Mesomere Kationen. IV—13C-NMR-Spektren von Uronium-, Thiouronium- und Guanidiniumsalzen,sowie einigen Guanidinen

The ¹³C chemical shifts of uronium-, thiouronium- and guanidinum salts are reported, and compared with the shifts of the corresponding uncharged ureas, thioureas and guanidines. The shifts for the aromatic carbon atoms in these compounds compared with related phenols and thiophenols give insight into the charge distribution in the systems.     

Mesomere Kationen: III—13C-Fourier-Transform-NMR-Spektren von Harnstoffen,Thioharnstoffen und Guanidinen

The ¹³C-chemical shifts of methyl substituted ureas, thioureas and related compounds are reported and discussed. π-Electron densities, as obtained by simple HMO calculations, do not reproduce the ¹³C-chemical shift trend. A correlation has been found for the carbonyl and thiocarbonyl resonances which has been discussed as being mainly influenced by the mean excitation energy in the paramagnetic screening term.     

The nature of the [C2H6N]+ and [CH4N]+ ions formed by electron impact on methylated formamides,acetamides, ureas,thioureas and hexamethylphosphoramide

By means of appearance potential measurements and metastable analysis, it is shown that the [C₂H₆N]⁺ and [CH₄N]⁺ ions formed from methylated formamides, acetamides, ureas and thioureas rearrange upon formation to structures similar to the [M? H]⁺ ions from mono- and dimethylamine.     

15N NMR spectroscopy 24—chemical shifts and coupling constants of α-amino acid N-carboxyanhydrides and related heterocycles

The chemical shifts of amino acid N-carboxyanhydrides (NCAs) and cyclic or linear urethanes are less sensitive to solvent effects than those of amides and lactams. The values of the one-bond ¹⁵N? ¹H coupling constants depend on the solvent and are 5-8 Hz larger than those of ureas and amides. The ¹⁵N? ¹³C coupling constant of the N? CO group is also unusually high, while that of the N—CH group lies within the range known for N-acylated aliphatic amines. The one-bond ¹⁵N? ¹³C coupling constant was found to be insensitive to conformational changes.     

Hindered rotation in thioureas: Steric effects and conformations

The temperature dependent spectra of several mono-, di- and trialkylthioureas have been recorded. Free energy barriers to internal rotation about the C? N bonds have been calculated. In thioureas that were unsymmetrically substituted, free energy barriers were found to be different for each C? N bond with the more substituted amino group exhibiting the higher barrier. The monosubstituted thioureas showed different rotational barriers for the NH₂ groups of the cis and trans isomers, respectively. The free energy barriers for the trans isomers were found to be substituent dependent and substantiate the reassignment of the high and low field substituent signals to the trans and cis isomers, respectively. The spectrum of 1-methyl-3-t-butylthiourea indicates restricted rotation of the t-butyl group at temperatures below 200 K.     

Calculation of some N and C nuclear shielding parameters for some ureas and thioureas

Molecular orbital calculations using CNDO/S parameters are employed to determine bond-orders, charge-densities and the shielding of the ¹⁵N and ¹³C nuclei of a series of ureas and thioureas. For the sterically uncrowded molecules a linear relationship is noticed between the ¹⁵N chemical shifts and C---N bond-orders. When steric crowding occurs this simple relationship is no longer applicable.     

Spontaneous cyclization of (acridin-9-ylmethyl)thioureas to spiro [dihydroacridine-9′(10′H),5-imidazolidine]-2-thiones,a novel type of acridine spirocycles

Novel acridine spirocompounds, spiro[dihydroacridine-9′(10′H),5-imidazolidine]-2-thiones have been prepared by the spontaneous cyclization of 1-substituted 3-(acridin-9-ylmethyl)thioureas, which were obtained from 1-(acridin-9-yl)methanamine, N-(acridin-9-ylmethyl)propan-1-amine, and N-(acridin-9-ylmethyl)benzylamine and alkyl/aryl isothiocyanates, as continuation of our previous studies on new acridine spirocycles. Imidazolidine-2-thiones thus obtained were subsequently transformed with mesitylnitrile oxide to imidazolidine-2-one analogues, some of which partly reopened to the corresponding (acridin-9-ylmethyl)ureas. An unusual spirocyclization via a CH carbanion instead of the N-1 nitrogen has been found for 3-(acridin-9-ylmethyl)-1-(acridin-9-yl)thioureas possessing two acridine rings. Structural modifications in positions 1, 3, and 4 of the spiro ring together with ¹H, ¹³C, and ¹⁵N NMR spectroscopy and X-ray crystallography have been employed to rationalize a general propensity of various 9-substituted acridines to undergo easy spirocyclization.     

