Carbon-13 nuclear magnetic resonance assignments and biosynthesis of ochratoxin A

The carbon-13 chemical shifts have been assigned to all the carbons in the isocoumarin portion of ochartoxin A. Incorporation of carbon-13 enriched acetate was used to confirm the biosynthesis of ochratoxin A.     

Assignment of the 13C nuclear magnetic resonance spectra of three 27-hydroxyfriedelanes

¹³C NMR spectra of 27-hydroxyfriedelane, kokoonol (27-hydroxyfriedelan-3-one) and kokoononol (27-hydroxyfriedelan-3,21-dione) have been recorded and signals assigned using the off-resonance decoupling, inversion recovery and lanthanide induced shift techniques.     

Carbon-13 nuclear magnetic resonance spectra of exo- and endo-2-norbornyltrimethyl-stannanes: Corrected assignments

The ¹³C NMR spectra of pure exo-2-norbornyltrimethylstannane and a mixture of the exo- and endo-isomers have been recorded. ¹H–¹³C polarization transfer spectra have been obtained and require the previously reported assignments for C-3 and C-4 in the exo-isomer to be reversed. The reported assignments for the endo-isomer are correct. The new assignment for C-4-exo [with J(¹¹⁹Sn,¹³C) vic=12 Hz, instead of the previously assigned J(vic)=23 Hz], has a very minor effect on the nature of the Karplus curve [for ³J(¹¹⁹Sn,¹³C)] generated previously.     

13C nuclear magnetic resonance studies of cellulose acetate

¹³C-NMR spectra of ring carbons and O-acetyl carbonyl carbons of cellulose acetate (CA) in dimethyl sulfoxide-d₆ were analyzed. The CA samples with the degree of substitution (DS) ranging from 0.84 and 1.91 were prepared by homogeneous acetylation of cellulose with acetic anhydride in a 10% LiCl/dimethyl acetamide solvent. It was found that the use of these low DS samples permitted easier assignments not only of the ring carbon but also of the O-acetyl carbonyl carbon signals. The assignments were confirmed by comparing with the ¹H-NMR spectra of the samples obtained by complete acetylation of the corresponding CA samples with acetyl-d₃ chloride. Two methods for determining the distribution of O-acetyl groups of CA, i.e., the relative DS at the three different types of hydroxyl groups, were developed. One is based on the measurements of the relative intensities of the signals for the ring carbons and the other is based on the measurements of the relative intensities of the signals for the O-acetyl carbonyl carbons.     

13C nuclear magnetic resonance data of bile acid derivatives

The 13C-NMR spectra of well over 100 bile acid derivatives have been analyzed and summarized. A diagnostic gamma-oxygen shielding effect has been identified.     

Solid-state 13C nuclear magnetic resonance characterization of MDI-based polyurethanes

¹³C solid-state nuclear magnetic resonance (NMR) experiments on linear polyurethanes and poly(ether-urethane) block copolymers demonstrate that ¹³C spin-lattice relaxation experiments in the laboratory [T₁(C)] and rotating [T_1p(C)] frames provide the most information about domain morphology in these microphase-separated polymer systems. T₁(H) TCH, and T_1p(H) data are less useful in a 4,4′-methylene bis(p-phenyl isocyanate)-1,4-butanediol (MDI/BD) hard-segment material, the MDI bridging methylene and the MDI urethane carbonyl T₁(C and T_1p(C) times fall in characteristic ranges for crystalline, amorphous, interfacial, and dissolved species. BD methylene carbons have short T_1p(C) for crystalline and long T_1p(C) for amorphous hard-segment aggregates. The distinct T_1p(C) and T₁(C) fractins observed are attributed to the presence of several crystalline polymorphs. Both T₁(C) results and DSC endotherms indicate that the crystalline polymorphs present in the poly(ether-urethane) are less ordered than the types seen in the pure hard-segment material. © 1993 John Wiley & Sons, Inc.     

