Carbon-13 NMR spectroscopy of sulfonium ylides

C-13 NMR data for a series of stabilized and unstabilized ylides indicate flattening at the ylidic carbon with respect to the parent salts. For unstabilized ylides association between the carbanionic center and the lithium cation cannot be neglected.     

Carbon-13 NMR spectroscopy of hop bitter substances

The ¹³C NMR spectra of the most important hop bitter substances are analysed. All individual carbon atoms are assigned, except the oxygen bonded sp² carbon atoms. Most assignments are made partly by comparison with analogous compounds and partly by reference to literature data. Cross-correlations with the ¹H NMR shifts are made for two main products and confirm the correctness of the assignments. All data are in agreement with the known structures.     

Carbon-13 NMR spectroscopy of some pentacyclic triterpenoids

Carbon-13 NMR signal assignment of a number of polyoxygenated triterpenes of the olean-12-ene and urs-12-ene series, carried out by considering the changes in chemical shift produced by the change of oxygenation pattern(s), and using methyl oleanolate as a model, are reported.     

Carbon-13 NMR of polybutadiene

By using the carbon-13 NMR technique, it is shown that there are no cis-1,4–trans-1,4 linkages in a n-BuLi-catalyzed polybutadiene. The polymer consists of “blocks” of cis-1,4 units and trans-1,4 units separated by isolated vinyl units. Preliminary evidence suggests this might also be true for other types of 1,3-diene polymerization. Some of the implications of this finding on the mechanism of polymerization are discussed. Tacticity triad distributions are readily determined in polybutadienes with high 1,2 addition contents.     

Carbon-13 NMR spectroscopy of some Strychnos alkaloids. Part 2

¹³C NMR data for a series of natural and semi-synthetic ar-hydroxy- and -methoxy-substituted Strychnos alkaloids are presented and are used to determine substituent-induced chemical shift (SCS) values for the various substitution patterns.     

Carbon-13 NMR of fluorocyclopropanes

Carbon-13 and hydrogen NMR data are presented for several cyclopropanes with one to four fluorine atoms attached to the ring. Substituents directly attached to a ring carbon are found to unshield that carbon to an extent increasing with their electronegativity, but remote substituents may have an opposite effect. Relative orientation of the substituent groups, especially if they are bulky, also has a strong influence on the ring-carbon shifts. Chemical shifts of carbons in methyl groups attached to the ring change to an extent which is additive in the effects of gem and cis substituents. One-bond coupling of ring carbons with fluorines is affected both by geminal and vicinal substituents, in a way generally related to their electronegativity. It is tentatively concluded that two-bond coupling between ring carbons and ring-attached fluorines depends in magnitude primarily on what groups are cis to the fluorine. Two-bond coupling between a carbon atom in a ring-attached methyl and the hydrogen gem to the methyl appears to be vanishingly small, but there is appreciable three-bond coupling of the methyl carbons to vicinal hydrogens.     

Carbon-13 NMR spectroscopy of acrylic monomer and Lewis acid complexes

¹³C NMR spectra of acrylic monomers complexed with a Lewis acid were measured and their electronic structures discussed in relation to their alternating copolymerizability. The β-carbon of acrylonitrile and methacrylonitrile showed a downfield shift due to the complex formation with the Lewis acid, while the α-carbon showed an upfield shift and the nitrile carbon showed no significant shift. The degree of shift of olefinic carbons decreased in the following order: AlCl₃ > EtAlCl₂ > Et_1.5AlCl_1.5 > Et₂AlCl > SnCl₄, EtOAlCl₂ > Et(EtO)AlCl, which seems to run parallel to the Lewis acidity and acid strength. On the other hand, the chemical shift of olefinic carbons of methyl acrylate, methyl methacrylate, and olefinic diesters was influenced little by complex formation with Lewis acids, whereas the carbonyl and alkoxyl carbons were deshielded significantly by the complex formation. These results are discussed in terms of electron distribution on the carbons and an alternating polymerization mechanism.     

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 of quinone methides

The ¹³C NMR spectra of four ortho- and seven para-quinone methides were assigned using chemical shift and long-range carbon-proton coupling information. The carbonyl shifts are compared with those in ortho- and para-benzoquinones. The chemical shifts of the carbonyls of the p-quinone methides are observed at δ 186.2–186.4 for the three ortho-di-tert-butyl-substituted compounds and at δ 180.7–181.5 for the four ortho-oxy-substituted compounds. In the three o-quinone methides with meta, para-dioxy substituents, the carbonyl signals are at δ 184.2–185.4. The carbonyl signal of the one o-quinone methide with no oxygen substitution is shifted downfield to δ 200.9, apparently as a result of hydrogen bonding to the nearby hydroxyl.     

Carbon-13 NMR studies of benzoquinones

The assignment of all ring carbons of p-benzoquinones derived from perezone and from thymoquinone was completed using gated decoupled spectra. The long range proton-carbon couplings are discussed in terms of the degree of substitution of the quinone ring. The tautomeric interconversion of the two energetically equivalent forms of 2,5-dihydroxy-1,4-benzoquinones has been studied in various solvents and at several temperatures.     

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 study of isoxazolopyridine systems

The ¹³C NMR spectra of all azabenzenoid isoxazolopyridines and some of their chloro derivatives are discussed. All ¹³C resonances were unambiguously assigned by means of gated decoupled spectra, from which one-bond and long-range ¹³C? ¹H coupling constants have been determined from line splittings.     

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 bromine-containing esters

¹³CHBrCOOR fragments of bromine-containing mono- and dicarboxylates show ¹³C NMR signals in the range 38–45 ppm, at 12–15 ppm to high field of their chloro analogues. The introduction of one bromine atom into the α-CH₂ group causes a 6–13 ppm shift of the carbon signal, depending on the type of ester. The replacement of H with Br in ¹³CH₂(COOR)₂ has only a negligible influence on the ¹³CH₂ signal position. The ¹³CBr₂COOR fragment signals appear in the range 50–54 ppm. ¹³C NMR data, including the chemical shift values, signal multiplicities and spin-spin couplings, make possible the identification of isomers present in isomer mixtures of bromine-containing esters. The ¹³COOR chemical shifts of diastereomers of α,β-dibromoesters differ noticeably from each other, and the ¹³C NMR spectra can thus be used to supply preliminary information about the stereochemistry of bromine addition to α,β-unsaturated acids.     

Carbon-13 NMR assignments in 1,3-cyclohexadiene

Unambiguous ¹³C-NMR assignments for the olefinic carbons of 1,3-cyclohexadiene (in CDCl₃: δ_C(1) = 126.02 ppm and δ_C(2) = 124.30 ppm downfield from TMS) have been made by comparison with regioselectively synthesized 2-deuterio-1,3-cyclohexadiene (in CDCl₃: δ_C(1) = 125.88, δ_C(3) = 124.22 and δ_C(4) = 126.02 downfield from TMS). ¹H-NMR chemical shifts and splittings are shown for both compounds.