Carbon-13 chemical shifts of α,β-unsaturated acids

Carbon-13 spectra of 59 α,β-unsaturated carboxylic acids have been measured. Large differential shieldings of ethylenic carbons in the cis and trans isomers of acrylic acid derivatives were found (altogether 15 isomeric Z- and E-pairs of acids were investigated). The origin of differential shieldings is complex and both molecular ground states as well as changes in excited states appear to be involved. All measured ¹³C chemical shifts can be described by additive parameters that provide a straightforward new technique by which structural assignments can be made for a wide variety of isomeric mono-, di- and tri- substituted α,β-unsaturated acids.     

13C Pulse fourier transform NMR of menthol stereoisomers and related compounds

¹³C NMR spectra of menthol stereoisomers have been determined. The correlations of chemical shifts of these ring carbons with those of stereoisomeric 2-isopropylcyclohexanols are examined. Observed chemical shifts of 1-Me carbons are compared with those predicted from the chemical shifts of stereoisomeric 1-methyl-4-t-butylcyclohexanes. ¹³C NMR spectra of menthyl acetates, and cis and trans p -menthanes have also been examined.     

Carbon-13 n.m.r. studies. Part 66—13C spectra of several methyl,hydroxymethyl and carboxylic acid derivatives of bicyclo[2.2.1]heptane and bicyclo[2.2.2]octane

The ¹³C n.m.r. spectra of a series of 52 substituted norbornanes, norbornenes, bicyclo[2.2.2]octanes and -octenes have been recorded to examine the stereochemical effects of closely neighboring substituents on the carbon shieldings of these well-defined molecular frameworks. These moderately rigid skeletons permit detailed examination of a variety of orientations of substituents and, in this study, attention has been focused on a series of vicinally substituted systems. The substituents include methyl, hydroxymethyl and carboxylic acid groups for comparison with earlier data for several bicyclic alcohols and hydrocarbons, from which conclusions regarding the conformational preferences of the hydroxyl group may be drawn. The deviations of the observed shieldings from those predicted by simple additivity follow definite patterns which should be useful for stereochemical assignments in related systems.     

Carbon-13 spectra of polycyclic systems Tricyclo[3.2.1.02,7]octanes as models for some tetracyclic sesquiterpenes and pentacyclic diterpenes. Ishwarane and trachylobane derivatives

The ¹³C spectra of tricyclo[3.2.1.0^2,7]octane, its 1,5-dimethyl derivative and ten tricyclo[3.2.1.0^2,7]octan-6-ones have been recorded as model compounds to aid the interpretation of the spectra of the naturally occurring polycyclic terpenes, ishwarane, ishwarone, ishwarol and methyl trachylobanate. The assignments for specific carbons in this series are based on the internal consistency of the observed shielding trends together with the effects of shift reagent on the shieldings of some of these derivatives.     

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.     

(1)H and (13)C NMR studies of asymmetrically substituted bis- and tris-Tröger's bases

The (1)H and (13)C NMR spectra of two stereoisomeric bis-Tr?ger's bases and four stereoisomeric tris-Tr?ger's bases asymmetrically substituted on the external aromatic rings were recorded and the corresponding signals assigned. The relative configuration of the stereogenic units has been unequivocally determined on the basis of homoallylic couplings and NOE experiments.     

Investigation of substituent effects of chalcones by 13C n.m.r. spectroscopy

¹³C chemical shifts for 23 para- and meta-substituted chalcones of the types 1 and 2 have been determined. The aromatic shieldings are compared with previous results for other aromatic derivatives. Correlations of the ¹³C chemical shifts of vinyl carbons and carbonyl carbons as well as ring carbons with Hammett σ parameters, π electron densities and the reactivity parameters of Swain and Lupton provide a consistent picture of electronic effects transmitted through the carbon framework of the compounds studied.     

Mass spectrometry in structural and stereo-chemical problems—CCXXIV: Fragmentation of cyclopropanes

Generalizations for interpreting the mass spectra of cyclopropanes are enumerated and are based on the examination of many deuterium labeled substrates. Most cyclopropyl fragmentations can be visualized as resulting from ionization of the most substituted cyclopropyl bond in which the positive charge is localized on the most substituted carbon atom. Subsequent hydrogen shifts and/or simple scissions produce an array of predictable fragment ions.     

