13C NMR spectra of diazastilbenes. I—trans-styryldiazines

The ¹³C NMR spectra of all trans-styryldiazines, methyldiazines and trans-styrylpyridines have been fully assigned. Empirical shift increments were obtained for the chemical shifts of the carbon atoms; these are discussed in the light of theoretical considerations.     

The NMR spectra of porphyrins. 27—proton NMR spectra of chlorophyll-a and pheophytin-a

The 500 MHz proton NMR spectra of chlorophyll-a in several non-aggregating solvents, at high dilution, are given and that of pheophytin-a in CDCl₃ at 360 MHz and at high dilution is reported. Under these conditions, the dispersion is such that separate resonances are observed and can be assigned for all protons in the molecules, except P-5 to P-15 in phytyl side-chain. The complex spin systems of the C-7 propionic ester side-chains, and of the P-1/P-2 fragment, are analysed and the couplings obtained are interpreted in terms of possible conformations of the side-chain.     

Saturated 5-membered ring conformations. 2—1H NMR study of protonated N-methylpyrrolidines

The ¹H NMR spectra of the protonated and unprotonated forms of cis- and trans-N-methylhexahydroisoindoline, of 1,3,3,4,4-pentamethylpyrrolidine and of 1,3,3-trimethylpyrrolidine were analyzed. The coupling constants between the NH⊕ and the CH protons α to the nitrogen allowed the study of the conformation of the cyclic systems in pseudorotation.     

Total assignment of the proton and carbon NMR spectra of 5′-NOR-anhydrovinblastine

Total spectral assignment of the bis-indole alkaloid Navelbine®, 5′-nor-anhydrovinbIastine, was accomplished using a combination of two-dimensional nmr techniques. Conventional homonuclear and inverse-detected experiments were supplemented with the HMQC-TOCSY experiment to unequivocally assign carbons directly bound to overlapped protons. In addition, the recently introduced GEM-COSY experiment was evaluated as a technique for the identification of geminal methylene pairs. Partial assignments proposed by Potier and co-workers, based on chemical shift considerations, are largely in agreement with the unequivocal assignments determined in this study.     

H and c NMR study of phosphopeptides : I—Acetylphosphoserine and acetylphosphothreonine

The ¹H and ¹³C NMR spectra of acetylserine, acetylphosphoserine and acetylphosphothreonine have been measured and completely analysed as a function of pD. For pD> 4 the NMR parameters are only slightly pD dependent. The second titration step of the phosphate group (pK₂ = 6.5) is much more difficult to detect in AcPSer (AcPTher) than in PSer (PThr). The ionization of the phosphate is of minor direct influence on the NMR parameters, their comparatively much larger changes in PSer (PThr) are due to conformational changes caused by the electrostatic interaction between the amino and phosphate group. The conformation in which the atoms H-C-Cβ-O-P form a planar W-type arrangement predominates at all pD values in the studied region from 4 to 14 in contrast to the non-cetylated phosphoamino acids in which its percentage passes a minimum at pD 8.     

NMR assignment methods for the aromatic ring resonances of phenylalanine and tyrosine residues in proteins

The unambiguous assignment of the aromatic ring resonances in proteins has been severely hampered by the inherently poor sensitivities of the currently available methodologies developed for uniformly 13C/15N-labeled proteins. Especially, the small chemical shift differences between aromatic ring carbons and protons for phenylalanine residues in proteins have prevented the selective observation and unambiguous assignment of each signal. We have solved all of the difficulties due to the tightly coupled spin systems by preparing regio-/stereoselectively 13C/2H/15N-labeled phenylalanine (Phe) and tyrosine (Tyr) to avoid the presence of directly connected 13C-1H pairs in the aromatic rings. The superiority of the new labeling schemes for the assignment of aromatic ring signals is clearly demonstrated for a 17 kDa calcium binding protein, calmodulin.     

