13H‐Dibenzo­[a,i]­fluoren‐13‐one

Molecules of 13H‐dibenzo­[a,i]­fluoren‐13‐one, C₂₁H₁₂O, strad­dle a crystallographic mirror plane and are essentially planar, with a dihedral angle of only 1.9 (1)° between the two naphtha­lene ring systems. Repulsive intramolecular C=O?H interactions therefore do not explain the larger distortions found in isomeric ketones.     

Conformational Analysis and CD Calculations of Methyl‐Substituted 13‐Tridecano‐13‐lactones

Conformational models covering an energy range of 3 kcal/mol were calculated for (13S)‐tetradecano‐13‐lactone ( 3 ), (12S)‐12‐methyltridecano‐13‐lactone ( 4 ), and (12S,13R)‐12‐methyltetradecano‐13‐lactone ( 8 ), starting from a semiempirical Monte‐Carlo search with AM1 parametrization, and subsequent optimization of the 100 best conformers at the 6‐31G*/B3LYP and then the TZVP/B3LYP level of density‐functional theory. CD Spectra for these models were calculated by the time‐dependent DFT method with the same functional and basis sets as for the ground‐state calculations and Boltzmann weighting of the individual conformers. The good correlation of the calculated and experimental spectra substantiates the interpretation of these conformational models for the structure–odor correlation of musks. Furthermore, the application of the quadrant rule in the estimation of the Cotton effect for macrolide conformers is critically discussed.     

A hydrogen‐bonded dimer of 13‐hydroxy‐13‐[(triisopropyl­silyl)­ethynyl]penta­cen‐6(13H)‐one

The title compound, C₃₃H₃₄O₂Si, has been obtained as a product in the synthesis of 6,13‐bis­[(triisopropyl­silyl)ethynyl]‐6,13‐dihydro­penta­cene‐6,13‐diol. The solid‐state structure reveals a dimer, with strong hydrogen bonds holding the two mol­ecules in a face‐to‐face arrangement [O⋯O = 2.746 (2) Å and O—H⋯O = 173 (2)°]. Within each dimer, the penta­cene units are π‐stacked (the distance between the mean least‐squares planes of 22 C atoms is 3.60 Å).     

1H‐ und 13C‐NMR‐Spektroskopie

The present state of the routine 1D ¹H‐ and ¹³C‐NMR spectroscopy is reported. After a short introduction into the basic theory the current spectrometer and software systems are discussed. Using an example from natural product chemistry the procedures during the analysis of the NMR spectra are explained.     

cis‐6‐Phenyl‐4bH,6H,11H,13H‐isoindolo[1,2‐c]benz[2,4]oxazepin‐13‐one

The title compound, C₂₂H₁₇NO₂, contains an isoindolinone moiety joined to a phenyl‐substituted benzoxazepine ring. The isoindolinone moiety is essentially planar and the oxazepine ring adopts a distorted chair conformation, with the phenyl substituent equatorial. Owing to the severe puckering of the central oxazepine ring, the mol­ecule as a whole is non‐planar; the benzene ring of the benzoxazepine fragment makes an angle of 67.7 (1)° with respect to the isoindoline ring.     

13C isotope effects on 1H chemical shifts: NMR spectral analysis of 13C‐labelled D‐glucose and some 13C‐labelled amino acids

The one‐ and two‐bond ¹³C isotope shifts, typically ?1.5 to ?2.5 ppb and ?0.7 ppb respectively, in non‐cyclic aliphatic systems and up to ?4.4 ppb and ?1.0 ppb in glucose cause effects that need to be taken into account in the adaptive NMR spectral library‐based quantification of the isotopomer mixtures. In this work, NMR spectral analyses of some ¹³C‐labelled amino acids, D ‐glucose and other small compounds were performed in order to obtain rules for prediction of the ¹³C isotope effects on ¹H chemical shifts. It is proposed that using the additivity rules, the isotope effects can be predicted with a sufficient accuracy for amino acid isotopomer applications. For glucose the effects were found strongly non‐additive. The complete spectral analysis of fully ¹³C‐labelled D ‐glucose made it also possible to assign the exocyclic proton signals of the glucose. Copyright © 2009 John Wiley & Sons, Ltd.     

Suppressing one‐bond homonuclear 13C,13C scalar couplings in the J‐HMBC NMR experiment: application to 13C site‐specifically labeled oligosaccharides

Site‐specific ¹³C isotope labeling is a useful approach that allows for the measurement of homonuclear ¹³C,¹³C coupling constants. For three site‐specifically labeled oligosaccharides, it is demonstrated that using the J‐HMBC experiment for measuring heteronuclear long‐range coupling constants is problematical for the carbons adjacent to the spin label. By incorporating either a selective inversion pulse or a constant‐time element in the pulse sequence, the interference from one‐bond ¹³C,¹³C scalar couplings is suppressed, allowing the coupling constants of interest to be measured without complications. Experimental spectra are compared with spectra of a nonlabeled compound as well as with simulated spectra. The work extends the use of the J‐HMBC experiments to site‐specifically labeled molecules, thereby increasing the number of coupling constants that can be obtained from a single preparation of a molecule. Copyright © 2014 John Wiley & Sons, Ltd.     