Configurations and conformations in acyclic,unsaturated hydrocarbons. A 13C NMR study

¹³C NMR (CMR) spectra of a number of di- and trisubstituted ethylenes have been measured. Very consistent values are found for the differential shieldings of allylic carbons in a number of linear, (Z)- and (E)-disubstituted ethylenes. The discrepancies between the several structural elements are explained tentatively by assuming different rotational preferences in the (Z)-isomers, the (E)-isomers, or both. The results obtained for some branched olefins are qualitatively in agreement with conclusions reached in previous ¹H NMR (PMR) investigations, with the possible exception of 2,5-diMe-3-hexene. Differential shieldings in trisubstituted ethylenes are larger than in disubstituted ethylenes, probably as a consequence of steric interaction between geminal substituents. It is confirmed, that API's recent relabelling of the isomers of 3-Me-2-hexene was justified. Furthermore, the (E)-configuration is assigned to the high-boiling isomer of 3,4,4-triMe-2-pentene. On the other hand, the high-boiling isomer of 3-Et-4-Me-2-pentene is shown to be the (Z)-isomer. For the isomeric pair of geraniol and nerol, the (E)-configuration is assigned to the former and the (Z)-configuration to the latter, in agreement with our recent PMR investigations.     

Synthesis of functionalized dihydrothiophenes from doubly activated cyclopropanes using tetrathiomolybdate as the sulfur transfer reagent

A number of doubly activated cyclopropanes were synthesized starting from various substituted bromosulfonium bromides in good yield. Regioselective ring-opening of cyclopropanes with tetrathiomolybdate as the sulfur transfer reagent gave dihydrothiophenes in excellent yield.     

Stereospecificity of the addition of hydrogen cyanide to the carbonyl group of substituted 4-piperidones. Synthesis and three-dimensional structures of substituted 4-hydroxy-4-cyanopiperidines

Mixtures of stereoisomers of the corresponding 4-hydroxy-4-cyanopiperidines were synthesized by the reaction of substituted 4-piperidones with hydrogen cyanide by an exchange reaction with acetone cyanohydrin. The ratios of the stereoisomers in the resulting mixtures of stereoisomeric 4-piperidone cyanohydrins were determined by means of the ¹³C NMR spectra, and the three-dimensional structures were established by direct determination of the orientation of the cyano group in their molecules. A dependence of the quantitative ratios of the stereoisomeric cyanohydrins of the substituted 4-piperidones on the reaction temperature was demonstrated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1359–1363, October, 1990.     

Tryptophan chemical shift in peptides and proteins: a solid state carbon-13 nuclear magnetic resonance spectroscopic and quantum chemical investigation

We have obtained the carbon-13 nuclear magnetic resonance spectra of a series of tryptophan-containing peptides and model systems, together with their X-ray crystallographic structures, and used quantum chemical methods to predict the (13)C NMR shifts (or shieldings) of all nonprotonated aromatic carbons (C(gamma), C(delta 2) and C(epsilon 2). Overall, there is generally good accord between theory and experiment. The chemical shifts of Trp C(gamma) in several proteins, hen egg white lysozyme, horse myoglobin, horse heart cytochrome c, and four carbonmonoxyhemoglobins, are also well predicted. The overall Trp C(gamma) shift range seen in the peptides and proteins is 11.4 ppm, and individual shifts (or shieldings) are predicted with an rms error of approximately 1.4 ppm (R value = 0.86). Unlike C(alpha) and N(H) chemical shifts, which are primarily a function of the backbone phi,psi torsion angles, the Trp C(gamma) shifts are shown to be correlated with the side-chain torsion angles chi(1) and chi(2) and appear to arise, at least in part, from gamma-gauche interactions with the backbone C' and N(H) atoms. This work helps solve the problem of the chemical shift nonequivalences of nonprotonated aromatic carbons in proteins first identified over 30 years ago and opens up the possibility of using aromatic carbon chemical shift information in structure determination.     

