Carbon-13 NMR spectra of some labdane diterpenoids

Carbon-13 signal assignments of the labdane diterpenoids phlogantholide-A, its diacetate and isophlogantholide-A are reported. The assignments of the ¹³C NMR signals of 14-deoxyandrographolide and its diacetate, made earlier, have now been confirmed by lanthanide shift studies on the former and also by ¹³C spectral studies on anhydroandrographolide diacetate and 14-deoxy-11,12-didehydroandrographolide diacetate.     

Carbon-13 NMR studies on some 5-substituted quinoxalines

Eleven 5-substituted quinoxalines (NO₂, NH₂, COOH, OCH₃, CH₃, OH, F, Cl, Br, I, CN, the latter five not reported previously) have been synthesized by standard methods. Their ¹³C NMR spectra have been measured in DMSO-d₆ and assigned on the basis of substituent parameters, by line widths and by intensities. The chemical shifts compare favorably with those calculated using benzene substituent parameters, and are very close to those of corresponding carbons in 1-substituted phenazines. The correlation with the chemical shifts of the corresponding positions in 1-substituted naphthalenes is also close except for those of carbons 4a and 8a in the quinoxalines which, due to their proximity to nitrogen, are downfield (in some cases 12 ppm) of the signals of the corresponding carbons in naphthalene. 5-Fluoroquinoxaline was also measured in CDCl₃, CD₃COCD₃, CD₃CN, CD₃OD, C₆D₆ and CD₃COOD. In all solvents an abnormally low ²J(CF) (～ 12 Hz) was found for C-4a and no C? F spin-spin splitting could be detected for the three-bond coupling of C-8a. Similar abnormalities were found in 2-fluoroaniline and 2-fluoroacetanilide. There are linear relationships between the Q parameter of the substituent and the chemical shift of carbons 4a, 5 and 6. A linear relationship also exists between the chemical shift of C-8 (‘para’ position) and the Hammett σ_p parameter of the substituent.     

Carbon-13 spectral studies of some chalcones and flavanones

Carbon-13 signal assignments of 2′,4′-dihydroxychalcone, flemichapparin, pashanone, isodidymocarpin, pedicellin, pedicinin, methylpedicinin, didymocarpin-A and the corresponding quinoflavanone dehydrodidymocarpin-A are reported.     

Carbon-13 magnetic resonance spectral studies of some bipyridine derivatives

Assignments of¹³C chemical shifts and¹³C-¹H coupling constants are presented for six bipyridine derivatives. Some earlier spectral assignments have been corrected. The spectral parameters of the bipyridines are generally very similar to those of the appropriate simple substituted pyridines. Long range interring coupling constants in these compounds have been discussed.

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¹³C-NMR einiger Bipyridine

Zusammenfassung Es werden die Zuordnungen der¹³C-Verschiebungen und der¹³C-¹H-Kopplungskonstanten für sechs Bipyridinderivate präsentiert. Dabei wurden einige früher getroffene Zuordnungen korrigiert. Das NMR-Verhalten der Bipyridine ist dem der analog substituierten Pyridine generell sehr ähnlich. Die Long-Range-Kopplungen zwischen den Ringen werden diskutiert.

     

Carbon-13 NMR studies of some paramagnetic transition-metal acetylacetonates some linewidth considerations

Carbon 13 isotropic shifts and linewidths have been measured for a series of paramagnetic transition-metal acetylacetonates. The results have been compared with the corresponding NMR parameters of the contiguous protons. The relaxation pathway of the ¹³C spins is controlled by hyperfine interactions with the unpaired electron spins and, in cases where the proton linewidth is also dominated by the hyperfine mechanism, good agreement is obtained between the experimental ¹³C/¹H linewidth ratio and that calculated on the basis of the contact shifts. Pseudocontact interactions can be important in determining the total isotropic shift for both the ¹³C and ¹H nuclei and contrary to previous studies, it is suggested that such effects are also important for V(III) and Mn(III) acetylacetonate.     

Carbon-13 NMR studies of some organotin(IV) compounds

The ¹³C chemical shifts and ¹³C−¹¹⁹Sn, ¹¹⁷Sn coupling constants for several organotin(IV) compounds R_xSnCl_4−x (R = Me, Buⁿ, Ph; x = 1−4) have been measured in both inert (CDCl₃) and donor (DMSO-d₆) solvents, as have ¹³C data for the compounds R_xSnR′_4−x (R = Me, Ph; R′ = Buⁿ and R = Me; R′ = Ph; x = 1−3) and the compounds Me₃SnX (X = pseudo halide). The δ and ¹J(C-Sn) values appear to depend mainly on the type and number of substituents on tin and the donor ability of the solvent. There are linear relationships between the number of substituents (x) and both δ and ¹J(C-¹¹⁹Sn) for almost the R_xSnX_4−x series (R = Me, Buⁿ, Ph; X = Cl and R = Me, Buⁿ; X = Ph; x = 1−4), when measured in a single solvent, e.g. CDCl₃. There is an excellent linear relationship between ¹J(C-¹¹⁹Sn) and ²J(¹HC-¹¹⁹Sn) for the compounds Me_xSnCl_4−x. Determination of ¹³C data for Me₃SnCl and Ph₃SnCl in a range of solvents reveals that the value of ¹J(C-Sn) increases with the donor ability of the solvent.The marked increase in the values of ¹J(C-¹¹⁹Sn) in DMSO-d₆ for the compounds R_xSnCl_4−x(R = Me, Buⁿ,Ph) on going progressively from x = 4 to x = suggest tin coordination numbers of 4, 5, 6 and 6, respectively. Some additional physical data are presented for the isolated complexes from DMSO and the compounds Ph_xSnCl_4−x(x = 1−3) and Me₃SnX with X = N₃ or OCOMe.     

