Synthesis of some aminopropynylpyrazoles

Conclusions A number of mono-, di-, and tri(aminopropynyl)pyrazoles was synthesized by the Mannich condensation from pyrazolylacetylenes, and by the condensation of N-substituted propargylamines with iodopyrazoles.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2524–2529, November, 1972.     

Mild regioselective iodination of pyrazoles using <Emphasis Type="Italic">n</Emphasis>-butyltriphenylphosphonium peroxodisulfate

A practical, efficient and inexpensive method of synthesis of iodopyrazoles by the reaction of pyrazoles with iodine using n-butyltriphenylphosphonium peroxodisulfate as an oxidant at room temperature is reported. The use of n-butyltriphenylphosphonium peroxodisulfate is feasible due to its easy preparation and handling, high stability and activity.     

Silica–Sulfuric Acid Catalyzed Nitrodeiodination of Iodopyrazoles

Abstract

We report here the synthesis of nitropyrazoles in good to excellent yields from iodopyrazoles over silica–sulfuric acid catalyst for the first time. The present procedure require less acid, offers a simplified workup procedure, and may be applied for the nitration of a wide variety of iodoazoles in drug and pharmaceutical industries.     

Nitrogen nuclear magnetic resonance spectroscopy. Nitrogen-15 and proton chemical shifts of methylanilines and methylanilinium ions

Nitrogen and carbon electron densities of the toluidines and xylidines have been recalculated by the INDO method; previously published errors have been corrected. Although the nitrogen-15 chemical shifts of these compounds still display the earlier suggested correlation with σ and total electron densities, the calculated inverse correlation with proton electron densities has been shown to be incorrect. Methyl proton chemical shifts of these compounds display no meaningful correlation with the nitrogen shifts. The nitrogen chemical shifts of the toluidinium and xylidinium ions correlate moderately well with the ¹³C chemical shifts of the analogous di- and tri-methylbenzenes.     

Empirically predicted 13C NMR chemical shifts for 8-hydroxyflavone starting from 7,8,4'-trihydroxyflavone and from 7,8-dihydroxyflavone

8-Hydroxyflavone is not found in nature. While the (13)C chemical shifts of 8-hydroxyflavone have been reported previously, the observed (13)C chemical shifts were not assigned. A previously reported empirical predictive tool has been applied in reverse in order to deconvolute the (13)C chemical shifts for 8-hydroxyflavone from each of those of 7,8,4'-trihydroxyflavone and 7,8-dihydroxyflavone together with those of 7-hydroxyflavone, 4'-hydroxyflavone, and flavone. The two sets of calculated (13)C chemical shifts for 8-hydroxyflavone are in good agreement with each other in that the average absolute difference is 0.4 ppm. The previously reported but unassigned (13)C chemical shifts for 8-hydroxyflavone have been assigned by matching them with the averages of the two sets of calculated (13)C chemical shifts for 8-hydroxyflavone such that the minimum average absolute difference is 0.63 ppm. The assigned (13)C chemical shifts of 8-hydroxyflavone may be used, along with the (13)C chemical shifts of the remaining monohydroxyflavones, as part of a predictive tool to rapidly assess the (13)C NMR spectra of C8-hydroxylated flavonoids.     

Comparison of experimental and DFT‐calculated NMR chemical shifts of 2‐amino and 2‐hydroxyl substituted phenyl benzimidazoles,benzoxazoles and benzothiazoles in four solvents using the IEF‐PCM solvation model

A comparative study of experimental and calculated NMR chemical shifts of six compounds comprising 2‐amino and 2‐hydroxy phenyl benzoxazoles/benzothiazoles/benzimidazoles in four solvents is reported. The benzimidazoles showed interesting spectral characteristics, which are discussed. The proton and carbon chemical shifts were similar for all solvents. The largest chemical shift deviations were observed in benzene. The chemical shifts were calculated with density functional theory using a suite of four functionals and basis set combinations. The calculated chemical shifts revealed a good match to the experimentally observed values in most of the solvents. The mean absolute error was used as the primary metric. The use of an additional metric is suggested, which is based on the order of chemical shifts. The DP4 probability measures were also used to compare the experimental and calculated chemical shifts for each compound in the four solvents. Copyright © 2015 John Wiley & Sons, Ltd.     

