Carbon-13 NMR studies of monohydroxylated and monochlorinated derivatives of Z- and E-p-menthanes

J(¹³C¹H) coupling constants for some methyl- and aminopyrimidines have been determined by ¹³C NMR. Both the one-bond and long-bond and long-range coupling constants follow general trends which can be summarized in a few simple rules. In particular, the ³J(C-i,H) coupling constants between a ring carbon C-i and the ring protons are larger than the ²J(C-i,H) coupling constants. The opposite is observed for the couplings between the ring carbons and the methyl protons: ³J(C,Me). These general rules are very useful for the assignment of resonances in complex ¹³C spectra of pyrimidines and seem to be valid for other 6-membered aromatic nitrogen heterocycles. Furthermore, the additivity of substituent effects on ¹J (CH) for monosubstituted pyrimidines allows the estimation of ¹J (CH) for polysubstituted pyrimidines with a very good accuracy.     

Compound 30.4, An unusual chlorinated 1,4-benzoxazin-3-one derivative from corn (zea mays).

An unusual chlorinated 1,4-benzoxazin-3-one derivative has been isolated from young corn roots. Its structure was established by spectroscopic techniques (¹H-, ¹³C-NMR, EI and CI-MS, UV and IR) and by correlation with known benzoxazinone derivatives. The assignments of all protons could be made by ¹H shift-correlated 2-D NMR techniques. The chlorinated benzoxazine derivative has been confirmed to be naturally occurring in a second carefully planned growth-chamber experiment.     

1H and 13C NMR of platinum porphyrins—II. Platinum complexes of meso-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl) porphyrin and 2,3,7,8,12,13,17,18-octaethylporphyrin

The ¹H and ¹³C NMR spectra of the title compounds are reported. In both porphyrins, long-range spin-spin coupling is observed between platinum and the porphyrin protons and carbon atoms. Two-bond Pt-“α-pyrrole” carbon couplings are less than the three-bond Pt-“fβ-pyrrole” carbon and Pt-“meso-carbon” couplings. Also, large upfield shifts of the pyrrole α-carbon resonances are noted. The coupling mechanism appears to proceed via Fermi contact through the porphyrin σ-framework for the carbons, and the π-system for the protons.     

Application of 1H and 13C NMR to the structural elucidation of tetrahydroquinoxalines condensed with five-membered heterocycles

¹H and ¹³C NMR spectral data for 21 N-methyltetrahydroquinoxalines annelated with furan, pyrrole, imidazole or thiazole rings are reported. Unambiguous assignments of the ring junction ¹³C resonances were made on the basis of selective decoupling experiments and with the aid of one-bond and long-range ¹³C–¹H coupling constants. The effects of five-membered heterocycles on the ¹H and ¹³C chemical shifts of the ring junction hydrogen and carbon atoms are considered. Values of one-bond ¹J(CH) and vicinal ³J(HH) coupling constants between the ring junction protons are also discussed as a diagnostic means for structural elucidation of tetrahydroquinoxalines condensed with five-membered heterocycles.     

Assignment of indirect 13C, 1H couplings in the 13C NMR spectra of some purine and pyrimidine nucleosides and their analogues by long-range selective 1H decoupling

The assignments of the long-range ¹³C, ¹H coupling constants in the ¹³C NMR spectra of the base moieties of several purine and pyrimidine nucleosides and their analogues were established by the application of long-range selective ¹H decoupling with low-power ¹H irradiation. The ³J values between the carbons and protons of the pyrimidine ring in these compounds were larger than the ²J values. The conformational preference of these nucleosides in solution was shown to be predominantly anti from the value of the vicinal ¹³C, ¹H coupling constants between H-1′ and the base carbons through the glycosidic bond.     

High resolution 13C and 1H NMR spectral analysis of 1-azafluorene

An analysis of the ¹H and ¹³C spectra of 1-azafluorene has been carried out using computer calculations and homo- and heteronuclear double resonance techniques. The ⁴J, ⁵J and ⁶J long-range coupling constants of the 9-CH₂ group protons with the protons of the pyridine and the phenylene rings have been observed and measured. The long-range J(CH) values in the ¹H coupled ¹³C spectrum have been assigned and measured on a first-order basis.     

Peptide conformations. Part 30. Assignment of the 1H-, 13C-, and 15N-NMR spectra of cyclosporin A in CDCl3 and C6D6 by a combination of homo- and heteronuclear two-dimensional techniques

