Configurational assignment and 1H NMR spectral parameters of isomeric 1,2-diacetoxy-4-alkyl-3-methylcyclopentanes

The ¹H NMR spectral parameters of all eight 1,2-diacetoxy-3,4-dimethylcyclopentanes and 1,2-diacetoxy-4-t-butyl-3-methylcyclopentanes are discussed.     

Configurational assignment of some isomeric 1,4-diacetoxy-2,3-dialkylcyclopentanes from 1H-n.m.r. spectral parameters

From the study of the ¹H n.m.r. spectral parameters of differently functionalized 1,4-diacetoxy-2,3-dialkylcyclopentanes it is shown that the magnitude of those parameters allows an easy configurational assignment within a set of four different configurations.     

Configurational assignment and 1H-NMR spectral parameters of the isomeric 1,4-diacetoxy-2,3-dimethylcyclopentanes

All six 1,4-diacetoxy-2,3-dimethyl-cyclopentanes have been prepared starting from 2, 3-dimethyl-4-hydroxy-2-cyclopentenone. ¹H-NMR spectral parameters, allowing configurational assignment, are discussed.     

Stereochemistry of organic sulphur compounds: 12—Configurational assignment of 2-methylsulphinyl-1-phenylethanol and some O-derivatives

The conformational analysis of the diastereomeric 2-methylsulphinyl-1-phenylethanol and its O-methyl and O-acetyl derivatives has been performed by ¹H NMR spectroscopy. The study of the influence of dilution and solvent polarity changes on the coupling constants has permitted the configurational assignment of each diastereoisomer. The role played by hydrogen bonding in the hydroxysulphoxides has also been studied by IR spectroscopy.     

Investigation of group 12 metal complexes with a tridentate SNS ligand by X-ray crystallography and 1H NMR spectroscopy

Two series of zinc triad complexes containing the ligand 2,6-bis(methylthiomethyl)pyridine (L1) were synthesized and characterized by X-ray crystallography and solution-state 1H NMR spectroscopy. The distorted meridional octahedral M(L1)2(ClO4)2 series includes the first structurally characterized Zn(II) and Cd(II) complexes with N2(SR2)4 coordination spheres. Coordination of HgCl2 and ZnCl2 with 1 equiv of ligand afforded mononuclear, five-coordinate species Hg(L1)Cl2 and Zn(L1)Cl2, respectively, with distorted square-pyramidal and trigonal-bipyramidal geometries. With CdCl2, the dimeric [Cd(L1)Cl(mu-Cl)]2 complex was obtained. The distorted octahedral coordination geometry of each Cd(II) center in this complex is formed by one tridentate ligand, two bridging chloride ions, and one terminal chloride ion. NMR spectra indicate that the intermolecular ligand-exchange rate of [M(L1)2](2+) decreased in the order Cd(II) > Zn(II) > Hg(II). Slow intermolecular ligand-exchange conditions on the chemical-shift time scale were found for 1:2 metal-to-ligand complexes of L(1) with Hg(II) and Zn(II) but not Cd(II). Slow intermolecular ligand-exchange conditions in acetonitrile-d(3) solutions permitting detection of (3-5)J(199Hg1H) were found for 1:1 and 1:2 Hg(ClO4)2/L1 complexes, but not for the related Cd(ClO4)2) complexes. The magnitudes of J(199Hg1H) for equivalent protons were smaller in [Hg(L1)2](2+) than in [Hg(L1)(NCCH3)x](2+). The relative intermolecular ligand-exchange rates of the zinc triad complexes investigated here suggest that the toxicity of Hg(II) is accentuated by the relative difficulty of displacing it from the coordination sites encountered.     

Configurational assignment of fragment ions by CID. Stereochemistry and mechanism of retro-Diels–Alder fragmentation accompanied by hydrogen transfer in gas-phase cations

Low energy CID mass spectra of m/z 173, C₈H₁₃O₄⁺, obtained from the diethyl ester of cis,syn,cis-l,2,3,3a,4,5,5a,6,7,8-decahydroindacene-4,5-dicarboxylic acid and cis,syn,cis-l,2,3,4,5,6,7,8,8a,9,10,10a-dodecahydrophenanthrene-9,10-dicarboxylic acid indicate they have the structure of protonated diethyl maleate. This finding together with previous deuterium labelling results suggest that the formation of this ion from both precursors takes place by migration of a hydrogen atom from an allylic δ-position followed by the concerted cleavage of two allylic C? C bonds in analogy to the ground state retro-Diels–Alder fragmentation.     

