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.     

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.     

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.     

The NMR spectra of porphyrins 20—proton and 13C characterization of porphyrin atropisomers

The preparation, isolation and characterization by proton and ¹³C NMR of the four possible atropisomers of meso-tetra(2-methoxy-1-naphthyl)porphyrin is described. Chemical shift differences due to atropisomerization effects are observed in the porphyrin and naphthyl rings. Comparison of the naphthyl chemical shifts with those of the model compound 1-isopropenyl-2-methoxynaphthalene allows the shifts due to the porphyrin ring current to be isolated. The observed Δδ values of the naphthyl protons agree well with those calculated from the previously described porphyrin ring current model, and allow both the angle of tilt of the naphthyl ring and the dihedral angle of the 2-methoxy substituent to be estimated. In contrast, the Δδ values for the naphthyl carbons bear no relationship to the calculated ring current shifts. Calculations of the total ring current contribution (porphyrin plus naphthyl rings) at the different naphthyl rings of the unsymmetric type III isomer show that at least part of the observed atropisomerism effects are due to the long-range current shifts of the naphthyl rings. The results also provide a clear demonstration of the identity of the porphyrin ring current in the free base and porphyrin dication.     

The NMR spectra of porphyrins. 27—proton NMR spectra of chlorophyll-a and pheophytin-a

The 500 MHz proton NMR spectra of chlorophyll-a in several non-aggregating solvents, at high dilution, are given and that of pheophytin-a in CDCl₃ at 360 MHz and at high dilution is reported. Under these conditions, the dispersion is such that separate resonances are observed and can be assigned for all protons in the molecules, except P-5 to P-15 in phytyl side-chain. The complex spin systems of the C-7 propionic ester side-chains, and of the P-1/P-2 fragment, are analysed and the couplings obtained are interpreted in terms of possible conformations of the side-chain.     

13C NMR spectra of metal σ-cyclopentadienyls

Proton decoupled ¹³C NMR spectra have been measured for the cyclopentadienyl compounds C₅H₅Si(CH₃)_nCl_3?n(n = 1, 2, 3), C₅H₅Ge(CH₃)₃, CH₃C₅H₄Ge(CH₃)₃, C₅H₅Sn(CH₃)₃, σ-C₅H₅Fe(CO)₂-π-C₅H₅ and C₅H₅HgCH₃. A fast metallotropic rearrangement occurring in the compounds causes the spectra to be temperature dependent for the Si, Ge, Sn and Fe derivatives. For the derivatives of silicon or germanium, the olefinic signals are unsymmetrically broadened by the 1,2-shift at lower migration rates. Line widths of the ring carbon signals have been measured to give an estimate for the activation parameters of the rearrangement in C₅H₅Ge(CH₃)₃ (E_a = 10·7 ± 0·9 kcal/mole, ΔG^? = 13·4 ± 0·9 kcal/mole) and C₅H₅Sn(CH₃)₃ (E_a = 6·8 ± 0·7 kcal/mole, ΔG^? = 7·1 ± 0·7 kcal/mole). At room temperature, the spectrum of C₅H₅HgCH₃ displays just one narrow signal responsible for the cyclopentadienyl ligand. The spectrum of CH₃C₅H₄Ge(CH₃)₃ at –30° demonstrates that two isomers containing methyl in the vinylic position are present, the ratio being ca. 2:1. The ¹³C spectra of the vinylic isomers have been analysed in the case of C₅H₅Si(CH₃)_nCl_3?n.     

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.     

13C NMR spectra of diazastilbenes. II—Torsion angles of trans-pyridinalanilines and trans-benzylideneaminopyridines

The torsion angles of trans-pyridinalanilines and trans-benzylideneanilines were derived from the chemical shifts of the para-carbon atoms. The N-torsion angle is between 33 and 59°. For the C-torsion angle, a value of approximately 10° is obtained for the benzylideneaminopyridines and the C-torsion angle for the pyridinalalines is estimated to be in the same range.     

