3]naphtha[5, 6, 7,ij][1, 4]dithiepin 1-oxide was also isolated. The structure was initially established by mass spectrometry. The proton spectrum was totally assigned using a combination of proton zero quantum coherence (ZQCOSY) and proton-carbon heteronuclear chemical shift correlation (HC-COSY) techniques, which also allowed the unequivocal assignment of the protonated carbon resonances. An X-ray crystal structure of the 1-oxide irrefutably confirmed the structure. The crystal was triclinic and the space group was Pl, and the data refined to a final R = 0.0353. The molecule was shown to be folded about the axis passing through the two sulfur atoms with a dihedral angle of 109.00°.     

Characterization of mechanically sheared ethylene–propylene copolymers by high resolution NMR

We report high-resolution solution-state NMR experiments on chain ends generated in ethylene–propylene copolymers by mechanical shearing in an extruder. The use of the higher resolution of the ¹³C-NMR spectrum, in a two-dimensional ¹H-¹³C chemical shift correlation experiment, has allowed the complete resolution and assignment of the olefinic chain-end region of the ¹H-NMR spectrum. Simultaneously, the assignments of the ¹³C olefinic resonances, previously identified [A. C. Kolbert, J. G. Didier, and L. Xu, Macromolecules, 29 , 8591 (1996)] are confirmed. An iterative method for calculating the average molecular weight, based on quantitative measurements of the olefinic ¹H-NMR peak intensities is introduced and these results are compared with measurements from ¹³C-NMR and size exclusion chromatography and correlated to reduced viscosities. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1955–1961, 1997