13C NMR Signals Research Articles

Synthesis and structural, spectroscopic, and electrochemical characterization of benzo[c]quinolizinium and its 5-aza-, 6-aza, and 5,6-diaza analogues

Four heterocyclic salts 1a– d were prepared by Ca 2+-assisted cyclization of fluoro derivatives 3, and investigated by spectroscopic (NMR and UV), electrochemical, and computational (DFT and MP2) methods. The mechanism for the formation of the cations was investigated at the DFT level of theory. 2-D NMR spectroscopy for 1[ ClO 4 ] in DMSO­ d 6 aided with DFT results permitted the assignment of 1H and 13C NMR signals in cations 1. The molecular and crystal structures for 1a[ ClO 4 ] [C 13H 10ClNO 4 triclinic, P−1, a=9.6517(12) Å, b=11.0470(13) Å, c=12.2373(15) Å, α=67.615(1)°, β=78.845(2)°, γ=87.559(2)°; V=1183.0(2) Å 3, Z=4] and 1d[ ClO 4 ] [C 12H 9ClN 2O 4 triclinic, P−1, a=5.9525(6) Å, b=8.3141(9) Å, c=12.2591(13) Å, α=73.487(1)°, β=83.814(1)°, γ=83.456(1)°; V=576.07(10) Å 3, Z=2] were determined by X-ray crystallography and compared with results of DFT and MP2 calculations. Electrochemical analysis gave the reduction potential order ( 1b> 1c∼ 1d> 1a), which is consistent with computational results.

First solid-state NMR analysis of uniformly 13C-enriched major light-harvesting complexes from Chlamydomonas reinhardtii and identification of protein and cofactor spin clusters

The light-harvesting complex II (LHCII) is the main component of the antenna system of plants and green algae and plays a major role in the capture of sun light for photosynthesis. The LHCII complexes have also been proposed to play a key role in the optimization of photosynthetic efficiency through the process of state 1–state 2 transitions and are involved in down-regulation of photosynthesis under excess light by energy dissipation through non-photochemical quenching (NPQ). We present here the first solid-state magic-angle spinning (MAS) NMR data of the major light-harvesting complex (LHCII) of Chlamydomonas reinhardtii, a eukaryotic green alga. We are able to identify nuclear spin clusters of the protein and of its associated chlorophyll pigments in 13C– 13C dipolar homonuclear correlation spectra on a uniformly 13C-labeled sample. In particular, we were able to resolve several chlorophyll 13 1 carbon resonances that are sensitive to hydrogen bonding to the 13 1-keto carbonyl group. The data show that 13C NMR signals of the pigments and protein sites are well resolved, thus paving the way to study possible structural reorganization processes involved in light-harvesting regulation through MAS solid-state NMR.

1H and13C NMR study of phase transition and molecular motion in ionic liquids forming lyotropic liquid-crystalline ionogels

The room temperature ionic liquid (RTIL) 1-decyl-3-methyl-imidazolium bromide [C10mim][Br], dissolved in water, was studied using 1H and 13C NMR spectroscopy. The manifestation of phase transitions and fine features of molecular motion in NMR spectra upon changing temperature and composition have been analyzed. The 1H NMR line shape typical for anisotropic fluids with zero biaxiality (asymmetry) of magnetic shielding and the chemical shift anisotropy (CSA) of ca 0.33 ppm was observed and attributed to water molecules. CSA values for 13C nuclei have been found in the range of 1.2–1.7 ppm. The difference between lyotropic liquid-crystalline (LC) ionogel phase and the solid one has been revealed, where the motions of RTIL and water molecules have been found to be dynamically segregated. The anisotropic 13C NMR signal shape at 16.89 ppm shows the difference between LC ionogel and the lamellar phases, where usually the decreasing order parameter moving along hydrocarbon chain from the polar head is observed. It indicates that the terminal –CH3 groups are more ordered and the supramolecular structures of [C10mim][Br], similar to some higher micellar RTIL aggregates are expected. In order to explain the experimental observations, the quantum chemistry DFT calculations of 1H and 13C magnetic shielding tensors of [C10mim][Br] and various H-bond structures of H2O were performed. Keywords: properties of molecules and molecular ions, line and bands widths, shapes and shifts, NMR and relaxation

One-pot microwave assisted synthesis under green chemistry conditions, antioxidant screening, and cytotoxicity assessments of benzimidazole Schiff bases and pyrimido[1,2-a]benzimidazol-3(4H)-ones

