Carbon-13 nuclear magnetic resonance spectra. VIII—18α- and 18β-glycyrrhetic acid derivatives

The ¹³C NMR data of six pairs of 18α/18β-glycyrrhetic acid derivatives are presented. It is shown that the configuration at C-18 can easily be recognized by inspecting the chemical shifts of several characteristic carbons, e.g. C-12, C-13, C-18 and C-28. The shifts of these carbons originated by the change of the D/E ring junction proved to be largely independent of the substitution at C-3 and C-20.     

Carbon-13 nuclear magnetic resonance spectra of some 2-substituted tetrahydropyrans

Fourier transform carbon-13 nuclear magnetic resonance spectra have been obtained and interpreted for some 2-substituted tetrahydropyrans. The effects of the substituents on α, β, γ and δ-carbon atoms are discussed. Using suitable reference compounds the γ-parameter can be used for quantitative conformational analysis.     

Mass spectra of some acyclic α,β-unsaturated aldehydes,ketones and esters

The mass spectra of a series of aliphatic acyclic α,β-unsturated aldehydes, ketones and ester have been examined. The spectra do not show evidence for McLafferty rearrangements, alkoxyl migration or for fragmentations which are dependent upon the s-trans or s-cis conformations of the compounds. There is some evidence for cis-trans isomerism about the double bond.     

The carbon-13 and nitrogen-15 nuclear magnetic resonance spectra of uroporphyrinogens I and III

Due to their sensitivity to light and air, porphyrinogens are not normally isolated, but are routinely analyzed by oxidation to the corresponding porphyrin. We report herein the ¹³C- and ¹⁵N-NMR spectra of uroporphyrinogens I and III in their “native state”, multiply labelled with ¹³C and ¹⁵N, and at natural abundance (¹³C only).     

The carbon-13 nuclear magnetic resonance spectra of anthraquinone,eight polyhydroxyanthraquinones and eight polymethoxyanthraquinones

The ¹³C NMR spectra of anthraquinone, eight hydroxyanthraquinones and eight methoxyanthraquinones are reported. Peak assignment for the carbon atoms of these compounds is achieved by using decoupling techniques and by intercomparison of the variously-substituted derivatives. Carbonyl chemical shifts in the hydroxyanthraquinones can be rationalized in terms of cross conjugation and intramolecular hydrogen bonding and in terms of cross conjugation in the methoxyanthraquinones. Hydroxy and methoxy substituent chemical shifts are also reported.     

Mass spectra of some α-substituted phenyl-α,α′-dimethoxyl ketones and their derivatives

The mass spectra of some α-substituted phenyl-α,α′-dimethoxyl ketones (compounds 1) and their 2,4-dinitrophenylhydrazones (compounds 2) and semicarbazones (compounds 3) have been studied. The characteristic fragments at m/z (M ? 73) from compounds 1, m/z (M ? 253) from compounds 2 and m/z (M ? 130) from compounds 3 are abundant and proposed to be [ArCROCH₃]⁺. Fragmentations yielding [M⁺ ? 49] from compounds 2 are abnormal and probably involve the methoxyl and nitro groups. The intense peak at m/z 130 due to [CH₃OCH₂CNNHCONH₂]⁺ from compounds 3 corresponds to α-cleavage of the molecular ion. Some other fragments from these new compounds are interpreted in this paper.     

The high resolution mass spectra of α,β-unsaturated and α-chloromercuri-α,β-unsaturated steroidal ketones