Accurate calculation,prediction, and assignment of 3He NMR chemical shifts of helium-3-encapsulated fullerenes and fullerene derivatives

Helium-3 NMR chemical shifts of various (3)He-encapsulated fullerenes ((3)He@C(n)()) and their derivatives have been calculated at the GIAO-B3LYP/3-21G and GIAO-HF/3-21G levels with AM1 and PM3 optimized structures. A good linear relationship between the computed (3)He NMR chemical shifts and the experimental data has been determined. Comparisons of the calculation methods of (3)He NMR chemical shifts show that GIAO-B3LYP/3-21G with AM1-optimized structures yields the best results. The corrected (3)He NMR chemical shifts were calculated from the correction equation that is obtained through linear regression fitting of the experimental and calculated (3)He NMR chemical shifts over a wide range of (3)He-encapsulated fullerene compounds. The corrected (3)He NMR chemical shifts match the experimental data very well. The current computational method can serve as a prediction tool and can be applied to the assignments and reassignments of the (3)He NMR chemical shifts of (3)He@C(n)() and their derivatives.     

A New Metric for Evaluating DFT Calculated Proton and Carbon Chemical Shifts of Natural Products and Organic Compounds

The calculation of DFT (density functional theory) chemical shifts have become an important technique for the verification of a proposed structure. An easily calculated metric developed for proton and carbon chemical shifts of natural products and organic compounds, the calculated chemical shift index (CCSI), has been developed, which uses the deviation of each pair of calculated and experimental chemical shifts. The mean absolute deviation (MAD), which is commonly used as the goodness of fit metric for DFT calculated chemical shifts, can conceal large deviations in the calculated data. A classification strategy is also proposed for the CCSI to highlight when further assessment of the NMR data is required.     

17O NMR investigation of acetyl and formylthiophenes,furans and pyrroles

The ¹⁷O nmr chemical shifts of 20 acetyl and formylthoiphenes, furans, and pyrroles are reported. The chemical shifts qualitatively correlate with the electronic character of the heterocyclic rings. The effect of steric factors are noted for the ¹⁷O chemical shifts of alkyl substituted acetylthiophenes and for N-methylpyr-roles.     

Carbon-13, nitrogen-15 and oxygen-17 NMR chemical shifts of substituted 1-phenyl-2-pyrrolidinones

The carbon-13 and nitrogen-15 NMR chemical shifts and the direct carbon—proton coupling constants of 1-phenyl-2-pyrrolidinone and its 2′-methyl, 3′-methyl, 4′-methyl, 2′-chloro, 3′-chloro, 4′-chloro, 3′-methoxy, 4′-methoxy and 4′-nitro derivatives were measured in dimethyl sulfoxide. The oxygen-17 NMR chemical shifts of some of the compounds were determined in acetone.The effect of substituents on the chemical shifts of carbonyl carbons correlates well with the Hammett substituent parameters and the nitrogen chemical shifts seem to follow a similar trend. The variation of the oxygen chemical shift due to the substituents is small. The chemical shifts of aromatic carbons can mainly be derived using the substituent parameters of benzene; some deviation probably due to steric effects is observable, however.     