The synthesis of a number of benzimidazole Schiff bases 3 and 3-oxo-pyrimido[1,2-a]benzimidazoles 4 in excellent yields by a one-step sequence from the reaction of 2-aminobenzimidazole under green chemistry conditions is described. Structural assignments of the new compounds as well as complete assignment of 1H and 13C NMR signals have been unambiguously achieved based on the analysis of their 1H and 13C NMR (1D and 2D), IR, MS and elemental analysis data. To the synthesized Schiff bases the E-configuration was assigned on the basis of comparison of experimental and calculated (DFT) 13C NMR chemical shifts. Compounds 3 and 4 were evaluated as inhibitors of lipoxygenase (LOX) and of lipid peroxidation (LPO). All the tested derivatives showed inhibition of lipid peroxidation, whereas most of them were found to have higher activation than the reference compound trolox; The Schiff bases 3e, 3h, and 3i, and the pyrimidobenzimidazoles 4a, 4e and 4f were found to be the most potent. The most potent LOX inhibitor within the subset of Schiff bases was found compound 3i, followed by 3f, whereas compounds 4a and 4g were found the most potent of the 3-oxo-pyrimido[1,2-a]benzimidazole group. Moreover, some cytotoxicity assessments were undertaken, whereupon it was found that Schiff base 3i and pyrimidobenzimidazoles 4e and 4f did not exhibit cytotoxicity at similar concentrations resembling thus the inhibitory activity of lipid peroxidation. The most cytotoxic Schiff base and pyrimidobenzimidazole were found to be 3d and 4c, respectively.

Structure of fomitellan A, a mannofucogalactan from the fruiting bodies of Fomitella fraxinea

Chemical structure of fomitellan A, a polysaccharide with a mitogenic effect isolated from the fruiting bodies of Fomitella fraxinea, has been assigned as a mannofucogalactan with a repeating unit of penta-saccharide, which was composed of a (1→6)-linked d-galactopyranosyl backbone having a C-2 position substituted with disaccharide units of 3-O-d-mannopyranosyl-l-fucopyranosyl residue. The 1H and 13C NMR signals of fomitellan A have been completely assigned by extensive NMR experiments.

1,5-Benzoxazepines vs 1,5-Benzodiazepines. One-Pot Microwave-Assisted Synthesis and Evaluation for Antioxidant Activity and Lipid Peroxidation Inhibition

Amino-1,5-benzoxazepines 2 and 5 and hydroxyl-1,5-benzodiazepines 3 and 6 have been synthesized in one-pot solvent-free conditions from 2,3-diaminophenol and ketones through microwave assisted acid catalysis, the benzoxazepine/benzodiazepine ratio depending on the R(1) and R(3) aryl substituents. The otherwise inaccessible and unknown 2,2-dimethyl-4-aryl-1,5-benzodiazepines 8 were also prepared in an analogous manner. The reaction mechanism was investigated by means of DFT calculations. Structural assignments of the new compounds as well as complete assignment of (1)H and (13)C NMR signals have been unambiguously achieved on the basis of the analysis of their (1)H and (13)C NMR (1D and 2D), IR, MS, and elemental analysis data, whereas the presence of an amino group in 5 and of a hydroxyl in 6 was confirmed by derivatization. Compounds 2, 3, 5f, 6a, 6c, 6d, 6f, 6h, 8c, and 12 were evaluated as antioxidants and lipid peroxidation inhibitors in vitro. Compound 6f was also evaluated as anti-inflammatory agent in vivo. Compounds 2 and 6f were found to be the most potent as inhibitors of lipoxygenase and of lipid peroxidation, respectively.

Characterization of uniformly and atom-specifically 13C-labeled heparin and heparan sulfate polysaccharide precursors using 13C NMR spectroscopy and ESI mass spectrometry