The high resolution mass spectra (500 eV) of some α,β-unsaturated steroidal ketones have been studied and compared with the spectra of the corresponding α-chloromercuri ketones. In the latter, the carbon-mercury bond frequently remains intact at the expense of the fission of two carbon-carbon bonds. The abundance of mercury-containing ions allows the use of the mercury atom fingerprint in confirming ring B fragmentation of the steroid nucleus at C(6)–C(7) and C(9)–C(10) for 5α-androst-1-ene-3,17-dione, 1,4-androstadiene-3,17-dione and their 2-chloromercuri derivatives; and at C(7)–C(8) and C(9)–C(10) for 1,4,6-androstatriene-3,17-dione, 1,4,6-androstarien-17 β-ol-3-one and their 2-chloromercuri derivatives. 2-Chloromercuri-1,4,6-androstatriene-3,17-dione and 2-chloromercuri-1,4,6-androstatrien-17 β-ol-3-one also give an abundant ion as the result of ring C fragmentation at C(8)–C(14) and C(11)–C(12), the chloromercuri group being replaced by a hydrogen atom. This ring C cleavage gives the only recognizable distinctive fragmentation ion for 1,4,6-pregnatriene-3,20-dione and 2-chloromercuri-1,4,6-pregnatriene-3,20-dione. For most of the mercurated steroids, the low resolution mass spectra (70 eV) are reported. In these spectra, the fragmentation patterns are similar to those obtained using the higher ionization energy employed for the high resolution spectra.     

Carbon-13 nuclear magnetic resonance spectra of some aromatic diterpenoids

¹³C n.m.r. studies of a series of tricarbocyclic ring C aromatic diterpenoids using proton-noise and single-frequency off-resonance decoupling, partially relaxed Fourier transform techniques, shift reagents and specifically labelled derivatives have permitted unequivocal assignments of almost all signals. The shieldings caused by oxygenation of C-18 and C-19 and by introduction of various substituents in the aromatic ring are discussed. It is concluded that the ¹³C n.m.r. data are sufficiently characteristic to allow stereochemical assignments. Some effects of deuterium substitution on the carbon resonances are presented.     

Carbon-13 nuclear magnetic resonance spectra of some heterocyclic D-homoandrostanes

The ¹³C chemical shift data of several D-homoandrostanes with heteroatoms (N,O) in the 17a-position are reported. Heteroatom effects on the shieldings of the carbons of rings C and D are discussed.     

Carbon-13 nuclear magnetic resonance spectra of 1,2,3,4-tetrahydroquinazoline and some 3-benzyl derivatives

The concerted use of the ¹H/¹³C shift correlation with full ¹H/¹H decoupling and the modified COLOC sequence for long-range X/H correlation 2D nmr spectroscopy allows the total assignment of the ¹³C nmr resonances of 1,2,3,4-tetrahydroquinazoline and 3-benzyl-1,2,3,4-tetrahydroquinazoline. From these data, the ¹³C nmr chemical shifts of a series of 3-benzylsubstituted-1,2,3,4-tetrahydroquinazolines are deduced.     

Hierarchical clustering of carbon-13 nulear magnetic resonance spectra

The problems of preprocessing of ¹³C-n.m.r. spectra for hierarchical clustering are discussed. Encoding of the spectra in nonequidistant intervals is proposed. To establish the optimal intervals, a Simplex method with variable-sized movements is used. The optimized parameter is the amount of information contained in the first two coordinates of the transforms, obtained by the application of principal component analysis to the ¹³C-n.m.r. spectra. The spectra encoded in optimized intervals are used for automated structure elucidation, based on a hierarchical organization of a collection of more than 2000 assigned¹³C-n.m.r. spectra. The hierarchical trees needed for the library search and prediction of some structural features were generated by a 3-distances clustering method. The retrieval and predictive abilities of the system are discussed.     

Stereoselective synthesis of cyclohexa-2,4-dien-1-ones and cyclohex-2-en-1-ones from phenols

A convenient synthetic method for the synthesis of substituted cyclohex-2-en-1-ones by the direct alkylation of phenols has been developed. Furthermore, enantiomerically enriched 2,6-dimethyl-6-(3-methylbut-2-enyl)-cyclohexa-2,4-dienone was prepared by the deprotonation of 2,6-dimethylphenol with a sparteine–lithium complex followed by alkylation with 1-chloro-3-methylbut-2-ene. 2,6-Dimethyl-6-(3-methyl-but-2-enyl)-cyclohex-2-enone was prepared from the corresponding cyclohexa-2,4-dien-1-one by selective hydrogenation of the 4,5-double bond. The method was extended to 2-methyl-naphthalen-1-ol and 1-methyl-naphthalen-2-ol resulting in 2-(R)-methyl-2-(3-methylbut-2-enyl)-2H-naphthalen-1-one and 1-(S)-methyl-1-(3-methylbut-2-enyl)-1H-naphthalen-2-one, respectively.     

