13C NMR spectral study of polyacetoxyxanthones. IV—13C NMR spectroscopy of substituted xanthones

The ¹³C NMR chemical shifts of nine acetoxyxanthones are reported and identified. The acetoxy substituent effects have been evaluated and the corresponding shift increments proposed.     

13C NMR spectroscopy of natural substances: IV—13C NMR studies of trachylobane diterpenes: Complete carbon assignment

The diterpene trachylobane and a series of derivatives have been completely analysed by FT ¹³C NMR spectroscopy. All ¹³C frequencies for trachylobanol have been unambiguously assigned by experimental techniques, i.e. by proton single-frequency selective decoupling and shift reagent experiments. Shift calculations have been performed for the parent compound by using known methods. The results are discussed in comparison with the experimental data. Satisfactory agreement between predicted and experimental values has been obtained.     

High resolution 13C spectroscopy by Fourier transform NMR. Thiete sulfone

The high resolution ¹³C NMR spectrum of thiete sulfone (obtained without wide-band ¹H decoupling) indicates that ring strain and re-hybridization in this small ring compound are significant. Pulsed-Fourier operation allows rapid determination of spectral parameters. Experimental acquisition time by pulsed NMR without ¹H decoupling is comparable to the experimental time required for swept ¹³C NMR with complete ¹H decoupling.     

High resolution 13C NMR spectroscopy V—structure dependence of the 13C,H coupling constants in aceheptylene,azulene and 5-azaazulene

In order to study the structure dependence of the ¹³C,H coupling constants in polycyclic conjugated π electron systems with 5-and 7-membered rings, the ¹H coupled ¹³C NMR spectra of aceheptylene, azulene and 5-azaazulene have been analysed. The results are discussed, including published data of monocyclic reference compounds, and are compared to theoretical values calculated by the FPT-INDO method. It is shown that, first, the geminal ¹³C,H couplings in the unsaturated 5-membered carbocycles are predominantly dependent on CC bond lengths. Second, the vicinal ¹³C,H coupling constants in 5- as well as in 7-membered unsaturated carbocyclic rings can be linearly related to the lengths of the central CC bonds. Third, the vicinal inter-ring couplings show a large variation and give information on ring junction, thus being of special value in structure determination and are governed mainly by the bond angles. Fourth, the calculated ¹³C,H couplings across one and two bonds are totally useless for practical purposes; those across three bonds, however, at least reflect the experimental trends.     

13C NMR spectra of cephalosporins. Signal assignments of free acids and esters

¹³C NMR signals were assigned for several cephalosporin free acids and esters as well as cephalosporinate ions using T₁ measurements, selective NOE, and pD-dependent chemical shifts to investigate the structure-reactivity relationship.     

13C NMR spectroscopy of cyclopropane derivatives. 1—monocyclic compounds

The ¹³C NMR data of an appreciable number of cyclopropane derivatives have been collected. Most of the spectra were recorded by ourselvesand some were taken from the Literature. With a view to furthering the useof ¹³C NMR spectroscopy as a diagnostic tool in this field, we havemeasured not only the chemical shifts,δ, but also the coupling constants ¹J(CH). It is shown that both embody valuable structural information.     

New triterpenes from Swietenia mahagoni. IV—the 13C NMR spectra of cycloswietenol, 30-norcycloswietenol and isocycloswietenol

¹³C NMR spectral data of cycloswietenol and 30-norcycloswietenol from Swietenia mahagoni and the rearranged product, isocycloswietenol, are presented and their specific structural features discussed.     

Effect of the orientation of substituents on the chemical shifts of 13C. IV—13C NMR spectra of N,N-diisopropylamides and -thioamides

N,N-diisopropylamides and -thioamides show hindered rotation around the N? CH bonds, and the presence of mixtures of conformational isomers can be demonstrated at temperatures below 273 K in solution. ¹H and ¹³C NMR spectra of these conformers are measured and assigned. The ¹³C data serve to study through-space effects on ¹³C chemical shifts, which strongly depend on the conformations of the isopropyl groups. For amides, a through-space shielding of the N-methine carbons is found to exist only for conformers in which the methine hydrogen atom is spatially close to the oxygen atom. Chemical shift differences between amides and thioamides can be rationalized in terms of through-bond and through-space contributions, and serve for a better understanding of the shift differences in N,N-dialkylamides and -thioamides.     