New and old NMR experiments for the resonance assignment of complex oligosaccharides--application to a cyclodextrin derivative

The complete assignment of the (1)H and (13)C sugar resonances in mono-3,6-anhydro-heptakis(2,3-O-methyl)-hexakis(6-O-methyl)-β-cyclodextrin, an asymmetrically functionalized β-cyclodextrin, was carried out by means of 2D NMR experiments. The TOCSY and the homonuclear multiple relay COSY spectra provided most of the (1)H assignments. The multiplicity edited HSQC and a set of F(1) selective HSQC-TOCSY and multiple relay HSQC-COSY spectra gave access to most of the (13)C chemical shifts. The latter were fully and accurately determined by means of a pair of complementary, highly folded HSQC-TOCSY spectra. The TOCSY-ROESY and ROESY-TOCSY spectra yielded the sequential assignment of the sugar units. A high resolution F(1) selective F(1) decoupled version of the TOCSY-ROESY experiment was recorded.     

1H and 13C NMR studies of the protonation of isomeric methoxysulmazole analogues

The major protonation sites of six cardiotonic isomeric 2-aryl-n-methoxy-1H-imidazo[4,5-b]- and -[4,5-c]-pyridines (n = 4–7) were determined by ¹H and ¹³C NMR methods. All the 1H-imidazo[4,5-c]pyridines and the 7-methoxy derivative of sulmazole were found to protonate at the pyridyl nitrogen. Protonation occurred at the imidazo nitrogen, however, for the 5- and 6-methoxy derivatives of sulmazole.     

Synthesis and characterization of two cyclic organosiloxanes by multinuclear NMR spectroscopy and X-ray crystallography

1,3-[2′,6′-Pyridinebis(methyleneoxy)]-1,3-bis(diphenyl)cyclodisiloxane (9) and 2,6-pyridinebis(1,1-diphenylethoxy)diphenylsilane (11) were obtained from 2,6-pyridinediol derivatives with dichlorodiphenylsilane. An N→Si interaction is present in 2,6-pyridinebis(1,1-diphenylethoxy)diphenylsilane, which also shows fluxional behavior. The activation energy of 13.2 kcal mol⁻¹ for 11 was obtained for the intramolecular exchange between the phenyl groups from a variable-temperature ¹H-NMR study. The compounds were characterized by ¹H-, ¹³C- and ²⁹Si-NMR and their structures were established by X-ray crystallographic studies.     

Conformational studies about the NN′ bond by NMR spectroscopy and their application in the configurational assignment to the Diels-Alder adducts of maleic annhydride

Non-planar stable conformations about the NN′ bond in N-(diacylamino)imide derivatives of naphthalene-maleic anhydride adducts 2 and 3 and their reduced products 4 and 5 have been studied by NMR spectroscopy and have been demonstrated as a means of assigning exo/endo configurations to the Diels-Alder adducts. While N-(diacetylamino)imide system has proven to be a suitable probe for configurational assignment to Diels-Alder adducts, which show clear cut magnetic effects on the N′-substituents, the N′-acetyl-N′-aroyl-N-aminoimide system has been shown to be a versatile probe, applicable even to simple adducts. The N-(diaroylamino) imide system has shown little effect on the cage-protons, whereby it has been presumed that the aryl-rings of the N′-aroyl groups preferably reside away from the cage-moiety.     

The assignment of the absolute configuration of 1,2-diols by low-temperature NMR of a single MPA derivative

[reaction: see text] The absolute configuration of 1,2-primary/secondary diols can be easily assigned by low-temperature NMR of a bis-MPA ester derivative. The assignment requires the analysis of just the methylene protons, is not limited by the absence of signals from the R group of the diol, and requires a very small and recoverable sample.     

NMR and X-ray crystallographic studies of the conformation of a 3,4,6-triphenyl-delta-lactone.

1-Oxa-3S,4S,6R-triphenyl-2-cyclohexanone and its enantiomer were synthesized, and the structure was determined by NMR and X-ray crystallography. The X-ray crystal structure showed that the delta-lactone adopts a boat conformation in the solid. The X-ray data showed a shortened C-O bond between the carbonyl carbon and the ether oxygen, consistent with delocalization involving the ester group. (1)H and (13)C NMR measurements in acetone-d(6) showed that the lactone is biased in favor of a boat conformation. In the less polar solvent chloroform-d(1), changes in the (1)H NMR coupling constants indicate a shift in the equilibrium in favor of a less rigid twist-boat conformation. The IR absorption of the lactone carbonyl at 1740 cm(-)(1) would suggest a half-chair conformation inconsistent with the dominance of the boat forms shown by NMR and X-ray.     