The biological actions of heparin and heparan sulfate, two structurally related glycosaminoglycans, depend on the organization of the complex heparanome. Due to the structural complexity of the heparanome, the sequence of variably sulfonated uronic acid and glucosamine residues is usually characterized by the analysis of smaller oligosaccharide and disaccharide fragments. Even characterization of smaller heparin and heparan sulfate oligosaccharide or disaccharide fragments using simple 1D 1H NMR spectroscopy is often complicated by the extensive signal overlap. 13C NMR signals, on the other hand, overlap less and therefore, 13C NMR spectroscopy can greatly facilitate the structural elucidation of the complex heparanome and provide finer insights into the structural basis for biological functions. This is the first report of the preparation of anomeric carbon-specific 13C-labeled heparin and heparan sulfate precursors from the Escherichia coli K5 strain. Uniformly 13C- and 15N-labeled precursors were also produced and characterized by 13C NMR spectroscopy. Mass spectrometric analysis of enzymatically fragmented disaccharides revealed that anomeric carbon-specific labeling efforts resulted in a minor loss/scrambling of 13C in the precursor backbone, whereas uniform labeling efforts resulted in greater than 95% 13C isotope enrichment in the precursor backbone. These labeled precursors provided high-resolution NMR signals with great sensitivity and set the stage for studying the heparanome–proteome interactions.

ChemInform Abstract: Oxidation of Spiroketones with DDQ - Synthesis of Tropone Derivatives and DDHQ Diesters.

Abstract Oxidation of spiroketones 3a–f with DDQ in dry benzene gave tropone derivatives 4a–f and DDHQ esters 5a–f ( cis - cis isomer 6a–f, trans - trans isomer 7a–f). While the aryl substituted spirokeone 17a gave a 2:1 mixture of 19a and the corresponding trans - trans isomer, the aryl substituted spiroketones 17b–d gave exclusively cis - cis isomers 19b–d. Heating acid chloride of acid 9c with DDHQ resulted in compounds 4a and 7a, thus confirming the structures assigned. Mechanism of formation of these compounds has been rationalised. A detailed study of 2D 1H-1H COSY, 1H-13C COSY, HMBC and 2D NOESY of compound 7d led to complete assignment of 1H and 13C NMR signals and its solution conformation.

A comparative study on the acetylation of wood by reaction with vinyl acetate and acetic anhydride

A comparative study on the acetylation of maritime pine wood by reaction with vinyl acetate (VA) and acetic anhydride (AA) has been undertaken. The reactivity of wood, cellulose and lignin with regards to VA or AA was examined using different techniques. The products dissolved in the reaction medium after acetylation of wood by the two different methods were analyzed by HPLC chromatography. Results suggested that the cellulose sites in wood were more attacked by VA than by AA in our experimental conditions. Besides, the unreacted cellulose sites of the AA-acetylated sample could be further esterified by vinyl propionate, which 13C NMR signals could be differentiated from the acetyl signals. The esterified materials obtained after modification of isolated cellulose and lignin were also characterized by FTIR, 13C CP-MAS NMR and 13P NMR spectroscopy and compared. Results indicated that VA could acetylate both biopolymers, but they also showed that AA reacted more readily with lignin than VA. Moreover, FTIR spectroscopy revealed that unexpected side reactions concurrently occurred in lignin when VA was used. It is hypothesized that some C-acetylation also took place between VA and a number of lignin aromatic rings during the treatment.

Synthesis and complete assignment of the 1H and 13C NMR signals of new acetamido and aminoflavonoid derivatives

The complete (1)H and (13)C NMR assignment of 9 acetamidochalcones, 18 acetamidoflavones, 18 aminoflavones, 9 acetamidoflavonols and 9 aminoflavonols has been performed using one- and two-dimensional NMR techniques including COSY, HMQC and HMBC experiments.

ChemInform Abstract: Definitive Structural Assignment of Condensation Products from Anthranilamide and 3-Amino-2-carbamoylthiophene with Ketones. Formation of Tetrahydroquinazolinones and Their Thiophene Isosteres.

Products formed from dehydrative cyclization of cyclopentanone, cyclohexanone, and cycloheptanone with anthranilamide (2) under neutral or acidic conditions are established as the spiro compounds 2,2-polymethylene-1,2,3,4-tetrahydroquinazolin-4-ones 6 by means of (a) the presence of a 13C nmr signal at 66–79 ppm for atom C-2 and (b) X-ray crystallography on the tetramethylene compound 6a. Analogously, products from reactions of these cycloalkanones with 3-amino-2-carbamoylthiophene are now shown by 13C nmr spectra to have structures isosteric to 6, i.e. 5,5-polymethylene-4,5,6,7-tetrahydrothieno[2,3-e]pyrimidin-7-ones 5, rather than the chelate ring structures previously proposed. Additionally, conflicting literature reports on product 3 from reaction of acetone with 2 are partially clarified. Correlation of 13C chemical shifts in systems 3, 5, and 6 is presented.