A solid state 13C NMR, crystallographic, and quantum chemical investigation of chemical shifts and hydrogen bonding in histidine dipeptides

We report the first solid-state NMR, crystallographic, and quantum chemical investigation of the origins of the 13C NMR chemical shifts of the imidazole group in histidine-containing dipeptides. The chemical shift ranges for Cgamma and Cdelta2 seen in eight crystalline dipeptides were very large (12.7-13.8 ppm); the shifts were highly correlated (R2= 0.90) and were dominated by ring tautomer effects and intermolecular interactions. A similar correlation was found in proteins, but only for buried residues. The imidazole 13C NMR chemical shifts were predicted with an overall rms error of 1.6-1.9 ppm over a 26 ppm range, by using quantum chemical methods. Incorporation of hydrogen bond partner molecules was found to be essential in order to reproduce the chemical shifts seen experimentally. Using AIM (atoms in molecules) theory we found that essentially all interactions were of a closed shell nature and the hydrogen bond critical point properties were highly correlated with the N...H...O (average R2= 0.93) and Nepsilon2...H...N (average R2= 0.98) hydrogen bond lengths. For Cepsilon1, the 13C chemical shifts were also highly correlated with each of these properties (at the Nepsilon2 site), indicating the dominance of intermolecular interactions for Cepsilon1. These results open up the way to analyzing 13C NMR chemical shifts, tautomer states (from Cdelta2, Cepsilon1 shifts), and hydrogen bond properties (from Cepsilon1 shifts) of histidine residue in proteins and should be applicable to imidazole-containing drug molecules bound to proteins, as well.     

相对化学位移值：获得CH基团中氢原子精确化学位移值的一种简单方法

为了解决CH基团中氢原子的精确化学位移值的问题,引入相对化学位移值概念．内标法和外标法用于测量全浓度范围的N-甲基乙酰胺水溶液．对于同一个分子来说,选择分子内某个基团的氢原子化学位移作为一个标准值,得到的其它基团氢原子相对化学位移值随温度和浓度的变化是和测试方法无关的。     

Understanding sterol-membrane interactions part I: Hartree-Fock versus DFT calculations of 13C and 1H NMR isotropic chemical shifts of sterols in solution and analysis of hydrogen-bonding effects

1H and 13C NMR chemical shifts are exquisitely sensitive probes of the local environment of the corresponding nuclei. Ultimately, direct determination of the chemical shifts of sterols in their membrane environment has the potential to reveal their molecular interactions and dynamics, in particular concerning the hydrogen-bonding partners of their OH groups. However, this strategy requires an accurate and efficient means to quantify the influence of the various interactions on chemical shielding. Herein the validity of Hartree-Fock and DFT calculations of the 13C and 1H NMR chemical shifts of cholesterol and ergosterol are compared with one another and with experimental chemical shifts measured in solution at 500 MHz. A computational strategy (definition of basis set, simpler molecular models for the sterols themselves and their molecular complexes) is proposed and compared with experimental data in solution. It is shown in particular that the effects of hydrogen bonding with various functional groups (water as a hydrogen-bond donor and acceptor, acetone) on NMR chemical shifts in CDCl3 solution can be accurately reproduced with this computational approach.     

Theoretical and experimental NMR study of protopine hydrochloride isomers

The 1H and 13C NMR chemical shifts of cis- and trans-protopinium salts were measured and calculated. The calculations of the chemical shifts consisted of conformational analysis, geometry optimization (RHF/6-31G** method) and shielding constants calculations (B3LYP/6-31G** method). Based on the results of the quantum chemical calculations, two sets of experimental chemical shifts were assigned to the particular isomers. According to the experimental results, the trans-isomer is more stable and its population is approximately 68%.     

14N and 13C NMR of tautomeric systems of mercapto- and amino-pyridines

Nitrogen chemical shifts are shown to provide a means of estimating equilibrium compositions of tautomeric systems of mercapto- and amino-derivatives of pyridine. Carbon chemical shifts can afford only qualitative information about such equilibria.     