13C and proton NMR spectra of 2(1H)pyrazinones

¹³C and proton NMR spectra data are given for eleven 2(1H)pyraziones. Assignments of chemical shifts were made by methods which included: deuterium exchange with certain protons of 3-alkyl substituents; change of chemical shifts of certain carbon atoms with change in pH; the use of long-range coupling constants for ¹³C to protons; and various correlations among assigned spectra.     

Complete analysis of the 1H NMR spectra of acetylated glycals—a conformational study

The complete analysis of the ¹H NMR spectra of the six acetylated glycals as well as that of tetra-O-acetyl-2-hydroxy-D -glucal has been carried out. The conformation of these compounds, as determined from the proton couplings (±0.05Hz) obtained, has been interpreted in terms of a rapid interconversion equilibrium between the two possible dihydropyran half-chair conformations. A computer treatment of all observed couplings has been carried out to obtain optimized values for the populations and couplings characteristic of the two alternative half-chair conformations. The rotamer populations of the acetoxy–methyl group are discussed on the basis of the measured ⁵J₁₆ and ⁵J₃₆ couplings.     

The NMR spectra of fluoroaromatics—IV: 1H and 19F spectra of some fluorinated benzofurans

The complete analyses of the ¹⁹F spectra of some fluorinated benzofurans are given. These provide an unambiguous determination of the substituent position in these compounds. Comparison of the ¹⁹F SCS and coupling constants in the benzofuran, benzothiophene and benzene series shows that ¹⁹F spectral parameters can be transposed from the benzene series with caution. A large increase in the para F? F coupling with ring closure of the hetero-ring is noteworthy. A long-range through-space ⁸J_H is reported which arises from the proximity of the 3 carboethoxy methyl group and the C₄ fluorine in these molecules.     

The NMR spectra and conformations of dihydropyran derivatives—I: The conformations of 2-alkoxy-5,6-dihydro-α-pyran-6-carboxylic esters

On the basis of NMR spectra of trans- and cis-2-alkoxy-5,6-α-pyran-6-carboxylic esters it was found that, at room temperature, the trans-compounds exist exclusively in the conformation with equatorial carbalkoxy and pseudoaxial alkoxy groups. The cis-isomers appear to be in conformational equilibrium between a form with equatorial carbalkoxy and pseudoequatorial alkoxy groups and that with axial and pseudoaxial substituents. In the latter case, the axial carbalkoxy group is bent out off its normal position by about 15°.     

1H NMR study of cyclo-L-Thr-L-Thr and cyclo-L-Thr-L-His conformations

Cyclodipeptides containing L -Thr and L -His residues have been studied by ¹H NMR in D₂O and DMSO-d₆. In the neutral form in D₂O as in DMSO-d₆, the folded form of the L -His residue is not unique. The diketopiperazine ring seems to be not strictly planar.     

Automatic assignment of 1H‐NMR spectra of small molecules

A novel data‐evaluation procedure for the automatic atom to peak or multiplet assignment of ¹H‐NMR spectra of small molecules has been developed using a fast and robust expert system. The applicability and reliability of the method are demonstrated by comparison of a manually assigned database of ¹H‐NMR spectra with the assignments produced by the automatic procedure. The results of this analysis show an excellent success ratio, indicating that this new algorithm can have a major impact as a time saving tool for the organic chemist. A new graphical feature used to illustrate both the stability and quality of the elementary assignments is also introduced. Copyright © 2013 John Wiley & Sons, Ltd.     

The NMR spectra and conformations of cyclic compounds—IV: The conformation of 3,4-cyclopropyl-proline

The ¹H magnetic resonance spectrum of the title compound has been completely analysed, using 60, 100 and 220 MHz spectrometers. The values of the chemical shifts and coupling constants obtained confirm the proposed structure and also provide information on the conformation. The molecule is slightly boat-shaped.     