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.     

13C n.m.r. of organosulphur compounds: II—13C chemical shifts and conformational analysis of methyl substituted thiacyclohexanes

¹³C n.m.r. spectra of methyl substituted thianes and thianium cations have been determined. The magnitude of the ¹³C substituent effects of an equatorial methyl group or of a gem-dimethyl grouping appear to depend in a systematic way on whether the carbon atom concerned is adjacent to, or removed from, the heteroatom. The shieldings are discussed in relation to the conformational properties of the thiane ring. Moreover, the average ¹³C substituent parameters obtained from conformationally biased systems are applied to potentially mobile systems to assess the position of the conformational equilibrium.     

Factors affecting the computation of the 13C shielding in disaccharides

Knowledge of the three‐dimensional structures of glycans and glycoproteins is useful for a full understanding of molecular processes in which glycans are involved, such as antigen‐recognition and virus infection, to name a few. Among the ubiquitous nuclei in glycan molecules, the ¹³C nucleus is an attractive candidate for computation of theoretical chemical shifts at the quantum chemical level of theory to validate and determine glycan structures. For this purpose, it is important to determine, first, which carbons can be used as probes to sense conformational changes and, second, all factors that affect the computation of the shielding, at the density functional theory (DFT) level of theory, of those carbons. To answer such questions, we performed a series of analyses on low‐energy conformations, obtained by sampling the glycosidic torsional angles (?, ψ) every 10°, of 12 disaccharides. Our results provide evidence that: (i) the carbons that participate in the glycosidic linkage are the most sensitive probes with which to sense conformational changes of disaccharides; (ii) the rotation of the hydroxyl groups closest to the glycosidic linkage significantly affects the computation of the shieldings of the carbons that participate in the glycosidic linkage; (iii) it is not possible to obtain the shieldings of one disaccharide from the computed values of a different disaccharide or from those disaccharides that differ in the anomeric state; and (iv) a proper basis set distribution, a functional, and a step size, with which to sample the conformational space, are necessary to compute shieldings accurately and rapidly. © 2014 Wiley Periodicals, Inc.     

Ab initio thermochemistry of some halogenated cyclopropanes

The standard enthalpies of formation for a series of chloro- and fluoro-substituted cyclopropanes have been calculated by using high-level ab initio G3/B3LYP methods. The relative stabilities of isomers and the influence of substituents on thermochemistry in several classes of substituted derivatives are discussed.     

13C NMR spectra of biologically active compounds.

The¹³C spectra of 25 new substituted cyclopropanes of the pyrethoid series have been investigated and a stereochemical assignment has been made of the stereoisomers formed. As the criterion for stereochemical assignment it is proposed to use the values of the chemical shifts for the -carbon atoms of the substituents in the cyclopropane ring.Institute of Chemistry, Bashkir Branch, USSR Academy of Sciences, Ufa. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 266–272, March–April, 1989.     

反-1,2-双[2-(5-取代苯基恶唑基)]环丙烷的合成与光性能的研究

合成了8个反 -1,2-双[2-(5-取代苯基恶唑基)]环丙烷和2个1,2-双[2-取代苯基恶唑基)]乙烷,其中9个为新化合物.讨论了化合物的结构与其电子光谱及荧光量子产率间的关系.发现恶唑环与三元环间存在一定程度的共轭,并解释了上述化合物荧光量子产率较低的现象.     

Diastereoselective michael initiated ring closure on vinyl sulfone-modified carbohydrates: a stereospecific and general route to alpha-substituted cyclopropanes

Suitably designed vinyl sulfone-modified furanosides act as substrates for Michael initiated ring closure reactions yielding cyclopropanated carbohydrates. The strategy is general in nature and gives access to cyclopropanes with predefined chiralities on three consecutive carbons with varying substitutions at the alpha-position.     

Organic photochemistry with 6.7eV photons: decomposition of 1,1-disubstituted cyclopropanes in solution

The course of the two-bond cleavage reactions of cyclopropanes in photolysis at 185nm in solution is strikingly influenced by disubstitution of one of the carbons in the ring.