Assignment of the 15N resonances of the antifungal agent posaconazole

The structurally complex antifungal agent posaconazole has been well characterized by conventional ¹H‐ and ¹³C‐NMR studies. In contrast, ¹⁵N resonance assignments have never been reported. We now wish to report the assignment of the eight ¹⁵N resonances of posaconazole using two‐dimensional long‐range ¹H‐¹⁵N GHMBCAD NMR data. The ¹⁵N resonance assignments were undertaken to facilitate the evaluation of the impact of ¹H‐¹⁵N heteronuclear shift correlation data in the Computer‐Assisted Structure Elucidation (CASE) of complex molecular structures. J. Heterocyclic Chem., (2011).     

Specificity of 15N NMR chemical shifts to the nature of substituents and tautomerism in substituted pyridine N-oxides

¹H, ¹³C, and ¹⁵N NMR chemical shifts have been measured for 2-aminopyridine N-oxide (1), its eleven derivatives (2–10, 13, 14), and 3-Cl and 3-Br substituted 4-nitropyridine N-oxides (11, 12). δ(¹⁵N) of pyridine ring nitrogen in 2-acetylaminopyridine N-oxides are 5.9–11.5 ppm deshielded from those in 2-aminopyridine N-oxides. When amino and acetylamino substituents are in 4-position, δ(¹⁵N) of ring nitrogen is 21.3 ppm deshielded in the acetylated derivative. The strong resonance interaction between 2-amino and 5-nitro groups reflects in the decrease of amino nitrogen shielding about 5.3–17.9 ppm. Also, ¹H and ¹³C NMR spectral data are in agreement with ¹⁵N NMR results reflected as deshielded amino protons and carbons C-2 and C-5. The pyridine nitrogen chemical shift in all amino- and acetylamino derivatives vary between ?101.2 and ?126.7 ppm, which has been connected with the tautomeric balance in our earlier studies.     

Aminodiphenylphosphanes: Isotope‐induced chemical shifts 1Δ14/15N(31P), coupling constants 1J(31P,15N), and chemical shifts δ15N and δ31P

A series of aminodiphenylphosphanes 1 [Ph₂P‐N(H)tBu ( a ), ‐NEt₂ ( b ), ‐NiPr₂ ( c )], 2 [Ph₂P‐NHPh ( a ), ‐NH‐2‐pyridine ( b ), ‐NH‐3‐pyridine ( c ), ‐NH‐4‐pyridine ( d ), NH‐pyrimidine ( e ), NH‐2,6‐Me₂‐C₆H₃ ( f ), NH‐3‐Me‐2‐pyridine ( g )], 3 [Ph₂P‐N(Me)Ph ( a ), ‐NPh₂ ( b )], and N‐pyrrolyldiphenylphosphane 4 (Ph₂P‐NC₄H₄) was prepared and studied by NMR (¹H, ¹³C, ³¹P, ¹⁵N NMR) spectroscopy. The isotope‐induced chemical shifts ¹Δ^14/15N(³¹P) were determined at natural abundance of ¹⁵N by using HEED INEPT experiments. A dependence of ¹Δ^14/15N(³¹P) on the substituents at nitrogen was found (alkyl < H < aryl; increasingly negative values). The magnitude and sign of the coupling constants ¹J(³¹P,¹⁵N) (positive sign) are dominated by the presence of the lone pair of electrons at the phosphorus atom. The X‐ray structural analysis of 2b is reported, showing the presence of dimers owing to intermolecular hydrogen bridges in the solid state. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:542–550, 2001     

A 15N NMR investigation of some azolopyridines

¹⁵N NMR chemical shift data are presented for 14 azolopyridines, together with the results of INDO/S-SOS calculations of nitrogen shieldings. Previous ¹⁴N NMR results for some of these compounds are reinterpreted. The ¹⁴N data and their assignments are shown to be reliable for the indolizine nitrogen atom from arguments based on relative line widths. The pyridine-type nitrogens are more reliably assigned from the ¹⁵N spectra combined with the results of the INDO/S-SOS calculations for individual molecules. A combination of ¹⁴N and ¹⁵N NMR spectra, together with the shielding calculations, provides a basis for unambiguous assignments of all the various nitrogen environments considered.     