Investigation of Substituent Effects on Proton and Carbon-13 Chemical Shifts of 4-Substituted Trans-Stilbenes

Proton and carbon-13 chemical shifts of para-substituted stilbenes have been measured. ¹H-¹H, ¹H-¹³C COSY spectra were obtained to analyze unambiguously the chemical shifts of protons and carbons. A long range coupling between 2-H and α-H was observed in a ¹H-¹H COSY spectrum. The observed chemical shifts have been correlated with Hammett substituent parameters. Among ethenyl protons and carbons, all but the chemical shifts of α-H show good correlation with both dual substituent parameters and single substituent parameters. In addition to this finding, the excellent linear correlations of C-l, and 4′-H of 4-substituted trans-stilbenes are also reported. Besides the correlations of chemical shifts with Hammett parameters, a good correlation between the chemical shifts and the calculated charges of position C-4′ are reported.     

The 1H and 13C chemical shifts for some tetrakis(para-substituted phenyl)ethylenes

The proton and carbon chemical shifts for a series of tetrakis(p-substituted phenyl)ethylenes are described. Assignments followed routine substituent chemical shift trends. Both proton and carbon chemical shifts ortho to the varying substituent follow the empirical parameter, Q. The ethylene carbon chemical shifts are proportional to those at the position para to the varying substituent.     

15N NMR chemical shifts for the identification of dipyrrolic structures

A variety of dipyrromethanes and dipyrromethenes have been prepared, and their 15N NMR chemical shifts have been measured by two-dimensional correlation to 1H NMR signals. The nitrogen atoms in five examples of dipyrromethanes consistently exhibit chemical shifts around -231 ppm, relative to nitromethane. Seven examples of hydrobromide salts of meso-unsubstituted dipyrromethenes consistently display 15N chemical shifts around -210 ppm, while their corresponding zinc(II) complexes exhibit chemical shifts around -170 ppm. The presence of electron-withdrawing substituents on one of the pyrrolic rings of dipyrromethenes affects the chemical shifts of both of the nitrogen nuclei in the molecule. Boron difluoride complexes of meso-unsubstituted dipyrromethenes display 15N chemical shifts around -190 ppm. Two examples of free-base dipyrromethenes bearing substituents at the meso-position exhibit 15N chemical shifts at approximately -156 ppm, and for the zinc complexes of these compounds at -162 ppm. One-bond nitrogen-hydrogen coupling constants, when measurable, were consistently in the range of -96 Hz. Since the measured 15N chemical shifts have such a high regularity correlated to structure, they can be used as diagnostic indications for identifying the structure of dipyrrolic compounds.     

NMR,MP2, and DFT study of thiophenoxyketenimines (o‐thio‐Schiff bases): Determination of the preferred form

Five new thiophenoxyketinimines have been synthesized. ¹H and ¹³C NMR spectra as well as deuterium isotope effects on ¹³C chemical shifts are determined, and spectra are assigned. DFT and MP2 calculations of both structures, chemical shifts, and isotope effects on chemical shifts are done. The combined analysis reveals that the compounds are primarily on a zwitterionic form with an NH⁺ and a S⁻ group and with a little of the neutral form mixed in. Very strong intramolecular hydrogen bonding is found and very high NH chemical shifts are observed. The theoretical calculations show that calculations at the MP2 level are best to obtain correct “C═S” chemical shifts.     

Effects of amino acid phi,psi propensities and secondary structure interactions in modulating H alpha chemical shifts in peptide and protein beta-sheet.

H alpha chemical shifts are often used as indicators of secondary structure formation in protein structural analysis and peptide folding studies. On the basis of NMR analysis of model beta-sheet and alpha-helical peptides, together with a statistical analysis of protein structures for which NMR data are available, we show that although the gross pattern of H alpha chemical shifts reflects backbone torsion angles, longer range effects from distant amino acids are the dominant factor determining experimental chemical shifts in beta-sheets of peptides and proteins. These show context-dependent variations that aid structural assignment and highlight anomalous shifts that may be of structural significance and provide insights into beta-sheet stability.