1H and 13C NMR spectral assignment of N,N′‐disubstituted thiourea and urea derivatives active against nitric oxide synthase

The ¹H and ¹³C NMR resonances of seventeen N‐alkyl and aryl‐N′‐[3‐hydroxy‐3‐(2‐nitro‐5‐substitutedphenyl)propyl]‐thioureas and ureas ( 1–17 ), and seventeen N‐alkyl or aryl‐N′‐[3‐(2‐amino‐5‐substitutedphenyl)‐3‐hydroxypropyl]‐thioureas and ureas ( 18–34 ), designed as NOS inhibitors, were assigned completely using the concerted application of one‐ and two‐dimensional experiments (DEPT, HSQC and HMBC). NOESY studies confirm the preferred conformation of these compounds. Copyright © 2016 John Wiley & Sons, Ltd.     

Dynamische NMR-Untersuchungen der behinderten Rotation am N?C(X)-Bindungsfragment. XVII. Einfluß des Zentralmetallatoms auf die Rotationsbarriere der behinderten C,N-Rotation und die 13C-NMR-Spektren von Chelatkomplexen des 1,1-Diethyl-3-benzoyl-thio(seleno)-harnstoffs

Dynamic N.M.R. Studies of Hindered Rotation on N? C(X) Bond Increment. XVI. Central Metal Atom Influence on Rotational Barriers of the Restricted C, N-Rotation and the ¹³C N.M.R. Spectra of the Metal Complexes of 1,1-Diethyl-3-benzoyl-thio(seleno) Urea The rotational barriers about the partial C, N-double bond Et₂N? C(X) in the metal complexes of 1,1-diethyl-3-benzoyl-thio(seleno) urea are submitted, corrected for the compounds of low Δν value. The barriers are dependent on the electric polarizibility of the metal ion and the chalcogen atom X (S, Se). The results are discussed. In the ¹³C n.m.r. spectra of the investigated compounds also the most electron attractive metal ions influence characteristic variations of ¹³C chemical shifts.     

Pathways of Intramolecular Cyclization of Phosphorylated Ureas and Thioureas: A Theoretical Study

The stability of phosphorylated ureas and thioureas containing various substituents at the P and N atoms and the stability of their cyclization products, the corresponding diazaphospholidines and/or oxaza(thiaza)phospholines, was evaluated by PM3, MNDO, and DFT calculations. The thermal effects of cyclizations involving phosphorylated ureas (or thioureas) and of separate steps of their synthesis were calculated. According to the calculations, diazaphospholidine structures are preferably formed from substituted ureas and thiazaphospholine structures, from their thio analogs; formation of five-membered heterorings with the P-N bond is thermodynamically preferable as compared to their six-membered analogs.     

Efficient Method of Synthesis of N,N′-Disubstituted Ureas/Thioureas by a Zinc Chloride Catalyzed Thermal Reaction

Symmetrically N,N′-disubstituted ureas/thioureas were synthesized by heating amines or phenyl hydrazine and urea/thiourea on a preheated hot plate at 80–85°C under solvent-free conditions in the presence of a catalytic amount of ZnCl₂ as a catalyst. The protocol has the advantages of not using toxic phosgene and other hazardous substrates or organic solvents. Increased reaction rate, good yield, and a simple workup procedure are involved.     

Domino reactions in the synthesis of tetrahydropyrimidin-2(1<Emphasis Type="Italic">H</Emphasis>)-ones(thiones) annelated at the N(1)–C(6) bond (Review)

Information relating to the synthesis of tetrahydropyrimidin-2(1H)-ones(thiones), annelated at the N(1)–C(6) bond, by cascade cyclization of substituted N-(3-oxoalkyl)ureas and thioureas is classified. The general relationships of this process are presented.     

Efficient Synthesis of N,N′‐Disubstituted Ureas/Thioureas Catalyzed by Iodine

Iodine is an efficient catalyst for the synthesis of symmetrically N,N′‐disubstituted ureas/thioureas by heating respective amines or phenyl hydrazine and urea/thiourea on a preheated hot plate at 90–95°C, under solvent‐free conditions. The yields are excellent, and the reactions go to complete within 5–10 min.     

13C and 15N spin–lattice relaxation and chemical shift studies of pentane-2,4-diamine

¹³C and ¹⁵N NMR chemical shift and spin–lattice relaxation data have been measured for both meso- and racemic-pentane-2,4-diamine. At high pH (12), relaxation is consistent with hindered rotation of the NH₂ group due, in part, to the formation of intramolecular hydrogen bonds. At low pH (2), relaxation is consistent with relatively unhindered rotation of the NH₃⁺ group. Rotational jump rates and barriers are reported, determined from the NT₁ ratios between ¹⁵N and ¹³C nuclei. In all cases, the ratios for the racemic diastereomer are higher than those of the meso compounds; this is interpreted in terms of conformationally more stable intramolecular hydrogen bond formation in the meso compound. Chemical shifts for the diastereomeric amines show that ¹⁵N shifts move downfield on protonation along with methyl and methylene carbons, while the methine carbon resonances move upfield.     