Heterogeneous Microtesla SABRE Enhancement of 15N NMR Signals

The hyperpolarization of heteronuclei via signal amplification by reversible exchange (SABRE) was investigated under conditions of heterogeneous catalysis and microtesla magnetic fields. Immobilization of [IrCl(COD)(IMes)], [IMes=1,3‐bis(2,4,6‐trimethylphenyl), imidazole‐2‐ylidene; COD=cyclooctadiene] catalyst onto silica particles modified with amine linkers engenders an effective heterogeneous SABRE (HET‐SABRE) catalyst that was used to demonstrate a circa 100‐fold enhancement of ¹⁵N NMR signals in ¹⁵N‐pyridine at 9.4 T following parahydrogen bubbling within a magnetic shield. No ¹⁵N NMR enhancement was observed from the supernatant liquid following catalyst separation, which along with XPS characterization supports the fact that the effects result from SABRE under heterogeneous catalytic conditions. The technique can be developed further for producing catalyst‐free agents via SABRE with hyperpolarized heteronuclear spins, and thus is promising for biomedical NMR and MRI applications.     

Low-temperature 15N NMR spectra of reduced 1,3-heterocyclic systems

The ¹⁵N NMR spectra of the O-inside cis-fused conformer of perhydropyrido[1,2-c][1,3]thiazine shows a shielding of the nitrogen of 23.0 ppm relative to the trans-fused conformer. In contrast, ¹⁵N shifts for the cis-and trans-fused conformers of perhydro-oxazolo[3,4-a]pyridine and perhydrothiazolo[3,4-a]pyridine show corresponding shieldings of only 0.6 and 2.5 ppm, respectively.     

Position-specific 15N isotope analysis in organic molecules: A high-precision 15N NMR method to determine the intramolecular 15N isotope composition and fractionation at natural abundance

The position-specific ¹⁵N isotope content in organic molecules, at natural abundance, is for the first time determined by using a quantitative methodology based on ¹⁵N Nuclear Magnetic Resonance (NMR) spectrometry. ¹⁵N NMR spectra are obtained by using an adiabatic “Full-Spectrum” INEPT sequence in order to make possible ¹⁵N NMR experiments with a high signal-to-noise ratio (>500), to reach a precision with a standard deviation below 1‰ (0.1%). This level of precision is required for observing small changes in ¹⁵N content associated to ¹⁵N isotope effects. As an illustration, the measurement of an isotopic enrichment factor ε for each ¹⁵N isotopomer is presented for 1-methylimidazole induced during a separation process on a silica column. The precision expressed as the long-term repeatability of the methodology is good enough to evaluate small changes in the ¹⁵N isotope contents for a given isotopomer. As observed for ¹³C, inverse and normal ¹⁵N isotope effects occur concomitantly, giving access to new information on the origin of the ¹⁵N isotope effects, not detectable by other techniques such as isotope ratio measured by Mass Spectrometry for which bulk (average) values are obtained.     

13C, 15N coupling constants in substituted 1,2,4-triazines-4-15N

The ¹³C,¹⁵N coupling constants in three 1,2,4-triazine-4-¹⁵N derivatives and in a substituted pyrimidine-1-¹⁵N have been determined and are compared with those in pyridine and quinoline. Of special interest are the data of the 5-(2-dimethylamino-1-propenyl)-3,6-dimethyl-1,2,4-triazine-4-¹⁵N, since one observes a characteristic alternation of the absolute value of J(¹³C,¹⁵N) along the sidechain and a hitherto unobserved very large ¹³C,¹⁵N coupling constant through four bonds of 3·9 Hz, the origin of which is attributed to the influence of the lone pair orbital on the ¹⁵N at position 4.     

A natural abundance 15N NMR investigation of bilirubin IX-α

Natural abundance ¹⁵N NMR spectra have been obtained for bilirubin IX-α using polarization transfer via the INEPT and SINEPT-2 techniques. The resonances for all four nitrogens are clearly resolved. ¹⁵N chemical shifts and ¹⁵N? ¹H coupling constants over one and three bonds are reported. Heteronuclear chemical shift correlation between ¹⁵N and ¹H for the four NH groups has been established by means of the SINEPT-2 pulse sequence.     

