Carbon-13 n.m.r. spectroscopy of some Strychnos alkaloids

¹³C N.m.r. spectra have been determined for strychnine and a series of fourteen derivatives. The assignments are based on previously established data and on off-resonance decoupled spectra. The shifts resulting from the alterations in molecular structure are discussed and explained, in part, as a consequence of conformational changes. This detailed study indicates that changes are required in some previously published assignments.     

Carbon-13 n.m.r. studies of methylquinolines and methylisoquinolines

Spectra are reported for all seven methylquinolines, nine dimethylquinolines, six methylisoquinolines and one dimethylisoquinoline. Substituent effects for the α, ortho, meta, para, vinylogous para and peri positions are reported for the monomethylcompounds. Additivity of these substituent effects is demonstrated for the dimethyl compounds which also exhibit a vicinal dimethyl steric effect.     

Carbon-13 n.m.r. studies. 65—13C spectra of some tricyclo[3.2.1.02,4]octane derivatives

The ¹³C n.m.r. spectra of 18 derivatives of the tricyclo[3.2.1.0^2,4]octanes have been determined. This series includes methyl, hydroxyl and oxo substituted examples to compare the effects of these substituents on the skeletal carbon shieldings with those observed for the corresponding norbornanes and bicyclo[3.2.1]octanes. In general, the trends are similar and the perturbations associated with closely neighboring groups follow a consistent pattern. The shielding data for the exo–exo and exo–endo isomers of tetracyclo[3.3.1.0^2,40^6,8]nonane are also reported.     

13C n.m.r. studies of some phenanthrene and fluorene derivatives

The ¹H and ¹³C spectra of fluorene, fluorenone, phenanthrene and their 4-methyl and 4,5-dimethyl derivatives have been examined. To complete the analyses for fluorenone and 4-methylfluorenone, ¹H spectra were recorded at 270 MHz. The results from the ¹H spectra permitted unequivocal assignments for the protonated aryl carbons by selective proton decoupling. A consistent set of assignments for the quaternary carbons was obtained through consideration of the dominant relaxation processes operative at these centres. This series of compounds was examined to investigate the shielding effects produced by the close approach of methyl groups separated by five bonds for comparison with the contrasting trends found for methyl carbons separated by three and four bonds. The results indicate that the relative orientation of the methyl groups is an extremely critical factor governing their shieldings and those of neighboring centres.     

Carbon-13 n.m.r. studies of substituted bisalkylamino-s-triazines in acidic media

The carbon-13 n.m.r. spectra of six substituted bisalkylamino-s-triazines are reported. The site of protonation of 2-methoxy-s-triazines is shown to be at N-1 or N-3. The effect of trifluoroacetic acid on 2-chloro-s-triazines is also discussed.     

Carbon-13 n.m.r. spectra of branched carboxylic acids and their derivatives

¹³C n.m.r. spectra have been measured for 39 compounds with branched structures including carboxylic acids, their methyl and ethyl esters, nitriles and chlorinated esters. The results obtained indicate that the ¹³ C n.m.r. technique is applicable to structure assignment of acids and their derivatives containing various numbers of substituents on the chain. The dependence of the carboxylic carbon chemical shift on the number and structure of α-positioned substituents has been determined. Calculation of the chemical shifts for branched carboxylic acids, esters and nitriles from the corresponding increments using the additivity scheme is shown to be possible in principle.     

Carbon-13 n.m.r. spectra of saturated heterocycles. Part VII—Thianium Salts

The ¹³C n.m.r. spectra of 53 thianium, S-methylthianium, S-alkylthianium and S-methyl-1-thiadecalinium salts, most of them substituted with methyl groups in the ring, have been recorded. The chemical shifts of the ring carbons in these thianium and S-methylthianium salts and the S-methyls in the S-methylthianium salts have been analyzed in terms of additive parameters of the methyl substituents which are compared to those previously determined for the parent thianes. Comparison is also made with other charged species.     