IDENTIFICATION AND SEQUENCING BY NMR SPECTROSCOPY OF THE CARBOHYDRATE MOIETY IN A SAPONIN FROM Barringtonia asiatica

A trisaccharide chain in a saponin from the seeds of Barringtonia asiatica has been identified and sequenced as {[b-D-galactopyranosyl(1®3)- b-D-glucopyranosyl(1®2)]-b-D-glucuronopyranosyloxy} using a combination of homonuclear and heteronuclear correlation NMR spectroscopy. The 1H and 13C NMR signals of the sugar residues can be determined and distinguished from one other by use of the HMQC-TOCSY technique. Anomeric configurations were unambiguously assigned from the vicinal coupling constants 3JH-1,H-2 of the anomeric protons. Inter-glycosidic linkage assignments were elucidated using HMBC. Keyword: Barringtonia asiatica, carbohydrate, saponin, NMR

X-ray structure, NMR and stability-in-solution study of 6-(furfurylamino)-9-(tetrahydropyran-2-yl)purine – A new active compound for cosmetology

The crystal and molecular structure of 6-(furfurylamino)-9-(tetrahydropyran-2-yl)purine ( 1) was determined at 150(2) K. The compound crystallizes in monoclinic P2 1/ c space group with a = 10.5642(2), b = 13.6174(3), c = 10.3742(2) Å, V = 1460.78(5) Å 3, Z = 4, R( F) = for 3344 unique reflections. The purine moiety and furfuryl ring are planar and the tetrahydropyran-2-yl is disordered in the ratio 1:3, probably due to the chiral carbon atom C(17). The individual 1H and 13C NMR signals were assigned by 2D correlation experiments such as 1H– 1H COSY and ge-2D HSQC. Stability-in-solution was determined in methanol/water in acidic pH (3–7).

13C hyperpolarization of a barbituric acid derivative via parahydrogen induced polarization

Significant 13C NMR signal enhancement by a factor of 5000 of a barbituric acid derivative (5-methyl-5-propenyl-barbituric acid) via parahydrogen induced polarization is presented. This hyperpolarization is achieved by hydrogenating 5-methyl-5-propargyl-barbituric acid with 98% enriched para-H 2 under elevated temperature and pressure and transferring the initially created 1H hyperpolarization with an INEPT-derived pulse sequence to 13C. The polarization can be selectively transferred to different carbons in the barbituric acid derivative by applying different pulse delays in the INEPT pulse sequence. These results demonstrate the potential of using hyperpolarized barbituric acid derivatives as “ active” contrast agents in MRI and visualizing their pharmacokinetics in vivo.

Complete 1H and 13C NMR signal assignments and chemical shift calculations of four 1,2,4-oxadiazole-based light-emitting liquid crystals

In this article, we describe the complete 1H and 13C NMR signal assignments of four 1,2,4-oxadiazoles possessing light-emitting liquid crystal properties. These results were obtained by using one- and two-dimensional NMR techniques as well as GIAO (PCM) calculations at B3LYP/6-311++G(d,p) level for compounds 1 and 2a–d. The computed values are in good agreement with the ones obtained experimentally. In addition, some previously unexplained thermotropic features of compounds 2a–d could be clarified with the help of the geometry optimization calculations carried out by us.

Effect of solvent on the dehydrogenation of poly(1,3-cyclohexadiene): Formation and characteristics of benzoquinone-aromatic hydrocarbon charge-transfer complexes

The effect of solvent on the dehydrogenation of poly(1,3-cyclohexadiene) (PCHD) with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) [or 2,3,5,6-tetrachloro-1,4-(p-)-benzoquinone (TCQ)] was examined to improve the reactivity of benzoquinones for this dehydrogenation reaction. The dehydrogenation of PCHD with DDQ (or TCQ) was strongly affected by the type of solvent, and aromatic hydrocarbon based solvents were appropriate for this dehydrogenation reaction. A charge-transfer complex between DDQ (or TCQ) and aromatic hydrocarbons was formed in the reaction mixture, and the reactivity of the complex was much higher than that of free DDQ (or TCQ). The formation of a DDQ–aromatic hydrocarbon complex, which has a large diamagnetic shift of the 13C NMR signals with respect to DDQ, was the primary factor for improvement of the reactivity of DDQ. For the TCQ–aromatic hydrocarbon complex, the existence of an electron-withdrawing group on the aromatic hydrocarbon was the major factor for improvement of the reactivity of TCQ. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 342–350, 2010