The ¹H-, ¹³C-, and ¹⁵N-NMR spectra of the immunosuppressive cyclic undecapeptide cyclosporin A ( 1 ) have been analyzed at 300 MHz in CDCl₃, C₆D₆, and mixtures of these solvents. A combination of different homo- and heteronuclear two-dimensional NMR techniques enable complete assignment of all H-, C- and 4 N-signals. Recognition of the proton spin systems has been achieved via ¹H,¹H–COSY and double-quantum-¹H-NMR spectroscopy. NOESY spectra yield some sequence assignments, but two techniques using coupling across amide bonds have been applied to get independent assignments of all amino acids in the sequence: (i) An ¹H,¹H-COSY spectrum optimized for small coupling constants enables the detection of long-range couplings from N-methyl groups to both α-protons attached to that amide bond. (ii) An ¹H, ¹³C-COSY spectrum optimized for C,H-long-range couplings (J = 5 to 10 Hz) to the eleven CO groups again yields coupling to both α-protons attached to that amide bond. Additionally these two experiments yield the assignment of N-methyl protons and carbonyl C-atoms. Normal and relayed ¹H,¹³C-COSY in both solvents have been applied to assign all C-atoms via their directly attached and remote protons. An ¹H,¹³C-COLOC spectrum at 500 MHz in CDCl₃, which uses H,C-long-range couplings confirms the assignment of all proton spin systems as well as the C-signals of each individual amino acid. Ambiguities in the assignment of the C(δ)'s of MeLeu have thus been removed. An ¹H,¹⁵N-COSY spectrum enables the assignment of the 4 NH N-atoms.     

1H and 13C chemical shifts and coupling constants of lupane. Application of two-dimensional NMR techniques

2D NMR techniques (J-resolved ¹³C, ¹³C? ¹³C correlated, ¹H? ¹³C correlated) were used to gather more chemical shift and coupling information on the pentacyclic triterpene, lupane. They confirm and complete ¹³C assignments made earlier, and corroborate the constitution and major configurational details of lupane. Lupane is a parent compound and spectral reference in the study of sedimental demethylated triterpenes.     

Rotational Phenomena on Substituent Phenyl Rings of (η5-Cyclopentadienyl)(η4-Tetraphenylcyclobutadiene)Cobalt

We used extended Hückel calculations, variable-temperature, homonuclear long-range shift-correlated 2-D ¹H NMR and dynamic NOE measurements to investigate rotational phenomena of substituent phenyl rings on (η⁵-cyclopentadienyl)(η⁴tetraphenylcyclobutadiene)cobalt (1). Two closely related compounds, (η⁵-cyclopentadienyl)(η⁴-1,3-diphenylcyclobutadiene)cobalt (A) and (η⁵-cyclopentadienyl)(η⁴-1,2-diphenylcyclobutadiene)cobalt (A′), were prepared. Energy minima appeared at conformations of which the dihedral angles between phenyl and cyclobutadiene rings are about 30° for I and 0° for A according to extended Hückel calculations. In 1, ortho protons of phenyl rings belong to one set of multiplet in ¹H NMR; meta and para protons belong to the other. It was supported by a long-range coupling 2-D ¹H NMR and NOE experiments. A sharp line due to phenyl rings was observed in the low-temperature ¹H NMR spectrum of A, which indicates that the five protons are magnetically equivalent at that temperature.     

Two-dimensional NMR characterization of propylene copolymers

Newly developed two-dimensional NMR techniques are used to characterize homopolypropylene and copolymers of propylene with ethylene and butylene. Both homonuclear and heteronuclear shift-correlated experiments have been attempted. The most useful technique for the copolymers is ¹³C-¹H shift correlation, whereby the one-dimensional ¹H-NMR spectra of these copolymers are completely interpreted for the first time. Although the ¹H resonances are heavily overlapped, the use of a computerized “analytical” approach permits pertinent information to be obtained from the one-dimensional ¹H-NMR spectra.     

13C, 1H spin coupling constants of dimethylacetylene

¹³C, ¹H spin coupling constants of dimethylacetylene have been determined by the complete analysis of the proton coupled ¹³C NMR spectrum. For the methyl carbon ¹J(CH) = + 130.6₄ Hz and ⁴J(CH) = + 1.5₈ Hz, and for the acetylenic carbon ²J(CH) = ? 10.34 Hz and ³J(CH) = +4.30 Hz. The ⁵J(HH) long-range coupling constant (+2.79 Hz) between the methyl protons was also determined.     

An 1H and 13C NMR study of pentane-2,4-dione tellurium(II) compounds; chemical shifts, 125Te?1H and 125Te?13C couplings

¹H and ¹³C NMR data are reported for pentane-2,4-dione tellurium(II) compounds which contain a 6-membered tellurane ring bearing methyl or ethyl substituents. The ¹²⁵TeCH coupling constants are particularly interesting in that they are highly stereospecific, viz. ～60 Hz and 0–8 Hz for equatorial and axial protons, respectively, in the chair conformation. ¹J(TeC) and ²J(TeCC) couplings lie in the ranges 118–146 Hz and 44–49 Hz, respectively. Geminal HCH couplings in these compounds (9–11 Hz) are lower than analogous values in tellurane or in cyclohexane-1,3-diones.     

