Carbon-13 nuclear magnetic resonance studies of anthraquinones Part II—hydroxymethoxyanthraquinones,acetoxymethoxyanthraquinones and naturally occuring anthraquinone analogues

The ¹³C NMR spectra of hydroxymethoxyanthraquinones, acetoxymethoxyanthraquinones and naturally occuring anthraquinone derivatives are reported and all chemical shifts assigned. Hydroxy, methoxy and acetoxy substitution parameters are additive except in the case of 1,2-disubstitution and for 1-hydroxyanthraquinones substituted on C-3 or on C-4 by an hydroxy or a methoxy group. In anthraquinones substituted on C-2, carbon shieldings are found to be sensitive to steric interference and to conjugative electron release by the 1-OCH₃.     

The assignment of the 1H and the 13C NMR chemical shifts of N-phenyl-2,4-dimethylbuta-1,3-diene-1,4-sultam

The assignment of the signals in the ¹³C and ¹H NMR spectra of N-phenyl-2,4-dimethylbuta-1,3-diene-1,4-sultam is difficult for the signal pairs C-2 and C-4, C-1 and C-3, (C-1)? H, (C-2)? CH₃ and (C-4)? CH₃. The ¹³C NMR spectrum recorded under gated decoupling conditions provide long-range couplings which make possible an unambiguous assignment of the ¹³C NMR signal pairs. Application of the ¹H CW off-resonance decoupling technique in recording the ¹³C NMR spectra enables the assignment information from the ¹³C NMR spectrum to be transferred to the ¹H NMR spectrum.     

13C NMR studies of some hydroxycoumarins and related compounds

The ¹³C chemical shifts and one-bond carbon-hydrogen coupling constants have been obtained for some hydroxycoumarins and their corresponding acetoxy and methoxy derivatives. The changes in the one-bond carbon-hydrogen coupling constants resulting from the conversion of a hydroxy group to an acetoxy group represent a simple method of assignment of the ¹³C NMR signals in coumarins which contain one or more hydroxy groups in the benzenoid ring.     

Stereochemical studies of substituted 6,7- benzomorphan derivatives by 13C and 1H NMR spectroscopy

The ¹³C NMR spectra of a series of 6,7-benzomorphan derivatives variously substituted at C-5 and C-9 by methyl and at C-3 by cyano, alkyl and aralkyl groups, together with certain 3-cyano, 3-allyl or benzyl congeners, are reported and chemical shift data analysed in terms of the configuration of isomeric pairs and compounds isolated as single diastereoisomers. Special attention is given to the consequences of γ-shielding interactions, the effects of the nitrogen lone-pair orbital and anisotropic shielding by the aromatic region of the molecule. Deductions of stereochemistry are supported by ¹H NMR data and the NMR features of the corresponding methiodide salts.     

Acetylation of secondary hydroxy groups of α- and β-cyclodextrines silyl derivatives

The application of acetyl chloride in the combination with different solvents and bases permitted the preparation of silyl derivatives of α- and β-cyclodextrines containing a definite amount of acetyl substituents on the secondary hydroxy groups. It was found that by means of the ¹H and ¹³C NMR spectroscopy it is possible to make an exact attribution of acetyl groups to C² or C³ carbon atoms of carbohydrate fragments of α- and β-cyclodextrines. Desilylation with ammonium fluoride in methanol gives acetyl derivatives of cyclodextrines containing free primary hydroxy groups.     

Carbon-13 NMR investigation of 2,5-diaryl-1,4-dithiins

The ¹³C NMR spectra of eight 2,5-diaryl-1,4-dithiins were recorded and signals were assigned. A linear correlation was observed between the electronegativity of the substituent groups on C-10,10′ and the chemical shifts of C-10,10′ after applying corrections for the magnetic anisotropic effect of the substituents. A Hammett correlation was found between the ¹³C chemical shifts of C-3,6 and C-7,7′ and the σ_p⁺ parameter associated with the substituents on C-10,10′. Extended electronic interaction between the π system of the aryl group and the π system of the dithiin ring was suggested by the observance of an alternating behavior in the magnitude of the substituent effects on the ¹³C shifts of C-2,5 and C-3,6. An alternating effect was also noted in the magnitude of the long-range ¹³C? F coupling constants for these same carbon signals in 2,5-(10,10′-difluoro)diphenyl-1,4-dithiin.     

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.     