13C nuclear magnetic resonance studies on some substituted isoxazoles

¹³C n.m.r. spectra of some substituted isoxazoles have been examined to ascertain the reactive site in the metallation of 4-substituted 3,5-dimethylisoxazoles. The results obtained indicate that metallation occurred exclusively at the C-5 methyl.     

Cyclometallated compounds II. Proton and carbon-13 nuclear magnetic resonance spectral assignments of cyclopalladated compounds

The ¹H and ¹³C NMR spectra of nineteen cyclopalladated acetylacetonate complexes are reported. Definitve spectral assignments are made on the basis of selective proton decoupling experiments, difference NOE spectra and both homo- and hetero-nuclear two-dimensional correlation spectroscopy. The Pd(acac) substituent is shown to induce characteristic chemical shift changes in both proton and carbon spectra. These effects, however, vary from those of differently substituted palladium and other metal substituents.     

Inhibition and enhancement of resonance as studied by 13C nuclear magnetic resonance spectroscopy

¹³C chemical shifts are reported for a series of 2-substituted 1,3-dimethylbenzenes: comparisons of these values with those for the corresponding monosubstituted benzenes reveal, in some cases, large differences in the para-carbon substituent chemical shifts, which are attributable to steric hindrance of resonance. The questions of steric enhancement of resonance, and methoxy group conformation in certain anisoles are also studied by the ¹³C NMR technique. Studies of selected 2-substituted fluorenes are also reported, and substituent chemical shifts at carbon-7 (traversing eight bonds) of greater than 2ppm are observed. These effects are consistently greater than those reported for the corresponding biphenyl compounds, and are associated with planarity-enforced enhancement of resonance.     

13C nuclear magnetic resonance spectra of several hydroxy diterpene derivatives

¹³C NMR spectra of several hydroxy diterpene derivatives having a pimarane, isopimarane and (13S, 8α)rosane skeleton have been recorded. The most significant effects caused by the introduction of hydroxyl groups in such a skeleton are demonstrated. The magnitudes of the γ- and δ-effects are large enough to allow their use in sterochemical assignments.     

Assignment of juvabione diastereomers by 13C nuclear magnetic resonance spectroscopy

The determination of structures and partial assignments of stereochemistry of juvabione and some of its analogues can be made on the basis of ¹³C nuclear magnetic resonance studies. The complete ¹³C n.m.r. spectral assignments for juvabione and five analogues are reported.     

13C nuclear magnetic resonance spectra of some naturally occurring friedelanones

¹³C NMR spectra of friedelan, friedelan-21-one, friedelan-6-one, friedelane-321-dione, friedelane-3,6-dione and friedelane-3,6,21-trione have been recorded and signals assigned using off-resonance decoupling, inversion recovery and lanthanide induced shift techniques.     

13C nuclear magnetic resonance spectra of polyacrylates and their model compounds

¹³C nuclear magnetic resonance spectra (¹³C-NMR) of poly(methyl acrylates) and poly(isopropyl acrylates) of various tacticities were measured at 25.1 MHz and analyzed. ¹³C-NMR spectra of model compounds for poly(methyl acrylate), poly(acrylic acid), and poly(sodium acrylate) were also determined. The spectra of the polymers were generally complicated owing to the splittings corresponding to triad, tetrad, or pentad placements, and the assignment for each peak was difficult. Groups of peaks were analyzed by triad or tetrad placements by assuming Bernoullian or first-order Markovian statistics.     

13C nuclear overhauser polarization-magic-angle spinning nuclear magnetic resonance spectroscopy in uniformly 13C-labeled solid proteins

A recently introduced (13)C polarization technique based on the nuclear Overhauser effect in rotating solid (nuclear Overhauser polarization-magic-angle spinning, NOP-MAS) (Takegoshi, K.; Terao, T. J. Chem. Phys. 2002, 117, 1700-1707) is applied to uniformly (13)C, (15)N-labeled proteins. NOP enhancement factors per scan of 1.5 approximately 2.0 are obtained, while that by cross polarization (CP) is less than 1.0. We show that uniform enhancement of all (13)C signals by CP is difficult to attain, while it is easily achieved by NOP, thus enabling quantitative comparison of signal intensities. NOP is easy to carry out under fast MAS and works well even for somewhat mobile molecules, for which CP does not work. Moreover, in labeled protein samples containing nonlabeled additives, NOP can eliminate the latter signals. For these features, NOP is superior to CP in many uniformly (13)C labeled proteins.     