Temperature dependence of proton–proton residual dipolar couplings in adenosine dissolved in charged phospholipid bicelles. Application of a two‐dimensional selective refocusing method

¹H NMR spectra of nucleoside adenosine were recorded in 7.5% (w/v) DMPC/DHPC bicelles doped with CTAB. Despite the small size of adenosine in comparison with the distances between discoidal bicelles, intramolecular ¹H,¹H residual dipolar couplings D were observed at 29.5–38°C. The absolute values of ¹H,¹H spin interactions Δ (sum of J‐ and D‐couplings) were measured selectively by a series of 2D SERF experiments as a function of temperature. At lower temperatures dipolar couplings are scaled down, which leads to simplification of the ¹H NMR spectra. For some pairs of protons, which are simultaneously J‐ and D‐coupled, the exact values of D can be determined from temperature dependence of Δ. Copyright © 2002 John Wiley & Sons, Ltd.     

Assignment of the 5,6-Double-Bond Configuration in Iloprost and Isoiloprost from 13C-NMR Shifts Determined by 2D Methods

The configurations of the 5,6-double bond in the carbacyclins iloprost ( 3 ; (E)) and isoiloprost ( 4 ; (Z)) are based on a complete assignment of the ¹³C and ¹H resonances determined by 1D and 2D ¹³C-NMR and ¹H-NMR methods.     

Two-dimensional NMR studies of cyperene

Combined utilization of ¹H? ¹H homonuclear and ¹H? ¹³C heteronuclear NMR chemical shift correlations, two-dimensional J-resolved ¹H NMR measurements and homonuclear ¹H double resonance experiments allowed specific assignments for both ¹H and ¹³C NMR frequencies to be made for the tricyclic sesquiterpene cyperene. The results also provided information on the conformation of the six-membered ring of this natural product, which was recognized as a distorted chair.     

Giant Residual Dipolar 13C–1H Couplings in High‐Spin Organoiron Complexes: Elucidation of Their Structures in Solution by 13C NMR Spectroscopy

High‐spin Fe^II–alkyl complexes with bis(pyridylimino)isoindolato ligands were synthesized and their paramagnetic ¹H and ¹³C NMR spectra were analyzed comprehensively. The experimental ¹³C—¹H coupling values are temperature (T^?1)‐ as well as magnetic‐field (B²)‐dependent and deviate considerably from typical scalar ¹J_CH couplings constants. This deviation is attributed to residual dipolar couplings (RDCs), which arise from partial alignment of the complexes in the presence of a strong magnetic field. The analysis of the experimental RDCs allows an unambiguous assignment of all ¹³C NMR resonances and, additionally, a structural refinement of the conformation of the complexes in solution. Moreover the RDCs can be used for the analysis of the alignment tensor and hence the tensor of the anisotropy of the magnetic susceptibility.     

The study of longifolene by two-dimensional NMR spectroscopy

The complete assignment of the ¹³C NMR spectrum of longifolene was achieved from double quantum coherence measurements, while combined evaluation of a ¹H? ¹³C heteronuclear chemical shift correlation diagram and a homonuclear ¹H J-resolved diagram provided all proton chemical shifts. Conformational information on the seven-membered ring of the tricyclic sesquiterpene was obtained from proton chemical shift considerations.     

Application of two-dimensional NMR techniques for the elucidation of the proton spectrum of the epoxy monomer,diglycidyl ether of bisphenol A

The application of two-dimensional Fourier transform NMR spectroscopy to the study of the epoxy resin diglycidyl ether of bisphenol A is discussed. The combination of the homonuclear shift-correlated and J-resolved experiments permits the complete elucidation of the proton spectrum of the glycidyl ether moiety.     

J‐Edited Pure Shift NMR for the Facile Measurement of nJHH for Specific Protons

We report a novel 1D J‐edited pure shift NMR experiment (J‐PSHIFT) that was constructed from a pseudo 2D experiment for the direct measurement of proton–proton scalar couplings. The experiment gives homonuclear broad‐band ¹H‐decoupled ¹H NMR spectra, which provide a single peak for chemically distinct protons, and only retain the homonuclear‐scalar‐coupled doublet pattern at the chemical‐shift positions of the protons in the coupled network of a specific proton. This permits the direct and unambiguous measurement of the magnitudes of the couplings. The incorporation of a 1D selective correlation spectroscopy (COSY)/ total correlation spectroscopy (TOCSY) block in lieu of the initial selective pulse, results in the exclusive detection of the correlated spectrum of a specific proton.     

High-Resolution Solid-State MAS13C- and 1H-NMR Spectra of Benzenoid Aromatics Adsorbed on Alumina and Silica: Successful Applications of 1D and 2D Pulse Experiments from Liquid-State NMR

Strong line-narrowing effects in solid-state, magic-angle-spinning (MAS) ¹³C- as well as ¹H-NMR spectra of benzenoid aromatics adsorbed at alumina or silica surfAccs indicate high mobility of the organic adsorbates. Even under modest spinning rales (1 kHz), dipolar couplings are sufficiently reduced to allow scalar ^13C,¹H couplings to be measured. Hetero- and homonuclear pulse sequences known from high-resolution NMR in liquids, like SEFT, ^J-RESOLVED, DEPT, COSY, and ¹³C,¹H shift-correlation experiments are successfully applicable. ¹³C spin-lattice relaxation limes are as short as 0.5 s (CH) and 1.1 s (C_q), and T₁(¹H) values are in the order of 0.3 s.