Acylation of the 4,5- and 5,6-double bond isomers of 3-steroidal thiazolidines: The chemical stability of spiro-3-steroidal 4,5- and 5,6-double bond isomers

It has been shown that the 5,6-double bond steroidal thiazolidines can be N-acylated if there is no substituent in the 4'-position on the thiazolidine ring; substituents in the 4'-position of the thiazolidine sterically hinder acylation. On the other hand, the 4,5-double bond steroidal thiazolidine isomers decomposed on attempted acylation. The lability of these 4,5-double bond isomers was attributed to the contribution of a hyperconjugative resonance form to the structure of the 4,5-double bond isomers.     

Electron structures and 13C NMR spectra of 3-substituted coumarin derivatives

Quantum-chemical calculation of the electron densities on the atoms and the Wiberg indexes in coumarin and 3-amino-3-hydroxy-, and 3-carboxycoumarin molecules within the CNDO/2 approximation was accomplished. The ¹³C NMR spectra of the indicated compounds were recorded. The effect of the nature of the substituent in the pyrone ring on the electron structures of 3-substituted coumarin derivatives was evaluated on the basis of the data obtained. The character and degree of the relationship between the chemical shifts and the electron densities on the carbon atoms were established by means of regression analysis.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1611–1614, December, 1988.     

Structure determination of di-O-benzylidene derivatives of L-iditol by 1H and 13C NMR spectroscopy

The configuration and stable conformation of two dibenzylidene-L -iditol isomers formed upon benzalation of L -iditol, as well as that of a third isomer obtained by partial hydrolysis of the tribenzylidene derivative—the main product of the benzalation reaction—were determined by ¹H and ¹³C NMR spectroscopy.     

Complete 1H and 13C NMR assignments and Z/E-stereoconfiguration determination of isomers of 1,4-diketone derivatives

Complete 1H and 13C NMR assignments and Z/E-stereoconfiguration determination for a series of new isomers of 1,4-diketone derivatives obtained via self-sorting tandem reaction were accomplished by means of one- and two-dimentional NMR experiments including 1H, 13C, gCOSY, gHSQC, gHMBC, and NOESY.     

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.     

The 5,6- and 4,5-benzo derivatives of 1-hydroxy-7-azabenzotriazole

[structure: see text]. Syntheses of the two benzo derivatives of HOAt are described. Conversion of the two isomers to the corresponding onium-style coupling reagents gave in one case a guandinium species 14 and in the other, presumably as a result of steric factors, a uronium species 15. The two systems are compared as to their effectiveness in peptide coupling processes.     

Comparative analysis of 13C shielding constants stereospecificity in the silicon and germanium derivatives of acetylenic aldehyde and ketone oximes based on the 13C NMR spectroscopy and GIAO calculations

In the acetylenic aldehyde oximes with substituents containing silicon and germanium, the ¹³C NMR signal of the C‐2 carbon of triple bond is shifted by 3.5 ppm to lower frequency and that of the C‐3 carbon is moved by 7 ppm to higher frequency on going from E to Z isomer. A greater low‐frequency effect of 5.5 ppm on the C‐2 carbon signal and a greater high‐frequency effect of 11 ppm on the C‐3 carbon signal are observed in the analogous acetylenic ketone oximes. The carbon chemical shift of the C?N bond is larger by 4 ppm in E isomer relative to Z isomer for the aldehyde and ketone oximes. The ²⁹Si chemical shifts in the silicon containing acetylenic aldehyde and ketone oximes are almost the same for the diverse isomers. The trends in changes of the measured chemical shifts are well reproduced by the gauge‐including atomic orbital (GIAO) calculations of the ¹³C and ²⁹Si shielding constants. Copyright © 2009 John Wiley & Sons, Ltd.     

The identification of vicinally substituted cyclohexane isomers in their mixtures by 1H and 13C NMR spectroscopy

The radical addition reactions of organobromine compounds, XBr (X = CH2COOMe, PhCH2, CHBr2 and CCl3) with cyclohexene afforded mixtures of cis/trans isomer pairs of 1-X-2-Br-cyclohexanes. In addition to benzyl benzoyloxy derivatives are formed also, when benzoyl peroxide is used as an initiator. Owing to the great difficulties in separating these cis/trans isomer pairs, they are identified directly in their mixtures by NMR spectroscopy. In addition to one-dimensional (ID) 1H, proton decoupled 13C and DEPT-135, also two-dimensional (2D) 13C-13C INADEQUATE as well as 1H-13C HMQC experiments have been used in assigning the signals of each compound in their mixtures. The identification of each isomer was based on comparison of experimental 3J(H,H) coupling constants with theoretical ones based on the well-known Karplus type relationship. The more stable conformation for each isomer was estimated using the semiempirical AM1 molecular orbital method. The calculations support the isomer pair elucidations.     

