Carbon-13 NMR spectra of some 4- and 5-substituted pyrimidines

Carbon-13 NMR chemical shifts and carbon-proton coupling constants for nine 4-substituted and six 5-substituted pyrimidines are reported. The carbon chemical shifts are correlated with π-electron densities. Carbon-proton coupling constants fail to correlate with substituent electronegativity.     

13C NMR spectra of some 4-substituted protoadamantanes

The ¹³C NMR spectra of some mono- and geminal disubstituted protoadamantanes have been assigned with the aid of shift reagents.     

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 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 NMR spectra of some o-carbonyl benzeneselenenyl derivatives

The ¹³C NMR signals have been assigned for some ortho-carbonyl benzene-selenenyl derivatives, COR.C₆H₄. Sex, where R = H, CH₃, OCH₃ and OC₂H₅ and X = Cl, Br, CN, SCN, SeCN and CH₃. The effects of the nature of R and X have been discussed.     

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 spectra of some chlorinated polycyclodiene pesticides

Carbon-13 NMR spectra have been obtained for aldrin, dieldrin, isodrin, endrin, heptachlor, heptachlor oxide and 24 of their degradation/conversion products. Chemical shifts were assigned on the basis of (1) comparison among the spectra, (2) single frequency, off-resonance decoupling experiments, (3) substituent effects derived for norbornanes and (4) spectra obtained with an added lanthanide shift reagent. The substituent effects previously reported for norbornanes were found to be applicable for establishing trends in the chemical shifts for many of the compounds studied.     

13C NMR spectra of some monosubstituted quinolizinium bromides

Chemical shifts are reported for quinolizinium bromide and 12 monosubstituted derivatives, carrying a bromohydroxy, diethylamino or piperidino group as the substituent. In addition, ¹J(CH) values and long-range coupling constants are given.     

Carbon-13 nuclear magnetic resonance spectra of some 2-substituted tetrahydropyrans

Fourier transform carbon-13 nuclear magnetic resonance spectra have been obtained and interpreted for some 2-substituted tetrahydropyrans. The effects of the substituents on α, β, γ and δ-carbon atoms are discussed. Using suitable reference compounds the γ-parameter can be used for quantitative conformational analysis.     

Carbon-13 NMR spectra of some epoxides derived from dicyclopentadiene

The ¹³C NMR spectra of a series of epoxides derived from endo- and exo-dicyclopentadiene and their partially hydrogenated compounds were determined to examine the substituent effects arising from the introduction of the oxirane ring in comparison with those found in other ring systems. The ¹³C signals of some epoxides were assigned by using lanthanide shift reagents. Characteristic substituent effects exerted by an oxirane ring were demonstrated. Marked steric γ-effects of ?8—13 ppm were observed at the bridge carbon signal in the bicyclo[2.2.1]heptane skeleton. Differences were found in the substituent effects between endo- and exo-dicyclopentadiene epoxides, and have been discussed in relation to the molecular geometry.     

Carbon-13 NMR spectra of some tetra- and pentacyclic triterpenoids

The carbon-13 NMR spectra of lanosta-8-en-3β-ol, lanosta-8, 24-dien-3β-ol, lanosta-7,9(11)-dien-3β-ol, lanostan-3β-ol, eupha-8-en-3β-ol, eupha-8,24-dien-3β-ol, ursa-12-en-3β-ol (α-amyrin) and oleana-12-en-3β-ol (β-amyrin) have been obtained and completely assigned. The results of this study provide chemical shift data for methyl, methylene, methine and quaternary carbon atoms in tetra- and pentacyclic triterpenoid spectra. The carbon-13 NMR spectrum of a triterpenoid provides a unique fingerprint for the molecule.     

Carbon-13 NMR spectra of rifamycins

The natural abundance ¹³C Fourier transform magnetic resonance spectra of rifamycin S and some of its derivatives have been studied. A combination of five different approaches has made unambiguous assignments for most of the resonances possible: (1) comparative study of the non- decoupled and noise-decoupled spectra; (2) ¹³C spectral characteristics; (3) spectral comparison between derivatives; (4) selective proton decoupling; (5) biogenetic evidence. Pulse and Fourier transform ¹³C NMR spectroscopy provides a more complete picture of these complex molecules than was previously obtained by ¹H NMR spectroscopy.     

Carbon-13 NMR spectra of some polychloro hydrocarbons and related compounds

Data derived from the carbon-13 NMR spectra of 37 organic polychloro compounds allow one to identify readily the ? CHCl₂, ? CCl₂? and ? CH₂Cl groups, the ¹³C signals of which are registered in the shift ranges of 67 to 78 (80), 85 to 96 and 38 to 55 (59) ppm (from TMS), respectively, and have the distinctive one bond spin-spin coupling constants ¹J(C? H) 170 to 184 Hz (for the ? CHCl₂ groups) and 147 to 158 Hz (for the ? CH₂Cl groups). The ? CCl₂CH₂CH₂Cl fragment features characteristic diamagnetic shieldings of the ? CCl₂? and ? CH₂Cl that may be related to increased electron density on both of these groups.     

Carbon-13 NMR spectra of kauranoid diterpenes

Carbon-13 NMR spectra of a number of (?)-ent-kauranoids have been studied. An unambiguous selfconsistent assignment of resonances has been made by considering the changes in chemical shifts produced by the change of substituent(s).     

Carbon-13 NMR spectra of ericaceous toxins

Assignments of the ¹³C NMR resonances for the toxic diterpenoids of ericaceous plants have been made. The data have proved to be a useful tool for the structural examination of the congeners. For example, the structure of asebotoxin-VII to which a 6-O-acyl moiety had previously been assigned has now been revised to contain a 7-O-acyl group.     

Carbon-13 NMR spectra of arylmethyl carbanions

Carbon-13 chemical shifts of three arylmethyl carbanions have been determined by changing solvents and counter ions. The charge distributions in the carbanions are discussed and compared with those obtained from the ¹H chemical shifts.     

Carbon-13 NMR spectra of monosubstituted pyrazines

Carbon-13 NMR chemical shifts and one-bond carbon–hydrogen coupling constants have been obtained at 15·09 MHz. The trends in the carbon chemical shifts obtained for the pyrazines parallel those of monosubstituted benzenes and 2-substituted pyridines, except for the direct effect of substitution where the pyrazines resemble pyridines not benzenes. The substituent effects on the ¹³C NMR spectra are generally quite similar to those in the ¹H NMR spectra. The ¹³C NMR spectrum of the tautomeric hydroxypyrazine has been compared with the ¹³C NMR spectra of 2-, 3- and 4-hydroxypyridines. Hydroxy compounds that can exist as a cyclic amide show a large meta substituent effect on the chemical carbon shift.