15N and 13C NMR chemical shifts of 6‐(fluoro,chloro, bromo,and iodo)purine 2′‐deoxynucleosides: measurements and calculations

The ¹⁵N and ¹³C chemical shifts of 6‐(fluoro, chloro, bromo, and iodo)purine 2′‐deoxynucleoside derivatives in deuterated chloroform were measured. The ¹⁵N chemical shifts were determined by the ¹H? ¹⁵N HMBC method, and complete ¹⁵N chemical‐shift assignments were made with the aid of density functional theory (DFT) calculations. Inclusion of solvation effects significantly improved the precision of the calculations of ¹⁵N chemical shifts. Halogen‐substitution effects on the ¹⁵N and ¹³C chemical shifts of purine rings are discussed in the context of DFT results. The experimental coupling constants for ¹⁹F interacting with ¹⁵N and ¹³C of the 6‐fluoropurine 2‐deoxynuleoside are compared with those from DFT calculations. Copyright © 2009 John Wiley & Sons, Ltd.     

Hindered rotation in five-membered cyclic nitrosamines: A 13C NMR study

The equilibrium rotamer populations and N? N rotational barriers of N-nitrosopyrrolidine (1), N-nitrosothiazolidine (2), N-nitrosooxazolidine (3) and their 2-methyl derivatives, 4, 5 and 6, were determined by ¹³C NMR spectroscopy. While equal rotamer populations occur in 1 and 2, the E rotamers predominate in the other four compounds, with the highest percentage (92%) of E rotamer occurring in N-nitroso-2-methyloxazolidine (6). The average barrier to N? N bond rotation varies over a range of 4.1 kcal mol^?1in these compounds, decreasing in the order N-nitrosopyrrolidine > N-nitrosothiazolidine > N-nitrosooxazolidine. The compounds which contain an exocyclic 2-methyl group have average rotational barriers which are 0.1–0.9 kcal mol^?1 higher than those of the corresponding unmethylated derivatives. The results are interpreted in terms of the relative effects of steric hindrance by the 2-methyl substituents and electron induction by the heterocyclic sulfur and oxygen atoms on both the rotamer populations and the N? N rotational barriers.     

Quinoxalines XIV. Synthesis, H, C, N NMR spectroscopic, and quantum chemical study of 1H-pyrazolo[3,4-b]quinoxalines (flavazoles)

The synthesis of a series of 1H-pyrazolo[3,4-b]quinoxalines (flavazoles) by acylation, alkylation, halogenation, and aminomethylation of the parent compound is reported and their structure is investigated by ¹H, ¹³C, and ¹⁵N NMR spectroscopy. The restricted rotation about the partial C, N double bond of the N-acyl derivatives 7-10 is studied by dynamic NMR spectroscopy and the barriers to rotation are determined. In order to assign unequivocally the ¹⁵N chemical shifts of N-4 and N-9, in case of 3-substituted flavazoles, exemplary the ¹H, ¹³C, and ¹⁵N NMR chemical shifts of 34, 35, and 39 are also theoretically calculated by quantum chemical methods [ab initio at different levels of theory (HF/6-31G* and B3LYP/6-31G*)].     

15N NMR studies of amino acids and their reaction products with formaldehyde

Natural abundance ¹⁵N NMR spectroscopy has been used to investigate the effect of pH on the ¹⁵N chemical shifts of lysine and of ε-hydroxymethyllysine. A computer calcualtion which fits the chemical shifts of both α-and ε-nitrogen atoms versus pH has been used to predict the pK_a values. ¹⁵N chemical shifts and some ¹J(¹⁵NH) values of some other amino acids and of their reaction products with formaldehyde are also reported.     

Simple and efficient synthesis of O-unprotected glycosyl thiourea and isourea derivatives from glycosylamines

Practical, facile and high-yielding one-pot syntheses of different O-unprotected glycopyranosyl thioureas and thioureido bolaamphiphiles (two-step synthesis) and of 2-amino-4,5-dihydro-(1,2-dideoxy-β-d-glucopyranoso)[1,2-d]oxazoles (three-step synthesis) from glycopyranosylamines are reported. The method involves the preparation of O-unprotected β-d-gluco (and d-galacto)pyranosyl isothiocyanates which are in equilibrium with the corresponding 1,2-cyclic thiocarbamates, coupling with amines to afford glycosyl thioureas and treatment with yellow mercury (II) oxide to give trans-fused bicyclic isoureas. A d-gluco trehazolin analogue is prepared in this way. In situ transformation of N,N-dialkyl, N′-glucopyranosyl thioureas into the corresponding ureas is also reported.     

Synthesis and Properties of N,N′-Disubstituted Ureas and Their Isosteric Analogs Containing Polycyclic Fragments: XII. N-(1,3,3-Trimethylbicyclo[2.2.1]heptan-2-yl)-N′-R-ureas and -thioureas

Russian Journal of Organic Chemistry - N,N′-Disubstituted ureas and thioureas containing a lipophilic optically active bicyclic fragment of natural origin were synthesized by reaction of (R)-...