Carbon-13 nuclear magnetic resonance spectra of some methyl and phenyl substituted 2H-azirines

Carbon-13 n.m.r. spectra have been obtained for some methyl and phenyl substituted 2H-azirines. The higher field resonance of C-2 than that of the corresponding aziridine carbon is interpreted in terms of ring strain. Substituent effects on the chemical shifts of the azirine ring carbons are discussed. A set of additivity parameters for the methyl and phenyl groups are obtained which can be used for the calculation of the chemical shifts of the azirine ring carbons. The substituent effect of an azirine ring on the chemical shift of benzene is also discussed in comparison with those of some other substituents. A high degree of s character (48.5%) in the exocyclic orbital of C-3 is indicated by a large J(¹³C-3,H) value (242.5 Hz).     

13C nuclear magnetic resonance spectra. XV—the γ-anti substituent effect and its dependence on substitution at the α- and γ-carbon atom

It is shown that the γ_α effects of hetero substituents on the ¹³C chemical shift (γ_α SCS) are enlarged by +2 to +5 ppm by substituting the α-hydrogen atom by any group or atom (e.g. CH₃, OR, F, Cl, Br). The same is encountered when the axial γ-hydrogen is replaced by CH₃, OH or F. If, however, the substituting atom at the γ_α-carbon atom is a higher-row halogen (Cl or Br), diamagnetic γ_α SCS for this signal are observed which may even exceed those for unsubstituted γ_α-carbon atoms. The removal of a 1,3-diaxial hydrogen-hydrogen interaction and the existence of a still unspecified ‘heavy halogen effect’–both diamagnetic contributions to the γ_γ SCS of hetero substituents–are responsible for these findings. Methyl groups do not behave like hetero substituents with respect to the γ_α SCS.     

Carbon-13 nuclear magnetic resonance spectra of some mesoionic xanthine analogs

Natural abundance ¹³C NMR chemical shifts have been experimentally determined for a series of mesoionic thiazolo[3,2-a]pyrimidine-5,7-diones. The spectral data are compared with those of related mesoionic dihydrothiazolo[3,2-a]pyrimidine-5,7-diones and mesoionic 1,3,4-thiadiazolo[3,2-a]pyrimidine-5,7-diones. Resonable correlation between the observed ¹³C NMR chemical shifts and CNDO/2 total charge densities have been obtained for the different carbon atoms of 8-methylthiazolo[3,2-a]pyrimidine-5,7-dione.     

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.     

Carbon-13 nuclear magnetic resonance spectra of nicotine metabolites and related compounds

The potential of ¹³C NMR techniques for studies of nicotine metabolism has been investigated by acquisition and assignment of ¹³C NMR spectra of nicotine, its metabolites and some related compounds. It is shown that C-2′ of nicotine would be the best site for ¹³C labelling when using the method to gain further insight into the metabolic pathways of nicotine.     

The mass spectra of some α-lactams

The electron-impact induced fragmentation of eight aziridinones has been studied by conventional as well as by high resolution mass spectrometry. All α-lactams exhibit a molecular ion. The major primary step, in the fragmentation, is the ejection of carbon monoxide from the molecular ion. Ions of the general formula R₁? NC and R₂R₃C?O were found in the mass spectra of all α-lactams investigated. A skeletal rearrangement to rationalize these ions is proposed. The fragmentation of the molecular ion is affected by the N-substituent. Exact mass measurement and specific deuterium labeling indicate the absence of McLafferty rearrangement from either the N- or C-substituent.