Chiral, Neutral, and Paramagnetic Gold Dithiolene Complexes Derived from Camphorquinone

AbstractReaction of racemic (dl)‐ or enantiopure (D)‐camphorquinone with P4S10in dimethylimidazolidinone followed by addition of NiCl2·6H2O or KAuCl4affords the corresponding bornylenedithiolato metal complexes [(dl‐bordt)2Ni], [(D‐bordt)2Ni], [(dl‐bordt)2Au], and [(D‐bordt)2Au] in their neutral form. Cyclic voltammetry and UV/Vis/NIR spectrometry show that the bornylenedithiolato ligand acts as a highly electron‐rich dithiolene ligand. Complete assignment of the1H and13C NMR signals of the diamagnetic enantiopure [(D‐bordt)2Ni] is reported thanks to a combination of 2D NMR spectroscopy experiments. X‐ray crystal structure determinations were performed for the four complexes, showing that the two diastereomeric mixtures, [(dl‐bordt)2Ni] and [(dl‐bordt)2Au], are isomorphous (space groupP21/c), as are also the two enantiopure complexes, [(D‐bordt)2Ni] and [(D‐bordt)2Au] (space groupP21). Positional disorder of the two (D)‐ and (L)‐bornylenedithiolate enantiomers on the same position is observed in the structure of the diastereomeric mixtures, with the complex located on the inversion center. The temperature dependence of the magnetic susceptibility of the paramagnetic gold complexes [(dl‐bordt)2Au] and [(D‐bordt)2Au] demonstrates the presence of antiferromagnetic interactions, to a larger extent in the diastereoisomeric mixture [(dl‐bordt)2Au] than in the enantiopure complex [(D‐bordt)2Au], as a consequence of stronger lateral S···S intermolecular interactions in the former. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Expeditious one-pot synthesis of highly substituted thiazolo[3,2- a]pyridines involving chromones

The 1,4-zwitterion derived from 4,5-dimethylthiazole and acetylenedicarboxylates has been shown to react at low temperature readily with 3-formylchromones (chromone-3-carboxaldehydes) resulting, after an unusual rearrangement, in the facile synthesis of thiazolo[3,2- a]pyridine derivatives; in the case of electron-donating substituents in the chromone ring tetracyclic chromenothiazolopyridines are isolated as the main reaction products. However, at higher temperature after an unexpected 1,2-aroyl migration 8-formyl-5 H-[1,3]thiazolo[3,2- a]pyridines are formed as a mixture of two rotamers. Structural assignments of the new compounds as well as complete assignment of 1H and 13C NMR signals were based on the analysis of their 1H and 13C NMR (1D and 2D), IR, MS and elemental analysis data. Plausible mechanisms are proposed.

NMR spectral enantioresolution of spirobrassinin and 1-methoxyspirobrassinin enantiomers using ( S)-(−)-ethyl lactate and modeling of spirobrassinin self-association for rationalization of its self-induced diastereomeric anisochromism (SIDA) and enantiomer self-disproportionation on achiral-phase chromatography (ESDAC) phenomena

The phytoalexin spirobrassinin is known to exhibit the unusual phenomenon of enantiomer self-disproportionation on achiral-phase chromatography (ESDAC) whereby the enantiomeric composition of a non-racemic mixture varies across the eluting chromatographic peak. However, spirobrassinin is exceptional since both early and late fractions are enantioenriched in comparison to the starting enantiomeric composition. This behavior is rationalized by the formation of two different complexes, substantiated by DFT calculations at the B3LYP/TZVP level of theory, both of which are in dynamic equilibrium with the free molecules. Concomitant with complexation under chromatographic conditions, complexation was also evident by NMR including the observation of distinct NMR signals for each enantiomer for some spins in a non-racemic mixture—the phenomenon of self-induced diastereomeric anisochromism (SIDA). Enantioresolution of various 1H, 13C and 15N NMR signals were also induced for the enantiomers of spirobrassinin and another phytoalexin, 1-methoxyspirobrassinin, by application of the chiral solvating agent ( S)-(−)-ethyl lactate, a cheap reagent with potentially broad applications. For spirobrassinin, the observed NMR signals were evaluated against values calculated using the GIAO method at the B3LYP/cc-pVTZ level of theory.

Hindered Ureas as Masked Isocyanates: Facile Carbamoylation of Nucleophiles under Neutral Conditions

Hindered Ureas as Masked Isocyanates: Facile Carbamoylation of Nucleophiles under Neutral Conditions