Albert Virgili

Full sensitivity and enhanced resolution in homodecoupled band-selective NMR experiments.

Chemical shifts and coupling constants (J) are fundamentals in the analysis and interpretation of NMR spectra. Multiplicity information and J values can be extracted from the analysis of the fine multiplet structure, and they can be related to structural parameters, such as the number of neighbouring spins, the trace of trough-bond connectivities or dihedral angle constraints. Over recent years, a significant interest has emerged to develop homodecoupled H NMR spectroscopy techniques that offer increased resolution by simplifying the homonuclear splitting pattern, and therefore reducing signal overlapping. The simplest approach for homodecoupling is the use of semiselective shaped pulse decoupling during signal detection, where the receiver and the decoupling are alternatively activated. If the semiselective pulse is applied in a region A of the spectrum, the multiplet structure of J coupled signals resonating in a different region B appear simplified while they are detected. However, this is not a broadband method because protons from a third region C would not be decoupled, and therefore the corresponding coupling splittings will remain in the partially decoupled spectrum. Although the use of sophisticated multiple-region decoupling using different and simultaneous decoupling waveforms could be applied, it is difficult to achieve a perfect decoupling for all resonances and, moreover, without the interference of undesired decoupling sidebands. Alternatively, the internal projection in the chemical shift dimension of J-resolved experiments or the diagonal signals in anti-z-COSY experiments have been also proposed to obtain broadband homodecoupled NMR spectra. They require the collection of more time consuming 2D/3D data and post-processing tasks can be further required. Some years ago, the so-called Zangger–Sterk (ZS) method based on the implementation of the spatially encoded concept along the z-dimension was also proposed. The ZS method has been further refined and several applications have been reported to obtain high-resolved pure-shift multidimensional NMR spectra. The main drawbacks of ZS methods are their low sensitivities because signal only comes from selected z slices and, on the other hand, the need for an FID reconstruction method by means of a time-consuming 2D/3D mode acquisition. Very recently, a new NMR detection scheme has been proposed for the instant and speed-up acquisition of ZS-decoupled spectra in a one-shot single-scan experiment. The instant technique greatly improves the sensitivity per time unit ratio although the attainable sensitivity is still far from a regular H spectrum. Analogous ZS methods incorporating isotopic C editing by using BIRD elements have been also reported to efficiently minimise the effects of strong coupling, but an important penalty in sensitivity remains due to the low natural abundance of C (1.1 %). Based on the instant ZS experiment, a novel NMR spectroscopy method for the fast acquisition of full-sensitive, homodecoupled band-selective (HOBS) NMR spectra is proposed here. It is noteworthy that the spatial encoding gradients applied simultaneously with the selective pulses in the original instant scheme are here omitted, avoiding sensitivity losses due to spatial slice selection. In addition, the HOBS method incorporates a number of advantages, such as: 1) an effective homodecoupling NMR block consisting of a pair of hard/selective 1808 pulses flanked by pulsed field gradients (Figure 1), 2) an excellent spectral quality related to the use of selective gradient echoes, 3) real-time data collection without need of additional reconstruction methods that also allows conventional FID data processing, and 4) an easy implementation in multidimensional experiments. In our hands, the best results in terms of selectivity and optimum

Self-assembly of cyclodextrins with meso-tetrakis(4-sulfonatophenyl)porphyrin in aqueous solution

Abstract The ability of α-CD, β-CD and γ-CD to break the aggregates in the solution of the title porphyrin (TPPS4) and to form inclusion self-assemblies has been studied by UV/Vis and 1H-NMR (ROESY). The behaviour of TPPS4 in the presence of α-CD, β-CD and γ-CD has been compared to that of sodium p-toluenesulfonate and sodium benzenesulfonate. TPPS4 in neutral media forms inclusion assemblies, through its meso-phenyl groups, with β-CD and γ-CD, but not with α-CD. Diprotonated TPPS4, which has a different geometry from the free base, forms an inclusion assembly only with β-CD. The inclusion assemblies of TPPS4 with β-CD and γ-CD show different geometry: introduction of the phenyl substituent through the cyclodextrin secondary face for β-CD and through the primary face for γ-CD.

Implementing homo- and heterodecoupling in region-selective HSQMBC experiments

An NMR method to enhance the sensitivity and resolution in band-selective long-range heteronuclear correlation spectra is proposed. The excellent in-phase nature of the selHSQMBC experiment allows that homonuclear and/or heteronuclear decoupling can be achieved in the detected dimension of a 2D multiple-bond correlation map, obtaining simplified cross-peaks without their characteristic fine J multiplet structure. The experimental result is a resolution improvement while the highest sensitivity is also achieved. Specifically, it is shown that the (1)H-homodecoupled band-selective (HOBS) HSQMBC experiment represents a new way to measure heteronuclear coupling constants from the simplified in-phase doublets generated along the detected dimension.