N7‐ and N9‐substituted purine derivatives: a 15N NMR study

The ¹⁵N NMR chemical shifts of N⁷‐ and N⁹‐substituted purine derivatives were investigated systematically at the natural abundance level of the ¹⁵N isotope. The NMR chemical shifts were determined and assigned using GSQMBC, GHMBC, GHMQC and GHSQC experiments in solution. ¹⁵N cross‐polarization magic angle spinning data were recorded for selected compounds in order to study the principal values of the ¹⁵N chemical shifts. Geometric parameters obtained by using RHF/6–31G** and single‐crystal x‐ray structural analysis were used to calculate the chemical‐shielding constants (GIAO and IGLO) which were then used to assign the nitrogen resonances observed in the solid‐state NMR spectra and to determine the orientation of the principal components of the shift tensors. Copyright © 2002 John Wiley & Sons, Ltd.     

Quantitative 15N NMR spectroscopy

Line intensities in ¹⁵N NMR spectra are strongly influenced by spin-lattice and spin–spin relaxation times, relaxation mechanisms and experimental conditions. Special care has to be taken in using ¹⁵N spectra for quantitative purposes. Quantitative aspects are discussed for the ¹⁵N NMR of molecules with different nitrogen functional groups and also mixtures of nitrogen-containing compounds. It is shown that, in general, quantitative data are obtainable from integration of ¹⁵N lines in proton decoupled ¹⁵N NMR spectra using NOE suppression. Addition of paramagnetic relaxation reagents (PARR) under controlled conditions is frequently needed to accomplish the experiment within reasonable time limits.     

A 15N NMR study of some azoles

¹⁵N NMR data are reported for 42 azoles, taken mostly at a standard concentration and in a common solvent (0.5 M dimethyl sulfoxide with a 0.01 M increment of Cr(acac)₃ for each nitrogen atom present). Signal assignments were assisted by comparison with ¹⁴N line widths, the use of ²J(¹⁵N¹H) couplings, and shielding calculations obtained by the INDO/S–SOS approach. The generally large differences in nitrogen-shielding changes permitted rather facile shift assignments.     

Synthesis and 15N NMR Signal Amplification by Reversible Exchange of [15N]Dalfampridine at Microtesla Magnetic Fields

Signal Amplification by Reversible Exchange (SABRE) technique enables nuclear spin hyperpolarization of wide range of compounds using parahydrogen. Here we present the synthetic approach to prepare ¹⁵N-labeled [¹⁵N]dalfampridine (4-amino[¹⁵N]pyridine) utilized as a drug to reduce the symptoms of multiple sclerosis. The synthesized compound was hyperpolarized using SABRE at microtesla magnetic fields (SABRE-SHEATH technique) with up to 2.0 % ¹⁵N polarization. The 7-hour-long activation of SABRE pre-catalyst [Ir(IMes)(COD)Cl] in the presence of [¹⁵N]dalfampridine can be remedied by the use of pyridine co-ligand for catalyst activation while retaining the ¹⁵N polarization levels of [¹⁵N]dalfampridine. The effects of experimental conditions such as polarization transfer magnetic field, temperature, concentration, parahydrogen flow rate and pressure on ¹⁵N polarization levels of free and equatorial catalyst-bound [¹⁵N]dalfampridine were investigated. Moreover, we studied ¹⁵N polarization build-up and decay at magnetic field of less than 0.04 μT as well as ¹⁵N polarization decay at the Earth's magnetic field and at 1.4 T.     

15N NMR spectroscopy. 19—spectroscopic characterization of cyclodipeptides (2,5-dioxopiperazines)

Various cyclodipeptides containing glycine, alanine, leucine, valine, phenylalanine, phenylglycine and sarcosine units were synthesized by cyclization of dipeptide pentachlorophenyl esters. The ¹³C and natural abundance ¹⁵N NMR spectra of these heterocycles were measured in trifluoroacetic acid and compared with the spectra of the corresponding amino acids and polypeptides. The ¹³C NMR carbonyl signals of all cyclodipeptides show a 1.5–4.0 ppm upfield shift relative to the corresponding polypeptides. The ¹⁵N NMR signals show no such consistent relationship. The substituent effects and the neighbouring residue effects observed in the ¹⁵N NMR spectra of the cyclodipeptides are different from those of polypeptides, while the one bond N? H coupling constant of cis and trans amide groups was almost identical. The nitrogen and the carbonyl signal of the Gly units in cyclo-Gly-Phe show an extraordinary downfield shift, reflecting the interaction of the phenyl group with the 2,5-dioxopiperazine ring.     