Total assignment of the proton and carbon NMR spectra of the alkaloid quindoline — utilization of HMQC-TOCSY to indirectly establish protonated carbon-protonated carbon connectivities

The reisolation of the indoloquinoline alkaloid quindoline (also known as norcryptolepine) from Cryptolepis sanguinolenta is reported. The structure was unequivocally confirmed by two-dimensional nmr methods; the proton and carbon spectra were assigned for the first time. Because of congestion in the proton spectrum HMQC-TOCSY was used as an alternative to the more familiar COSY experiment. In addition to establishing proton-proton connectivities, HMQC-TOCSY affords the added benefit of providing, in an indirect sense, connectivity information between protonated carbons.     

Group IV organometal derivatives of pyridine. I—A 1H and 13C NMR investigation of trialkylmetal derivatives of pyridine

The proton and carbon-13 NMR spectra of thirteen trialkylmetal derivatives of pyridine, several of which were previously unknown, have been recorded and analysed. The proton NMR spectra show variations in proton chemical shifts but not in proton-proton coupling constants when the metal substituent is changed; the ring proton-metal coupling constants ⁿJ(M? H) in the tin and lead derivatives correspond closely with the corresponding proton-proton couplings ⁿJ(H? H) in pyridine. The carbon-13 chemical shifts of the carbons bound to the metal can apparently be correlated with the electron-donating ability of the trialkylmetal group. In the trimethylstannylpyridines the value of ¹J(Sn? C_ring) varies greatly with the position of the Me₃Sn group.     

Reassignment of the 1H NMR spectrum of fusidic acid and total assignment of 1H and 13C NMR spectra of some selected fusidane derivatives

Reassignment of the ¹H and ¹³C NMR spectra of fusidic acid and the total assignment of lumifusidic acid and some important di‐ and tetra‐hydrofusidic acid derivatives are reported. Copyright © 2002 John Wiley & Sons, Ltd.     

1H NMR spectra,configuration, and conformations of 2-mono- and 2,4-disubstituted 1,3-dioxolanes

The stereochemistry of 2-furyl-substituted 1,3-dioxolanes was studied. The ethane fragment of 2-monosubstituted dioxolanes displays the spectrum that is typical for an AA'BB' spin system, while the spectra of the 2,4-disubstituted compounds are typical for an ABC system. The contribution of the various conformational forms to the pseudorotation cycle of the dioxolanes is discussed on the basis of an analysis of the spectra by means of an iteration program.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No, 12, pp. 1613–1617, December, 1980     

Complete assignment of the 1H and 13C NMR spectra of garciniaphenone and keto-enol equilibrium statements for prenylated benzophenones

This article reports the structural elucidation by IR, UV and MS spectroscopic data along with 1H and 13C NMR chemical shift assignments of two benzophenones isolated from the fruit pericarp of Garcinia brasiliensis Mart. (Clusiaceae): garciniaphenone, (1R,5S,7S)-3-benzoyl-4-hydroxy-6,6-dimethyl-5,7-di(3-methyl-2-butenyl)bicyclo[3.3.1]non-3-ene-2,9-dione, a novel triprenylated benzophenone; and 7-epi-clusianone, a tetraprenylated benzophenone that has already been extracted from another species of the same family. Furthermore, the keto-enol tautomeric equilibrium at solution-state was described for these compounds by 1D and 2D NMR spectral methods and one attempt to rationalize the different ratios between the noted tautomers was based on stereochemical features.     

1H NMR spectra,dipole moments,and conformations of 3-oxo-6,7-benzo-1,5,3-dioxaphosphepines

¹H NMR spectroscopy, measurements of dipole moments, and the Kerr effect have been used to study the conformational structure in solutions of 3-X-3-oxo-6,7-benzo-1,5,3-dioxaphosphepines (X=Me, Et, Ph, Cl, NEt₂, OEt, OPh). On the basis of x-ray diffraction data, by means of the Dillen-Geise method, possible conformers of the seven-membered ring have been described quantitatively: chair, twist, and twist-boat. It has been shown that the compounds are characterized by a three-component equilibrium of chair and flexible forms. The relative populations of the conformers depend on the nature of the substituent X. The relationships in internal rotation around the P-Ph and P-O(R) bonds are discussed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1517–1523, July, 1991.