Synthesis and structural characterization of new stiff rod oligomeric domains by X-ray crystallography and NMR

The monomers N,N'-dibenzylbenzene-1,4-diamine (1), N,N'-dibenzylnaphthalene-1,5-diamine (2), and N,N'-dibenzylanthracene-1,9-diamine (3) were reacted with phosgene in the presence of a base to produce the corresponding N,N'-dibenzyl-1,4-bis(chlorocarbonylamino)benzene (4), N,N'-dibenzyl-1,5-bis(chlorocarbonylamino)naphthalene (5), and N,N'-dibenzyl-9,10-bis(chlorocarbonylamino)anthracene (6). These monomers were used to create zigzag type stacks, in a stepwise fashion, of trimers and 9-mers of either 1,4-diureidobenzenes ((Phe)K(3) and (Phe)K(9)) or 1,5-diureidonaphthalenes ((Nap)K(3) and (Nap)K(9)). A byproduct in the formation of (Phe)K(9) was a cyclic hexamer (Phe)K(6). NMR gave evidence of the structure in solution while X-ray crystallographic information was obtained for 5, 6, (Nap)K(3), and the cyclic (Phe)K(6).     

Stereochemistry of organic sulphur compounds. part 13. Configurational assignment of diastereoisomers of 2-methylsulphinyl-1,2-diphenylethanol and of their O-methyl and O-acetyl derivatives

The synthesis, isolation and conformational analysis of the diastereomeric 2-methylsulphinyl-1,2-diphenylethanol and of its $\text{O}$-methyl and $\text{O}$-acetyl derivatives are reported. Chemical correlations and the study of the influence of solvent polarity changes on the coupling constants have permitted the configurational assignment. Lanthanide shift reagents have been used also. to this effect. The role of hydrogen bonding in the hydroxysulphoxides has been evaluated in diluted solutions by IR and NMR spectroscopy. A donor-acceptor interaction between oxygen and sulphur has been invoked to explain the differences in conformational behaviour between epimeric sulphoxides at sulphur atom.     

Assignment of the ferriheme resonances of the high-spin forms of nitrophorins 1 and 4 by 1H NMR spectroscopy: comparison to structural data obtained from X-ray crystallography

In this work, we report the assignment of the majority of the ferriheme resonances of high-spin nitrophorins (NPs) 1 and 4 and compare them to those of NP2, published previously. It is found that the structures of the ferriheme complexes of NP1 and NP4, in terms of the orientation of the histidine imidazole ligand, can be described with good accuracy by NMR techniques and that the angle plot proposed previously for the high-spin form of the NPs (Shokhireva, T. Kh.; Shokhirev, N. V.; Walker, F. A. Biochemistry 2003, 42, 679-693) describes the angle of the effective nodal plane of the axial histidine imidazole in solution. There is an equilibrium between the two heme orientations (A and B), which depends on the heme cavity shape, which can be altered by mutation of amino acids with side chains (phenyl vs tyrosyl) near the potential position where a heme vinyl group would be in one of the isomers. The A:B ratio can be much more accurately measured by NMR spectroscopy than by X-ray crystallography.     

Structure assignment in the solid state by the coupling of quantum chemical calculations with NMR experiments: a columnar hexabenzocoronene derivative.

We present a quantum chemical ab initio study which demonstrates a new combined experimental and theoretical approach, whereby a comparison of calculated and experimental (1)H NMR chemical shifts allows the elucidation of structural arrangements in solid-state molecular ensembles, taking advantage of the marked sensitivity of the (1)H chemical shift to intermolecular interactions. Recently, Brown et al. have shown that, under fast magic-angle spinning (MAS) at 35 kHz, the resolution in a (1)H NMR spectrum of the solid phase of an alkyl-substituted hexabenzocoronene (HBC) derivative is sufficient to observe the hitherto unexpected resolution of three distinct aromatic resonances ( J. Am. Chem. Soc. 1999, 121, 6712). Exploiting the additional information about proton proximities provided by (1)H double-quantum (DQ) MAS NMR spectroscopy, it was shown that the results are qualitatively consistent with the aromatic cores packing in a manner similar to that in unsubstituted HBC. Using the HBC-C(12) molecule as an example, we show here that the new combined experimental and theoretical approach allows the observed (1)H chemical shifts to be related in a quantitative manner to the intermolecular structure. In the quantum chemical calculations, a series of model systems of stacked HBC oligomers are used. On account of the marked dependence of the (1)H chemical shift to ring currents arising from nearby aromatic rings, the calculated (1)H chemical shifts are found to be very sensitive to the stacking arrangement of the HBC molecules. Moreover, the ring current effect is found to be particularly long range, with a considerable influence of the second neighbor, at a distance of 700 pm, being observed.     

NMR study of antipyrine adducts with lanthanide shift reagents. Molecular structure of 2,3-dimethyl -1-phenyl- 5-ethoxypyrazole

The complex formation of antipyrine with LSR in CCl₄ is studied. Recommendations for producing the limiting lanthanide induced shifts characteristic of the 11 complex are given and the geometry of the complex formed is determined. Results of an x-ray structure study of the adduct which is formed upon reaction of antipyrine and the ethyl ester of benzenesulfonic acid are presented.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 191–196, February, 1989.