Carbon-13 n.m.r. analysis of selected strychnos alkaloids

The carbon-13 chemical shifts of strychnine and ten closely related analogues have been assigned and confirmed in several cases using selective proton decoupling techniques. Where additivity relationships are used the importance of having data on an array of structural analogues is emphasized. Quaternization at N-19 exerts long range polarization effects throughout the molecular framework. The effects of N-oxide formation on the carbon-13 shifts of 5 to 9-membered ring heterocycles and strychnine are discussed as aids to structural assignment. A Zwitterionic structure for strychnic acid is deduced.     

Carbon-13 n.m.r. characterization of indenyllithium in solution

¹³C N.m.r. chemical shifts obtained for indenyl- and cyclopentadienyllithium in several, mostly ethereal, solvents are presented. The chemical shift changes induced by varying the solvent are assumed to probe the actual π-electron density distribution, thus providing an insight into the structure of the ion-pair. The increase in the estimated π-electron densities obtained for the protonated aromatic carbons of indenyllithium by increasing the coordinating ability of the solvent gives evidence of a change of the ion-pair equilibrium towards solvent-separated species. The average chemical shifts for indenyl- and cyclopentadienyllithium were independent of the choice of solvent, a fact which supports the ion-pair model in all studied media.     

Carbon-13 n.m.r. study of naturally occurring xanthones

The ¹³C n.m.r. spectra of 36 naturally occurring xanthones are reported and all chemical shifts assigned. The shifts in substituted xanthones can be predicted from substituent effects evaluated for simple derivatives. The agreement between calculated and observed shifts decreases as the extent of substitution in the xanthone increases.     

Carbon-13 n.m.r. and CNDO/S study of 1,2-indandione and heterocyclic analogues

¹³C n.m.r. spectra are reported for 1,2-indandione, coumarandione and thionaphthenequinone in CDCl₃ solution and for isatin in dimethyl sulphoxide solution. The chemical shifts are assigned and discussed in connection with the influence of the heteroatom on the dicarbonyl group and the aromatic ring. The ¹³C shieldings have also been calculated by means of Pople's perturbation theory using CNDO/S wave functions.     

Carbon-13 n.m.r. studies: 68—diastereomeric nonequivalence and the characterization of some bisnorbornylpinacols

The sodium-ammonia reduction of racemic norcamphor, exo-3-methylnorcamphor and 3,3-dimethyl-norcamphor (camphenilone) leads to substantial yields (60–90%) of the corresponding 2,2′-bisnorbornyl-2,2′-diols as diastereomeric mixtures. A combination of their ¹H, ¹³C and infrared spectra together with the results of periodic acid and lead tetraacetate oxidations permits stereochemical assignments for these six pinacols.     

13C n.m.r. spectra of some polychloroalkenes

¹³C n.m.r. spectra have been measured for thirty-two polychloroalkenes including (i) monosubstituted compounds CH₂?CHCCl_nH_2?nX, where ? X stands for ? H, ? Cl, alkyl, and trisubstituted alkenes CCl₂?CHAlk, none of which form geometric isomers; (ii) disubstituted compounds RCH?CHR′; (iii) and (iv) trisubstituted compounds of the types RCCl?CHR′ and CHCl?CClR, respectively. Compounds (ii) to (iv) represent either individual isomers or mixtures of the Z and E forms. In the case of compounds (ii) and (iii), the ordering of chemical shifts is δ_E > δ_Z for the sp²-carbon atoms and δ_E < δ_z for the adjacent tetrahedral ones. On the contrary, the signals of the sp²-carbon atoms of compounds (iv) obey the rule δ_E < δ_z. The effect of vinyl and allyl groups as substituents on the ¹³C chemical shifts of chlorine-containing groups is discussed. The dependence of the sp²-carbon spin–spin coupling constants J(¹³C? ¹H) on the number of chlorinated substituents in the molecule is also considered.     