Determination of the geometric configuration of the polyene chain of mono-cis c40 carotenoids II : A 13CNMR study of mono-cis luteins and mono-cis capsanthins

Systematic chromatography has resulted in the isolation of four geometric isomers as products of iodine-catalysed isomerisation of lutein, ¹³C NMR spectral analysis has established their stereochemistry as mono-cis luteins with a cis-configured double bond at C-9. C-9', C-13 and C-13', respectively. Carbon-13 NMR has also provided unambiguous stereochemical assignment for the previously isolated four mono-cis capsanthins.     

Inside Cover: Parahydrogen-Induced Carbon-13 Radiofrequency Amplification by Stimulated Emission of Radiation (Angew. Chem. Int. Ed. 5/2023)

The feasibility of Carbon-13 Radiofrequency (RF) Amplification by Stimulated Emission of Radiation (C-13 RASER) is demonstrated on a bolus of liquid hyperpolarized ethyl [1-¹³C]acetate. Hyperpolarized ethyl [1-¹³C]acetate was prepared via pairwise addition of parahydrogen to vinyl [1-¹³C]acetate and polarization transfer from nascent parahydrogen-derived protons to the carbon-13 nucleus via magnetic field cycling yielding C-13 nuclear spin polarization of approximately 6 %. RASER signals were detected from samples with concentration ranging from 0.12 to 1 M concentration using a non-cryogenic 1.4T NMR spectrometer equipped with a radio-frequency detection coil with a quality factor (Q) of 32 without any modifications. C-13 RASER signals were observed for several minutes on a single bolus of hyperpolarized substrate to achieve 21 mHz NMR linewidths. The feasibility of creating long-lasting C-13 RASER on biomolecular carriers opens a wide range of new opportunities for the rapidly expanding field of C-13 magnetic resonance hyperpolarization.     

Amphiphilic conjugates of β-cyclodextrin with acetylsalicylic and 2-(4-isobutylphenyl)propionic acids

New amphiphilic β-cyclodextrin derivatives containing pharmacologically important aromatic and aliphatic monocarboxylic acid fragments at both primary and secondary hydroxy groups were synthesized with the use of palmitic, acetylsalicylic, and 2-(4-isobutylphenyl)propionic acid chlorides. The position of the acyl groups in the carbohydrate fragments of β-cyclodextrin was determined by ¹³C NMR spectroscopy.     

A revised assignment of the 13C NMR spectra of α- and β-pinenes

α-Pinene and β-pinene ¹³C enriched at the C-10 position were prepared by an unambiguous synthesis. ¹³C NMR spectral analysis provided evidence for a revised assignment for α-pinene at the C-9 and C-10 carbon atoms.     

13C chemical shifts and conformational analysis of S-methylthiolanium cations

The ¹³C NMR spectra of all cations obtained by methylation at sulphur of the mono-and dimethylthiolanes are reported. The methyl substituent on sulphur affects the shieldings of the adjacent carbons in a manner which allows easy identification of the cis and trans isomers. For most compounds the ¹³C pattern is consistent with a half-chair ring conformation with maximum staggering at C-3, C-4. Only with methyl groups at the 1,2-or 1,2,3-positions is the half-chair appreciably deformed. It is suggested that in these cases the preferred conformation is a quasi-envelope with C-3 at the top.     

α-Cyclodextrin compounds containing benzoic,acetylsalicylic, and 2-(4-isobutylphenyl)propionic acid residues

Selectively substituted derivatives of α-cyclodextrin and hexa-O-tert-butyldimethylsilyl-α-cyclodextrin containing a definite number of acylated primary or secondary hydroxy groups were synthesized using benzoic, acetylsalicylic, and 2-(4-isobutylphenyl)propionic acid chlorides in various solvents in the presence of different bases. The acyl residues were assigned to the C², C³, or C⁶ atoms in the carbohydrate fragments on the basis of ¹³C NMR data. Desilylation of the silyl derivatives with ammonium fluoride in methanol gave the corresponding O-acyl derivatives having free primary hydroxy groups in the α-cyclodextrin skeleton.     

Application of two-dimensional NMR spectroscopy to the complete analysis of the 1H and 13C NMR spectra of d-biotin in aqueous solution

The ¹H and ¹³C NMR spectra of d-biotin were observed at 400 and 100 MHz, respectively. Various types of two-dimensional NMR spectroscopy were performed to assign the spectra. The previous assignment of ¹³C NMR spectrum of d-biotin reported by Bradbury and Johnson was modified, and the dihedral angles between the C? H bonds of the ring were determined. The populations of the conformers produced by internal rotation around the C-2? C-δ bond were estimated.     