High resolution NMR spectroscopy of heteroaromatic cations. II—pyrylium,thiapyrylium and seleninium cations

The barriers to internal rotation about the C? N bond in several thiopiperidides have been determined by the NMR technique. It was found that the barrier height (ΔG^?) increases in the series cinnamoylthiopiperidide<thiobenzoylpiperidide<phenylthioacetylpiperidide. This trend was discussed in terms of the electronic structure, using HMO calculations. For m-and p-substituted cinnamoylthiopiperidides the barriers were shown to increase with the electron withdrawing character of the substituent in the aromatic ring.     

Accurate assignments in PMR and <Superscript>13</Superscript>C NMR spectra of anhydrolycoctonine using 2D spectroscopy

Resonances in PMR and ¹³C NMR spectra of anhydrolycoctonine were fully assigned based on a series of 1D and 2D NMR experiments. The conformation of ring A was concluded to be a distorted boat with H-1β from a comprehensive analysis of chemical shifts, SSCC, and the NOE for two possible conformations. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 267–269, May–June, 2008.     

Warifteine and methylwarifteine: 1H and 13C assignments by two‐dimensional NMR spectroscopy

The identity and structures of warifteine and methylwarifteine, two tertiary bisbenzylisoquinoline alkaloids of an unusual type, were elucidated and verified by 2D NMR spectroscopy. Complete ¹H and ¹³C NMR spectral assignments were obtained. Copyright © 2003 John Wiley & Sons, Ltd.     

13C NMR spectra of natural products. 2—eudesmanolides

The ¹³C NMR spectra of eight eudesmanolides have been measured and the chemical shifts assigned. Among the compounds studied are the naturally occurring α-cyclocostunolide, santamarin, arbsculin A and reynosin.     

1H and 13C NMR signal assignments of carboxyl‐linked glucosides of bile acids

Complete ¹H and ¹³C resonance assignments were carried out for a new type of carboxyl‐linked glucosides of chenodeoxycholic (3α,7α‐dihydroxy‐5β‐cholan‐24‐oic) and hyodeoxycholic (3α,6α‐dihydroxy‐5β‐cholan‐24‐oic) acids by using several homonuclear (¹H–¹H) and heteronuclear (¹H–¹³C) 2D NMR techniques. Differences in the ¹H and ¹³C resonances between the α‐ and β‐anomers of the ester glucosides of bile acids were clarified for the first time. A comparison of the ¹H and ¹³C signal shifts induced by β‐D ‐glucosidation at the 24‐carboxyl and 3α‐hydroxyl groups in the parent 5β‐cholanoic acid was also made. Copyright © 2003 John Wiley & Sons, Ltd.     

13C NMR of camphene and α-pinene: Revised assignments

The ¹³NMR signals of camphene (1) and α-pinene (2) are reassigned unequivocally. J(CH) values of 1, 2, bornene, norbornene and norbornadiene are reported.     

Complete 1H and 13C NMR assignments of the epimeric menthane-1-carboxylic acids

Complete NMR analyses with full assignments for (1)H and (13)C NMR spectral data for both epimers of menthane-1-carboxylic acid are described. The NMR properties of the recently synthesized axial isomer had not been previously described, and through use of a variety of 1D and 2D techniques, additional information is provided for the equatorial isomer. As well as assignments of chemical shifts, homonuclear coupling constants were determined for the equatorial isomer and most of coupling constants were measured for the axial isomer.     

1H and 13C NMR spectral assignments of 2′‐hydroxychalcones

Chalcones are of interest to medicinal chemists because their structures can be easily modified with various functional groups. The syntheses and biological activities of chalcones from natural sources are well known. In this study, 24 2′‐hydroxychalcones bearing methoxy substituents were synthesized, among which five are new. The NMR data for all synthesized chalcones are described for the first time. The complete assignments of the ¹H and ¹³C NMR data can be used for the identification of newly discovered and widely isolated, synthesized chalcones. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

13C NMR spectra of thiophenes. III—Bromothiophenes

The ¹³C NMR spectra of all bromo substituted thiophenes have been obtained at 15·085 MHz. ¹³C signal assignments for monobromothiophenes have been confirmed by comparison with the spectra of their partially deuteriated derivatives; a revision is made for the assignment in 3-bromothiophene. The substituent effect of bromine is generally additive for the ¹³C chemical shifts. The substituent effect on various types of ¹³C, ¹H coupling constants is also obtained and discussed.