Carbon-13 NMR spectra of simple glycidic esters

The line separations from geminal and vicinal CH-couplings in simple glycidic esters are found to be within narrow ranges. They can be used for the assignment of carbon resonances and epoxide configurations.     

Carbon-13 NMR spectra of N-methylimidazole-substituted sterol esters

The ¹³C NMR spectra of two N-methylimidazole-substituted sterol esters are discussed. The calculated shifts are compared to those obtained experimentally. For one of the sterol esters the experimental and calculated data show good agreement, and the substituent increments can be evaluated. This does not apply in the case of the second sterol ester, where the steric and/or electronic effects of a further substituent—a hydroperoxy group in close proximity to the N-methylimidazole group—prohibits the additive calculation of substituent increments.     

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 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.     

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 several dicyclopentadiene derivatives

The ¹³C shielding data for 69 derivatives of dicyclopentadiene are reported. These derivatives include compounds from both the exo and endo series having both saturated and unsaturated trimethylene bridges.     

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 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 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 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.     

Carbon-13 NMR spectra of ferrocenes and ferrocenyl-alkylium ions

The proton-coupled and proton-decoupled pulsed Fourier-transform ¹³C NMR spectra of a series of neutral ferrocene derivatives and ferrocenylalkylium ions have been recorded and analysed. The influence of a substituent group upon the ¹³C shifts of the carbons of the ferrocene rings has been investigated. The spectra of the ferrocenylalkylium ions are discussed in relation to the distribution of positive charge and to the various structural models which have been proposed for such species.     

Carbon-13 NMR spectra of hydroxylated bile acid stereoisomers

Carbon-13 NMR signals were assigned for the complete set of the 26 theoretically possible isomers of methyl 5β-cholanates having one to three hydroxy groups at positions C-3, C-7 and/or C-12 in the nucleus. Substituent effects on the ¹³C NMR shielding data serving to characterize the position and configuration of the hydroxy groups are discussed.     

Carbon-13 NMR spectra of fusicoccin and its derivatives

The ¹³C NMR spectra of fusicoccin, some of its cometabolites and derivatives were studied. Using standard Fourier transform techniques, T₁ relaxation time measurements and lanthanide shift reagents, the resonances of individual carbon-13 nuclei were assigned.     

Carbon-13 NMR spectra of chromium pentacarbonyl carbenoid complexes

The carbon-13 NMR spectra of six carbenoid complexes of the type (OC)₅Cr-(CXX′) have been recorded. The carbenoid carbon atoms are all markedly deshielded (chemical shifts vs. TMS in the range ?271 to ?360). With only one minor inversion of uncertain significance, the chemical shifts correlate well with the expected ability of the X and X′ groups to engage in dative π bonding to the carbenoid carbon atom. The longitudinal relaxation times for both carbenoid and carbonyl carbon atoms in (OC)₅Cr[C(CH₃)(OC₂H₅)] are 1 – 2 sec. The chemical shift difference for carbonyl carbon atoms cis and trans to the carbenoid ligand is essentially invariant in the six compounds.