Carbon-13 magnetic resonance spectral studies of some bipyridine derivatives

Assignments of¹³C chemical shifts and¹³C-¹H coupling constants are presented for six bipyridine derivatives. Some earlier spectral assignments have been corrected. The spectral parameters of the bipyridines are generally very similar to those of the appropriate simple substituted pyridines. Long range interring coupling constants in these compounds have been discussed.

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¹³C-NMR einiger Bipyridine

Zusammenfassung Es werden die Zuordnungen der¹³C-Verschiebungen und der¹³C-¹H-Kopplungskonstanten für sechs Bipyridinderivate präsentiert. Dabei wurden einige früher getroffene Zuordnungen korrigiert. Das NMR-Verhalten der Bipyridine ist dem der analog substituierten Pyridine generell sehr ähnlich. Die Long-Range-Kopplungen zwischen den Ringen werden diskutiert.

     

Carbon-13 n.m.r. of the 8-phosphabicyclo[3.2.1]octane system

¹³C Chemical shifts and ¹³C? ³¹P nuclear spin coupling constants have been determined for 26 8-phosphabicyclo[3.2.1]octane derivatives, namely phosphines, phosphine oxides, phosphine sulphides and one phosphonium salt. The influence of the phosphorus configuration on δ and ²J(PC) values was examined and other factors influencing the ²J(PC) coupling constant are discussed.     

Carbon-13 n.m.r. investigation on the nitrogen methylation of the mono- and diazanaphthalenes

The ¹³C n.m.r. spectra of the N-methylated mono- and diazanaphthalenes have been recorded and analysed. It has been shown that N-methylation as well as N-protonation in cinnoline occur predominantly at the β-nitrogen atom. N-methylation and N-protonation show a similar effect on the ¹³C chemical shifts.     

The structure of 1,2-dithioles studied by 13C n.m.r. spectroscopy

The charge separation between the dithiole ring and oxygen in 3-(1′,2′-dithiole-3′-ylidene)-6-methyl-2,3-dihydropyran-2,4-diones and 3-(1′,2′-dithiole-3′-ylidene)-2,3-dihydrobenzo[b] pyran-2,4-diones has been determined by ¹³C and ¹⁹F n.m.r. spectroscopy. By both methods it is found that these compounds are relatively polar. A correlation is established between the ¹³C chemical shift of some carbon atoms and the S2p binding energies measured by e.s.c.a. in the 1,2-dithiole derivatives. This correlation shows that these ¹³C shifts depend on the positive charge taken by the 1,2-dithiole ring and can then constitute a convenient evaluation of this charge.     

Carbon-13 n.m.r. and CNDO/S study of 1,3-indandione and heterocyclic analogues

¹³C n.m.r. spectra are reported for 1,3-indandione, phthalic anhydride, thiophthalic anhydride, and phthalimide in chloroform-d solution. The ¹³C chemical shifts have also been calculated by means of Pople's perturbation theory using CNDO/S wave functions; the agreement between theoretical and experimental values is improved when an empirical charge-dependent term is added to the calculated diamagnetic and paramagneitc contributions.     

Carbon-13 n.m.r. investigation of 2-arylaminothiazolines and analogous thiazines,thiazepines and their amides

The ¹³C n.m.r. spectra of 2-arylaminothiazoline, -thiazine and -thiazepine derivatives, and also those of some N-substituted isomeric pairs were recorded and assigned. The structure of the individual isomers, which as a result of contradictory spectroscopic data had earlier been ambiguous, was established. In addition, the tautomeric structure of some N-unsubstituted compounds was determined.     

13C n.m.r. spectroscopy: Comparison of the spectra of some dimeric Catharanthus alkaloids and their derivatives

The ¹³C n.m.r. spectra of some dimeric Catharanthus alkaloids are reported and assigned. Methods devised to aid in the assignment of resonances in complex molecules are described. The ¹³C n.m.r. spectra of several derivatives of vinblastine are discussed.