One‐bond 13C–13C coupling constants in alkyl‐substituted cyclopropenes: experimental and theoretical studies

Measurements of one‐bond carbon–carbon coupling constants, ¹J(C, C), were performed for two series of compounds, alkyl‐substituted cyclopropenes and cyclopropanes. The experimental data were complemented by a set of DFT‐calculated J couplings for the parent cyclopropene ( 1 ), its methyl and silyl derivatives and, additionally, for 1‐methylcyclobutene ( 3 ), 1‐methylcyclopentene ( 4 ) and 1‐methylcyclohexene ( 5 ) and good agreement was observed between the experimental and the calculated data; all the trends are perfectly maintained, including a dramatic decrease in the couplings across endocyclic single bonds in cyclopropene and its derivatives, and a significant decrease in the corresponding couplings in cyclobutene. Using the data obtained, the s characters of the carbon hybrid orbitals involved in the formation of the cyclopropene were calculated. The results clearly show that the ring closure and the related strain exerted upon the cyclopropene molecule only slightly disturb the electron structure of the double bond. The s character of the corresponding carbon orbital is 0.314 in cyclopropene vs the theoretical value of 0.333 in ethene. This is at variance with the endo‐ and exocyclic single bonds, where the s characters of the orbitals forming the endocyclic single bonds are much smaller than those of the bonds in the open‐chain compounds, i.e. 0.229 (C‐1 and/or C‐2) and 0.166 (C‐3). The s values calculated for the exocyclic CH bonds are 0.334 for C‐3 and 0.456 for C‐1 and/or C‐2. Copyright © 2002 John Wiley & Sons, Ltd.     

New Synthetic Analogs of Retinoids: Synthesis of Aromatic Analogs of 9‐Methylidene‐ and 13‐Demethyl‐9‐methylidene‐retinol, ‐retinal,and Ethyl 13‐Demethyl‐9‐methylideneretinoate

Aromatic analogs of 9‐methylidene and 13‐demethyl‐9‐methylideneretinol, ‐retinal, and ethyl 13‐demethyl‐9‐methylideneretinoate were synthesized via a new β‐methylidene‐aldehyde synthon.     

Efficient Hyperpolarization of U‐13C‐Glucose Using Narrow‐Line UV‐Generated Labile Free Radicals

Free radicals generated by UV‐light irradiation of a frozen solution containing a fraction of pyruvic acid (PA) have demonstrated their dissolution dynamic nuclear polarization (dDNP) potential, providing up to 30 % [1‐¹³C]PA liquid‐state polarization. Moreover, their labile nature has proven to pave a way to nuclear polarization storage and transport. Herein, differently from the case of PA, the issue of providing dDNP UV‐radical precursors (trimethylpyruvic acid and its methyl‐deuterated form) not involved in any metabolic pathway was investigated. The ¹³C dDNP performance was evaluated for hyperpolarization of [U‐¹³C₆,1,2,3,4,5,6,6‐d₇]‐d ‐glucose. The generated UV‐radicals proved to be versatile and highly efficient polarizing agents, providing, after dissolution and transfer (10 s), a ¹³C liquid‐state polarization of up to 32 %.     

Additional insights from very‐high‐resolution 13C NMR spectra of long‐chain n‐alkanes

New information has been obtained from very‐high‐resolution ¹³C NMR studies of a series of long‐chain n‐alkanes. These compounds are fundamentally important in the petroleum industry and are essential to the life of some plants, flowers, and insects. At least partial resolution of the ten different ¹³C NMR signals of n‐C₂₀H₄₂ is observed at 11.7 T for solutions in C₆D₆ or C₆D₅CD₃. A ¹³C T₁ inversion‐recovery experiment provides much more detailed information than in previous studies of long‐chain n‐alkanes, demonstrates a steady increase in the relaxation times of the ten different carbons proceeding from the middle to the end of the chain because of segmental motion, and thus confirms the assignments for the interior carbons. In contrast, there is significant overlap for the signals for C‐7 and the more interior carbons in a solution of n‐C₁₆ or longer chain alkanes in CDCl₃. Not only are the chemical shifts sensitive to the solvent used, but also the relative chemical shifts change. Signals for the interior carbons of the odd‐number alkanes in CDCl₃ are better resolved than in the spectra of their even‐number counterparts. Some mixed aromatic solvent systems give increased dispersion of the cluster of C‐6 through C‐10 signals of n‐C₂₀H₄₂, n‐C₂₁H₄₄, and n‐C₂₂H₄₆. However, none of the solvents used could even partially resolve the C‐10 and C‐11 signals of n‐C₂₁H₄₄ or n‐C₂₂H₄₆ at 11.7 T, which may result from a different distribution of conformers for n‐C₂₁H₄₄ or n‐C₂₂H₄₆ than for n‐C₂₀H₄₂ and shorter n‐alkanes. Copyright © 2013 John Wiley & Sons, Ltd.     

13C‐NMR data of daphnane diterpenoids

Daphnane diterpenoids are mainly distributed in Thymelaeaceae and Euphorbiaceae and have various bioactivities. About 100 daphnane diterpenoids have been isolated from natural plants. In this review, we systematically summarize the ¹³C‐NMR data of daphnane diterpenoids isolated from natural plants over the past several decades and briefly discussed their biological activities and basic structural–activity relationship. Copyright © 2013 John Wiley & Sons, Ltd.