13C nuclear magnetic resonance spectroscopy to determine olive oil grades

¹³C nuclear magnetic resonance spectroscopy was used in a first attempt to differentiate olive oil samples by grades. High resolution ¹³C NMR Distortionless Enhancement by Polarization Transfer (DEPT) spectra of 137 olive oil samples from the four grades, extra virgin olive oils, olive oils, olive pomace oils and lampante olive oils, were measured. The data relative to the resonance intensities (variables) of the unsaturated carbons of oleate (C-9 and C-10) and linoleate (L-9, L-10 and L-12) chains attached at the 1,3- and 2-positions of triacylglycerols were analyzed by linear discriminant analysis. The 1,3- and 2- carbons of the glycerol moiety of triacylglycerols along with the C-2, C-16 and C-18 resonance intensities of saturated, oleate and linoleate chains were also analyzed by linear discriminant analysis. The three discriminanting functions, which were calculated by using a stepwise variable selection algorithm, classified in the true group by cross-validation procedure, respectively, 76.9, 70.0, 94.4 and 100% of the extra virgin, olive oil, olive pomace oil and lampante olive oil grades.     

13C nuclear magnetic resonance studies on acetophenones: Barriers to internal rotation

The barrier to internal rotation in a series of p-substituted acetophenones has been determined by means of low temperature carbon-13 n.m.r. and total bandshape analysis, resulting in: ΔG = 5·4 ± 0·1 kcal mol^?1 (22·4 ± 0·4 kJ mol^?1) for the unsubstituted acetophenone. The substituent effects on the barrier are found to be the same as for the corresponding benzaldehydes. The barrier height is discussed in terms of contributions from resonance and steric effects.     

13C nuclear magnetic resonance of organophosphorus compounds XI—aminophosphines

Natural abundance ¹³C NMR studies have been carried out on a series of organophosphorus compounds possessing P? N bonds. For the first time a one-bond temperature-dependent ¹³C—³¹P nuclear spin coupling was observed for the P-phenyl carbons in bis(N,N-dibenzylamino)phenylphosphine (0-9 Hz) and bis(N,N-diethylamino)phenylphosphine (0–2 Hz). This temperature-dependent behavior can be rationalized in terms of free rotation about the phenyl phosphorus bond with concomitant hindered rotation about the P? N bonds. A conformational preference for the nitrogen and phosphorus lone pairs to exist in the trans orientation is indicated. In the similarly substituted 5-membered heterocyclic ring compound, 1,3-dimethyl-2-phenyl-1,3-diazaphospholidine, the phenyl one-bond coupling increases to (?) 42.1 Hz and becomes temperature independent. These data suggest that ¹J(PC) is very responsive to electronic effects.     

17O and 13C nuclear magnetic resonance study of polyfluorinated carbonyl compounds

δ¹⁷O- and δ¹³C values are reported for 15 polyfluorinated carbonyl compounds. If the R_f group is separated from the carbonyl group by less than two carbon atoms a marked increase in the nuclear shielding of ¹³C(C=O) and a marked decrease in the nuclear shielding of the ¹⁷O(C=O) nucleus is observed. This is ascribed to the differing effects of the R_f group on the σ and the π system of the carbonyl unit. The effect on the σ-manifold leads to increase in shielding but it may be offset (as in the case of the ¹⁷O nucleus) by destabilization of the π system. UV spectroscopic data for some polyfluorinated carbonyl compounds support these arguments.