13C and 1H NMR spectra of monosubstituted ferrocenes containing a chiral centre in the substituent

The ¹³C-{¹H} and ¹H NMR spectra of ferrocenyl chalkones (β-benzoylvinylferrocene add cinnamoylferrocene) and their iron carbonyl complexes were recorded. Splitting of the resonances of both α- and β-substituted cyclopentadienyl ring atoms was found in ¹³C-{¹H} NMR spectra of all complexes. In ¹H NMR spectra splitting of the resonances of the substituted cyclopentadienyl ring α-protons was detected only in the case of (cinnamoylferrocene)iron tricarbonyl. The splitting effect mentioned can be due to thesochsochronism (magnetic nonequivalence) of all carbon and hydrogen nuclei because of the iron carbonyl fragment coordination oauses the substituent to become the chiral group. The coordination of iron carbonyl group results in a reduction of the conjugation in the α,β-unsaturated ketone system. This phenomenon is discussed on the basis of ¹³C-{¹H} and ¹H NMR data.     

13C NMR substituent effects in tetracyclic diterpenoids

The ¹³C NMR chemical shifts of ent-beyer-15-ene, and one hydroxy and four acetoxy derivatives are reported. The substituent effects of the acetoxy groups upon the saturated and olefinic carbon atoms are discussed. Conformational aspects of the molecules are considered in the light of these effects.     

Méthylène-indolines,indolénines et indoléniniums—XIII: L'hydroxy-16 déhydro-1 vincadifformine,intérmédiaire dans le réarrangement biomimétique de la vincadifformine en vincamine

The title compound is a crucial intermediate in the biomimetic conversion of vincadifformine 1 into vincamine 7. Its configuration at C-16 is established by a combination of chemical and spectroscopic evidence. Iodine oxidation converts 14 into the bridged lactam 18, thus proving a β configuration for the hydroxy group at C-16. The same reaction applied to vindoline 19 gives 21 identical with one of the compounds obtained by microbiological transformation of 19. The ¹³C NMR spectra of derivatives 3 and 8 (obtained by oxidation of vincadifformine) show that oxidation proceeds with introduction of the substituent at C-16, with a β-configuration. The alcohol 3 however, posesses a different conformation due to strong hydrogen bonding with N-4.     

Nine sesquiterpenes from Solanum torvum

Three new sesquiterpenes, namely 3β,11-dihydroxy-4,14-oxideenantioeudesmane (1), 1β,10β,12,14-tetrahydroxy-allo-aromadendrane (2) and 1β,10β,13,14-tetrahydroxy-allo-aromadendrane (3), along with six known sesquiterpenes (4–9), were isolated from the roots of Solanum torvum. Compound 4 and 5 are epimers, their main difference lies in the C-11 configulation. Normally, epimers do not make a huge difference in C NMR spectra, but in this kind of structure of A, B, C rings, and C ring is sterically strained structure, stericall effects influence strongly the ¹³C NMR chemical shifts, when C-11 configulation changed, it makes a huge difference in the three ring of structure, such as C-6, C-7, C-11. New compound 2 and 3 are epimers and similar to compound 4 and 5, their just increase a hydroxy in C-1 and have a same regular pattern in C NMR spectra, otherwise, compound 5 was firstly confirmed by single-crystal X-ray diffraction.     

The structure of 1,6,6aλ4-trithiapentalene and substituted 1,6,6aλ4-trithiapentalenes studied by means of NMR spectroscopy

¹³C NMR spectra of unsubstituted 1,6,6aλ⁴-trithiapentalene and a series of methyl and phenyl substituted 1,6,6aλ⁴-trithiapentalenes have been recorded. The spectra have been assigned by comparison with ¹H NMR data based on coupling constants from undecoupled spectra. From the chemical shift of C-3a it is concluded that the double bond character of the C-3a—S-6a is low. The relaxation times for C-2 and C-3 only show small differences; this seems not to be in favour of a fast interconversion between two thiocarbonyl forms. The NMR data obtained seem thus to be in accordance with a bicyclic naphthalene-like structure with ten delocalised π-electrons.