The 13C NMR spectra of homoisoflavonoids

The study of a set of 3-benzylidene-4-chromanones, 3-benzyl-4-chromanones, 3-benzyl-3-hydroxy-4-chromanones and 3-benzylchromones (homoisoflavonoids) by ¹³C NMR spectroscopy shows the influence of the structure of these molecules on the chemical shifts of the more characteristic carbon atoms at positions 2, 3 and 4.     

Stereochemistry of alkyl 3-substituted 4-halotetrahydro-2-oxo-3-furancarboxylates: 2—1H and 13C NMR spectra

The ¹H NMR parameters of methyl 3-substituted cis-4-halotetrahydro-2-oxo-3-furancarboxylates are reported, with assignments of the ring protons based on solvent-induced changes in the vicinal trans coupling constants, ³J(H-4, H-5). Preferred conformations, ce with a pseudo-equatorial halogen for the cis isomers and ta with a pseudo-axial halogen for the trans isomers, have been suggested on comparison of the magnitudes of J(trans) and J(gem) in both series. The ³J(¹³CH₃, H-4) values measured for methyl cis-4-bromotetrahydro-3-methyl-3-furancarboxylate, methyl trans-4-bromotetrahydro-3-methyl-3-furancarboxylate and trans-3,4-dibromodihydro-3-methyl-2(3H)-furanone have confirmed the stereochemical assignments.     

The NMR spectra of the porphyrins: 24—The NMR spectra of some tetraarylchlorins

The synthesis and complete assignment of the ¹H NMR spectra of 5-(o-pivaloylaminophenyl)-10,15,20-triphenylporphyrin (PIVTPP) and its two chiral dihydro adducts 3,4-dihydro-(PIVPTPC-I)- and 7,8-dihydr- (PIVPTPC-II)-porphyrins are reported. The use of the zinc complexes of the chlorins as chiral shift reagents with optically active bases is discussed. Comparison of the observed shift differences between the chlorins and the parent porphyrin with those calculated by a ring current model shows that a decrease in the ring current occurs on chlorin formation, and also specific effects occur at the reduced pyrrole ring, presumably reflecting different steric constraints.     

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.     

Carbon-13 NMR studies. 96—carbon-13 spectra of several polyhydroxylated 9,10-dihydrophenanthrene and phenanthrene derivatives

The ¹³C spectra of eleven 9,10-dihydrophenanthrene and twelve phenanthrene derivatives were recorded in order to examine the effects of hydroxy and methoxy substitution on the skeletal carbon shieldings. For each series, the parent compounds were the 2,4-disubstituted derivatives with the others bearing 5-, 6- and/or 7-substituents, including the naturally occurring orchinol and loroglossol. The data should be helpful for examinations of related compounds. Some examples of particularly facile hydrogen-deuterium exchange at aryl positions were encountered. Since these occur under apparently neutral conditions, suitable caution may be required in examinations of related compounds.     

Linear and angular annelation effects on the 13C NMR spectra of tricyclic aromatic ketones. 1—annelated indanones

Carbon-13 NMR chemical shifts are reported for nine tricyclic aromatic ketones formally derived from indanone. The influences of remote ring size, as well as linear, angular exo and angular endo ketone orientation are examined. Results are compared with available ¹H NMR data. For indanone itself, based on selective ¹H decoupling experiments, a recently reported CIDNP derived ¹³C signal assignment is shown to be in error.     

13C NMR spectra of alkenylsilanes

Conclusions The¹³C NMR spectra of 12 alkenylsilanes were measured and the values of the increments of the chemical shifts of the (CH₃)₃Si group were determined for some allylsilanes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 938–939, April, 1978.     

13C NMR spectra of S-methylquinolines

The ¹³C chemical shifts and the increments () of the SCH₃ group in the NMR spectra of S-methylquinolines were measured. It is shown that the chemical shifts of S-methylquinolines correlate satisfactorily with the total charges on the carbon atoms calculated within the CNDO/2 approximation. The coupling of the SCH₃ group with the quinoline ring was examined on the basis of the values.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1665–1671, December, 1979.The authors thank É. É. Liepin' for his discussion of the results.Original article submitted October 1, 1978.