1H and 13C NMR spectra and three-dimensional structures of 1,2,5-trimethyl-4-phenylaminopiperidine isomers

The three-dimensional structures of two isomers of 1,2,5-trimethyl-4-phenylaminopiperidine were established on the basis of an analysis of their ¹H and ¹³C NMR spectra. The piperidine ring has a chair conformation in both isomers. The isomer has the 1,2e,5a, trimethyl-4e-phenylaminopiperidine structure, while the isomer has the 1,2e,5e-trimethyl-4e-phenylaminopiperidine structure.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1512–1515, November, 1982.     

A study of sesquiterpene lactones by13C NMR spectroscopy. I.13C NMR spectra of badkhyzin and its derivatives

The¹³C NMR spectra of badkhyzin and its derivatives have been studied.     

13C NMR spectra of 1-(α-aroyloxyarylideneamino)-1,2,3-triazoles. Identification of 4,5-unsymmetrically substituted derivatives

The ¹³C nmr spectra of the title compounds are reported. Chemical shifts of C-4 and C-5 carbons of the triazole ring are used for structural assignment of the 4,5-unsymmetrically substituted 1-(α-aroyloxyarylideneamino)-v-triazoles (triazolylisoimides). A complete assignment of the shifts of the α-aroyloxyarylideneamino group is given. Some J_C-H values are also reported.     

Magnetic and micellar effects in photoreactions. : 13C NMR determination of selective 13C enrichment in a dibenzyl ketone photoproduct.

The photolysis of dibenzyl ketone (DBK) in micellar solution results in formation of 1-phenyl-4-methyl acetophenone (PMAP) as a minor product; quantitative integration of the ¹³C NMR spectrum of PMAP produced by photolysis of DBK in micellar solution demonstrates that significant selective ¹³C enrichment has occurred at three carbon atoms.     

Catalytic mechanism of S-ribosylhomocysteinase (LuxS): direct observation of ketone intermediates by 13C NMR spectroscopy

S-Ribosylhomocysteinase (LuxS) catalyzes the cleavage of the thioether linkage of S-ribosylhomocysteine (SRH) to produce l-homocysteine and 4,5-dihydroxy-2,3-pentanedione (DHPD). This is a key step in the biosynthetic pathway of the type II autoinducer (AI-2) in both Gram-positive and Gram-negative bacteria. Previous studies demonstrated that LuxS contains a catalytically essential Fe2+ ion. The catalytic mechanism of LuxS was investigated using 2- and 3-13C-labeled SRH as substrate and 13C NMR spectroscopy. These studies revealed the presence of 2- and 3-keto intermediates in the catalytic pathway. The 2-keto intermediate was chemically synthesized, and its chemical and kinetic competence was demonstrated. The results support a catalytic mechanism in which the metal ion catalyzes an internal redox reaction, shifting the carbonyl group from the C-1 position to the C-3 position. Subsequent beta-elimination at the C-4 and C-5 positions releases homocysteine as a free thiol. The results also suggest that Cys-84 and Glu-57 are the possible general acids/bases for proton transfer during catalysis and that the keto intermediates are released from the enzyme active site before rebinding and completion of the reaction.     

13C NMR spectroscopy of isomerism in series of arylazo derivatives of sulfonaphthols and aminosulfonaphthols

¹³C NMR has been used in a study of the isomers formed in azo coupling of 1,8-dihydroxy-3,6-naphthalenedisulfonic acid (chromotropic acid), 1-hydroxy-8-amino-3,6-naphthalenedisulfonic acid (H acid), 1-hydroxy-6-amino-3-naphthalenesulfonic acid (J acid), and 1-hydroxy-7-amino-3-naphthalenesulfonic acid (gamma acid); and indicators have been established through which individual isomers in mixtures can be recognized spectroscopically.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2721–2728, December, 1989.     

The crystalline structures of ethylene-dimethylaminoethyl methacrylate copolymers as studied by solid-state high-resolution 13C NMR spectroscopy

The crystalline structures of ethylene-dimethylaminoethyl methacrylate (EDAM) copolymers, which were either melt-quenched (mq) or isothermally crystallized (iso), were studied by solid-state high-resolution ¹³C NMR spectroscopy. It revealed that the crystalline structures of EDAM copolymers are greatly dependent on the comonomer content, crystallization condition and the storage time after treatment. The ratio of monoclinic to orthorhombic crystal (M/O) increases with the increase in the dimethylaminoethyl methacrylate content. Higher crystallinity and lower monoclinic content were observed for iso samples compared to the mq ones. The monoclinic crystal was found to melt at lower temperatures compared to the orthorhombic one during the heating process. The degree of crystallinity as well as the contents of monoclinic and orthorhombic crystals and the M/O value are found to increase after storage at room temperature for a month.