The conformational dependence of 15n13c spin spin coupling constants

2-Azabicyclo(2.2.2)octan-3-One 1, N-isopropylpivaloylamide 2 and 4-azatricyclo(4.3.1.1^3,8)undecan-5-one 3 have been prepared with a ¹⁵N label. The amides were reduced to the corresponding amines 2-azabicy-clo(2.2.2)octane 4, N-isopropyl-N-neopentylamine 5 and 4-azatricyclo(4.3.1.1^3,8)undecane 6. The ¹³C spectra of these compounds and their hydrochlorides were measured and the ¹⁵N¹³C spin coupling constants interpreted in terms of their conformational dependence.     

The conformation of succinic acid in aqueous solution studied by 1h and 13c nmr

The vicinal H-H coupling constants of succinic acid were obtained as a function of pH and related to its conformation. The syn-clinal arrangement of the two CO₂H groups appears as more stable than the anti-periplanar conformation at variance with previous work but in agreement with recent data on similar molecules. The vicinal¹³CO₂H-¹H coupling constant was also obtained and is found to support that conclusion. The analysis was extended to solvents of variable dielectric constant and no significant effect was found.     

Carbonyl carbon chemical shifts in the 13c spectra of cyclic ketones

The ketonic ¹³C NMR signals of the keto lactones 1 and haplophytine (4) occur at exceptionally high field. These upfield shifts are interpreted in the context of a general consideration of the structural factors affecting the carbonyl carbon chemical shifts of cyclic ketones. It is concluded that dipole-dipole interactions are the major sources of the upfield shifts in the cases of both 1 and 4.     

Structural assignment of n3-acylated uridine derivatives by means of 13c nmr spectroscopy

Diisopropylethylamine was effective as a base for acylation of 2′,3′,5′-tri-O-acetyluridine with various acid chlorides. The ¹³C NMR spectra of the products and related compounds showed clearly that the acyl groups intoduced to the uracil moiety are attached to the N³-nitrogen.     

Photolyse von 2-oxo-[2-13c]-1-diazocyclohexan. Ein beitrag zum oxiren-problem

The synthesis of 2 - oxo - [2 - ¹³C] -1 - diazocyclohexan (10) starting with K¹³CN is described. Photolysis of 10 in dioxan-water yields cyclopentancarboxylic acid containing all the label in the carboxy group, which has been proved by C-13 NMR spectroscopy and electron impact induced fragmentation. The absence of isotope scrambling in the photolytic ring contraction excludes an oxiren participation.     

Identification of halogen substituents in natural products by measurement of 13c spin-lattice relaxation times and integrated intensities

A method is proposed for differentiating brominated carbons from chlorinated carbons by means of natural-abundance ¹³C NMR spectroscopy. The basis of the method is that the spin-lattice relaxation behaviour of brominated carbons is influenced by carbon-bromine scalar interactions, which can lead to shortened ¹³C spin-lattice relaxation times and reduced values of the nuclear Overhauser enhancement. C-Cl scalar interactions make a negligible contribution to the spin-lattice relaxation of chlorinated carbons. These effects are illustrated by measurement of the ¹³C spin-lattice relaxation times and integrated intensities of chloro-, bromo and iodobenzene and chloro-, bromo- and iodocyclohexane. The method is then tested on four polyhalogenated marine natural products. The results indicate that ¹³C relaxation measurements can be used to distinguish brominated carbons from chlorinated carbons in the case of halogenated quaternary carbons, sp² hydridized methine carbons and some sp³ hydridized methine carbons, but not in the case of halogenated methylene carbons or gem-dihalo substituted methine carbons.     

Oxidation of fusidic acid

Summary 11-Oxofusidic acid — a previously unknown keto derivative of the antibiotic — has been isolated from the products of the oxidation of fusidic acid by chromium trioxide.All-Union Scientific-Research Institute of Antibiotics, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 66–69, January–February, 1979.     

The dynamics of the carbonyl groups in phospholipid bilayers from a study of their 13c chemical shift anisotropy

Proton-enhanced carbon-13 magnetic resonance measurements have been made of the carbonyl carbons in hydrated liquid-crystalline pliospholipid bilayers The spectra indicate a superposition of two individual resonances assigned to the carbonyls of the γ and β chains. The shift anisotropies of these signals are ?25 ±2 and ?5 ±2 ppm. A model for the motion undergone by these groups is proposed, based on a measurement of the rigid lattice principal shielding values derived from anhydrous phosphollpid powders and the principal shielding directions taken from the data of Pines, Chang and Griffin.     

Electrophilic reactions of chlorin derivatives and a comprehensive collection of 13c data of these products and closely related compounds

Vilsmeier-formylation of the copper(II) complex of chlorin-e₆ trimethyl ester (2), under mild conditions, gives selective substitution in the 3-vinyl group. In contrast chlorination of 2 is shown to lead to selective substitution at position 20 of the macrocycle. A similar result is found for [3-ethyl]-isochlorin-e₄ dimethyl ester (17) although further reaction leads to more highly chlorinated products which have been isolated and identified. ¹³C NMR data for some of these compounds and several related chlorin derivatives are reported. In particular, after correction of the literature, many of the quaternary carbon signals of the macrocycle are assigned and substituent effects assessed. Consideration of the shifts of the α-pyrrolic carbons confirms that chlorin and its derivatives exist in a tautomeric form with the two inner H atoms on diagonally opposite pyrrole rings A and C. Such a form allows a satisfactory explanation of the substituent chemical shifts of the formyl group at positions 15 and 20 to be made.     

The stereochemistry of fusidic acid

Evidence is presented which shows that the antibiotic fusidic acid possesses the relative and absolute stereochemistry depicted in 2. The biogenetic implications of this result are briefly discussed.     

Microwave spectra of [15n] and [13c] pyridines,quadrupole coupling constants,dipole moment and molecular structure of pyridine

The microwave spectra of two ring-substituted species of pyridine have been remeasured, and the spectra of the two remaining species investigated and assigned.¹⁴N quadrupole coupling splittings were analyzed and used to determine the quadrupole coupling constants, while the derived center frequencies were fitted to give improved values for the rotational constants.A complete substitution structure can now be presented.The dipole moment of pyridine was redetermined from Stark measurements in the spectrum of [¹⁵N]pyridine.     

Nitrato- and fluoroboracites M3B7O13NO3 and M3B7O13F

New boracites containing nitrato- or fluoroanions that appear to be true low-pressure phases have been synthesized at superatmospheric pressures. The M₃B₇O₁₃NO₃ compounds (M = Co, Ni, Cu, Zn, Cd) transform rapidly and reversibly in the temperature region 300–500°C between probable orthorhombic and face centered cubic symmetry, while the M₃B₇O₁₃F compounds (M = Mg, Mn, Fe, Co, Zn) appear to maintain rhombohedral symmetry up to their decomposition temperatures of 800–900°C. True high-pressure boracite-like phases containing F and Cr, Mn, Fe, or Co that decompose upon heating to M₃B₇O₁₃F have also been isolated.     

13C-NMR spectra of fluorinated molecules using 19F-13C polarization transfer

¹³C-NMR spectra of fluorinated molecules are difficult to observe under conventional broad-band proton decoupling. The large coupling constants involved make polarization transfer between fluorine and carbon, using INEPT or DEPT experiments very effective while broad band fluorine decoupling collapses the multiplet pattern.     

13C13C spin spin coupling constants in azulenes

[4-¹³C]- Azulene and [4-¹³C]-4-methylazulene have been synthesized. The ¹³C¹³C spin coupling constants have been measured and interpreted in terms of πMO theory.