15N nuclear magnetic resonance spectroscopy. Natural abundance 15N NMR of monosubstituted indoles

¹⁵N chemical shifts of twenty-four substituted indoles have been determined in natural abundance (in organic solvents) using Fourier transform NMR. The overall chemical shift range is 27 ppm, with groups in the 2-, 3- and 5-ring positions showing the largest substituent effects. Substituents capable of resonance interaction with the indole nitrogen give shifts in the expected directions but they cannot be correlated with known substituent parameters. Compounds measured in DMSO give 0·2 to 10·2 ppm downfield shifts with respect to the same compound measured in CDCl₃. ¹³C NMR data for previously unreported compounds are also reported.     

Characterization of para‐substituted benzamidoximes and benzamidinium salts by 15N NMR spectroscopy

¹⁵N NMR data for a series of 12 para‐substituted benzamidoximes and benzamidinium salts were determined in dimethyl sulfoxide. For the amino group of benzamidoximes ¹J(N,H) coupling constants were determined using polarization transfer techniques; the other ¹⁵N atoms were not detectable owing to fast exchange processes and, thus, standard proton noise decoupled spectra had to be measured. The ¹⁵N NMR chemical shifts of the oxime‐type nitrogen atom and the benzamidinium amino group (with two exceptions) correlate with Hammett σ° values (r²>0.95). ¹⁵N NMR shift data are a suitable and sensitive means for characterizing far‐ranging electronic substituent effects in these functional groups. Additionally, ¹³C NMR data in dimethyl sulfoxide solution are given. All spectroscopic data will be used for investigations into the mechanisms of the enzymes involved in the metabolic cycle of oxidation and reduction of benzamidines and benzamidoximes. Copyright © 2002 John Wiley & Sons, Ltd.     

2D relayed15N,1H correlated NMR spectroscopy of a pentadecapeptide at natural abundance of15N

Summary Proton-detected H-relayed N,H correlation NMR spectroscopy at natural abundance of¹⁵N has been used to demonstrate the enormous value of heteronuclear NMR spectroscopy for the proton assignment of medium-sized oligopeptides.

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Zweidimensionalerelayed ¹⁵N,¹H-korrelierte NMR-Spektroskopie an einem Pentadecapeptid bei natürlicher Häufigkeit von¹⁵N

Zusammenfassung Anhand¹H-detektierterrelayed-N,H-korrelierter NMR-Spektroskopie bei natürlicher Häufigkeit von¹⁵N wird die große Bedeutung der heteronuklearen Kernresonanzspektroskopie für die Protonenzuordnung mittlerer Oligopeptide demonstriert.

     

15N and17O NMR spectra of vinyl ethers of pyridine and quinoline

Conclusions ¹⁵N (¹⁴N) and¹⁷O NMR spectra have been obtained on vinyl ethers of pyridine and quinoline, with unenriched samples. The principal factor determining the chemical shifts is p- conjugation of the unshared pairs of the heteroatom with the unsaturated fragments of the molecule; an additional contribution in the case of the¹⁵N signals comes from interaction of the nitrogen atom with a proton through space.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 769–773, April, 1987.     

13C‐15N Connectivity networks via unsymmetrical indirect covariance processing of 1H‐13C HSQC and 1H‐15N IMPEACH spectra

Long‐range ¹H‐¹⁵N heteronuclear shift correlation methods at natural abundance to facilitate the elucidation of small molecule structures have assumed a role of growing importance over the past decade. Recently, there has also been a high level of interest in the exploration of indirect covariance NMR methods. From two coherence transfer experiments, A→B and A→C, it is possible to indirectly determine B?C. We have shown that unsymmetrical indirect covariance methods can be employed to indirectly determine several types of hyphenated 2D NMR data from higher sensitivity experiments. Examples include the calculation of hyphenated 2D NMR spectra such as 2D GHSQC‐COSY and GHSQC‐NOESY from the discrete component 2D NMR experiments. We now wish to report the further extension of unsymmetrical indirect covariance NMR methods for the combination of ¹H‐¹³C GHSQC and ¹H‐¹⁵N longrange (GHMBC, IMPEACH‐MBC, CIGAR‐HMBC, etc.) heteronuclear chemical shift correlation spectra to establish ¹³C‐¹⁵N correlation pathways.