Carbon-13 NMR studies of benzoquinones

The assignment of all ring carbons of p-benzoquinones derived from perezone and from thymoquinone was completed using gated decoupled spectra. The long range proton-carbon couplings are discussed in terms of the degree of substitution of the quinone ring. The tautomeric interconversion of the two energetically equivalent forms of 2,5-dihydroxy-1,4-benzoquinones has been studied in various solvents and at several temperatures.     

Carbon-13 nuclear magnetic resonance spectra of some aromatic diterpenoids

¹³C n.m.r. studies of a series of tricarbocyclic ring C aromatic diterpenoids using proton-noise and single-frequency off-resonance decoupling, partially relaxed Fourier transform techniques, shift reagents and specifically labelled derivatives have permitted unequivocal assignments of almost all signals. The shieldings caused by oxygenation of C-18 and C-19 and by introduction of various substituents in the aromatic ring are discussed. It is concluded that the ¹³C n.m.r. data are sufficiently characteristic to allow stereochemical assignments. Some effects of deuterium substitution on the carbon resonances are presented.     

1H and 13C NMR spectral assignments of some natural abietane diterpenoids

An NMR study of 11 naturally occurring abietane diterpenoids is described. In addition to one‐dimensional NMR methods, including DPFGSE 1D‐NOE spectra, two‐dimensional shift‐correlated experiments [¹H,¹H COSY, ¹H,¹³C‐gHSQC ¹J(C,H) and ¹H,¹³C‐gHMBC ⁿJ(C,H) (n = 2 and 3)] were used for the complete and unambiguous ¹H and ¹³C chemical shift assignments of these substances. Copyright © 2003 John Wiley & Sons, Ltd.     

Carbon-13 NMR spectra of some phosphorane-amino compounds

The ¹³C NMR spectra of a series of compounds of the type R¹P(NR₂²)_nPh_3?n X^? are reported. A net trend toward larger ¹J(PC) values with an increasing number of dialkylamino substituents is observed.     

Carbon-13 NMR signals of some benzodiazepine drugs

Some carbon-13 NMR signal assignments of diazepam, flurazepam, clonazepam and chlordiazepoxide have been revised and the assignments of nitrazepam are reported. The assignments of the various resonances were made by considering the changes in chemical shifts produced by the change of substituents and also by comparison with the chemical shifts of model compounds.     

Carbon-13 NMR spectroscopy of some pentacyclic triterpenoids

Carbon-13 NMR signal assignment of a number of polyoxygenated triterpenes of the olean-12-ene and urs-12-ene series, carried out by considering the changes in chemical shift produced by the change of oxygenation pattern(s), and using methyl oleanolate as a model, are reported.     

Carbon-13 NMR studies on 3-phenyl-as-triazine 4-oxides

The substituent chemical shifts of a series of para substituted-3-phenyl-as-triazine 4-oxides were studied using the Swain-Lupton F and R values. This data was compared to that obtained for parasubstituted biphenyls, phenyl pyrroles and phenyl furans. It was found that the substituent chemical shifts in the triazines behaved in a similar manner to that in the biphenyl system.     

Carbon-13 NMR studies of twenty-four methyl-substituted morpholines

Carbon-13 NMR spectra of all the isomers of monomethyl-, 2,3-, 2,5-, 2,6-, 3,5-dimethyl-, 2,3,5-, 2,3,6-trimethyl- and 2,3,5,6-tetramethylmorpholine have been obtained at both ambient (25 °C) and low temperature (～ ?100 to ?120 °C). The ring carbon shifts appear to be additive with respect to the position of the methyl groups. A good correlation between predicted and experimental shift values was obtained (r = 0.998₉). The values were used in an attempt to assign, conformationally, the ‘all cis’ isomer 2,3,5,6-tetramethylmorpholine, which from ¹HNMR spin–spin coupling studies has been unsuccessful. Methyl carbon shifts to high field were found for axially oriented carbons. The extracted ‘steric shift’ values for such carbons were compared to their corresponding proton shift data.     

Carbon-13 NMR studies of naturally occurring maytansinoids

The ¹³C NMR spectra of the antileukemic ansa macrolide maytansine and eight naturally occurring homologues were recorded. Signals were assigned to specific carbon atoms of the ansa macrolide ring and C-3 ester side chain using data from off-resonance decoupling, single frequency decoupling, and 180°–τ–907° double pulse experiments. The assignment of each carbon is discussed for maytansine and tabulated for the other eight maytansinoids.     

Carbon-13 NMR spectra of some polychlorinated telomerisation products

The study of 28 specially selected compounds containing chlorine shows that ¹³C NMR signals of the CCl₃, CCl₃CH₂ and CH₂Cl groups are registered in the ranges of 93 to 105, 49 to 71 and 38 to 59 ppm (from TMS), respectively and are readily identified in ¹³C NMR spectra of polychloro compounds. The weak electron withdrawing ability of the CCl₃CHCl and CH₂Cl groups, as well as the extremely strong influence of CCl₃ and CH₂Cl groups on the methylene group between them in 1,1,1,3-tetrachloropropane, is noted.     

Carbon-13 NMR study of some isomeric n-nitroindazole derivatives

The ¹³C NMR spectra of 1,5- and 2,x-dinitroindazoles “x=4-7” and 3-bromo-1,x-dinitroindazoles are recorded and discussed. The data on he 2,x-dinitroindazoles confirm our previous structurea assignment based on ¹H NMR spectroscopy. The ¹³C NMR spectra established that in the novel 3-bromo-1,x-dinitroindazoles the N-nitro group is attached at N-1.