Simultaneous Multi-Slice Excitation in Spatially Encoded NMR Experiments

Improved sensitivity: A novel strategy to enhance the experimental sensitivity in spatially encoded NMR experiments has been developed. The use of a multiple-frequency modulated pulse applied simultaneously to an encoding gradient can afford a substantial sensitivity gain with respect to single-slice selected experiments.

Pyrazolic palladium compounds containing alcohol functionality: hindered rotation around PdN bond

Abstract Reactions of the ligands 1-hydroxymethyl-3,5-dimethylpyrazole (HL1), 1-(2-hydroxyethyl)-3,5-dimethylpyrazole (HL2) and 1-(3-hydroxypropyl)-3,5-dimethylpyrazole (HL3) with [PdCl2(CH3CN)2] lead to trans-[PdCl2(HL)2] complexes. The ligands are co-ordinated to the Pd(II) by a N atom of the pyrazolic ring in a trans disposition. Two chlorides complete the environment of the metallic atom. These Pd(II) complexes were characterised by elemental analyses, conductivity measurements and IR, 1H and 13C NMR spectroscopies and X-ray diffraction. The NMR study of the complexes is consistent with a very slow rotation of ligands around the PdN bond, so that two conformational isomers can be observed in solution. The crystal structures of [PdCl2(HL)2] (HL=HL1, HL2) complexes are described. These complexes react with AgNO3 to give [Pd(NO3)(HL)2]NO3 compounds.

Stereoselective free-radical cyclization on a sugar template the sulphonyl radical as a synthetic tool for functionalized glycosides

Abstract The addition of a sulphonyl radical to the external double bond of an allyl glycal generates a pro-chiral carbon radical which adds to the glycal double bond with a high diastereoselectivity. Through three consecutive radical steps, the configurations of three new asymmetric centers are controlled.

Enantiodifferentiation through Frequency‐Selective Pure‐Shift 1H Nuclear Magnetic Resonance Spectroscopy

A frequency-selective 1D (1) H nuclear magnetic resonance (NMR) experiment for the fast and sensitive determination of chemical-shift differences between overlapped resonances is proposed. The resulting fully homodecoupled (1) H NMR resonances appear as resolved 1D singlets without their typical J(HH) coupling constant multiplet structures. The high signal dispersion that is achieved is then exploited in enantiodiscrimination studies by using chiral solvating agents.

1,3-Dipolar cycloadditions of 3,4,5,6-tetrahydropyridine 1-oxide to α,β-unsaturated lactones : stereochemical assignment and conformational analysis of the products

Abstract We have investigated the 1,3-dipolar cycloaddition of nitrone 1 to seven, six, and five membered α,β-unsaturated lactones ( 2 , 3 , and 4–6 ) and 2,5-dihydrofuran, 18 . In all these reactions, exo adducts are obtained as major or exclusive products. To elucidate the stereochemistry of the obtained heterocyclic compounds, an accurate conformational analysis has been performed. Important differences in their conformational behavior in solution have been found as a function of the lactone ring size and the presence or absence of a carbonyl group. This study points out the existence of an attractive interaction between the nitrogen lone electron pair and a carbonyl group at a fitting distance, which induces the preference for the cis fusion of type E in the furanonic exo adducts ( 10 , 12 , 13 , 15 , and 16 ). In all the other cases, where this distance is too long ( 7 , 8 , 9 , 11 , 14 ) or no carbonyl group is present in the molecule ( 20 ), the trans invertomers predominate.

β-Cyclodextrin inclusion complex with adamantane intermolecular 1H{1H} NOE determinations and molecular mechanics calculations

Abstract NOE experiments on the β-cyclodextrin (CD) /adamantane complex unequivocally demonstrate that the guest is inside the host cavity. Molecular mechanics calculations indicate the absence of preference by the guest for one single orientation. The host/guest supermolecule has been fully optimized and results show that adamantane is in the narrower half of the β-CD torus.

Preparation of enantiomers of 1-(1-naphthyl)-2,2-dimethylpropylamine and their behaviour as chiral solvating agents: study of diastereochemic association by Job's plots and intermolecular NOE measurements

Abstract Enantiomers of 1-(1-naphthyl)-2,2-dimethylpropylamine have been obtained and considered as chiral solvating agents against several compounds. The formed complexes have been studied with the aid of the nuclear Overhauser effect and its stoichiometry by the method of continuous variations. Two diastereoisomeric complexes present similar geometry of association by π–π-stacking of the aromatic rings and by hydrogen bonding of the functional groups.