Two-Dimensional Proton Chemical Shift Correlated NMR Spectroscopy of Digitoxose1

Abstract

The hydroxyl proton coupled ¹H NMR spectra of solutions of β-d-digitoxopyranose and of an equilibrated mixture of the four ring forms of d-digitoxose in dimethylsulfoxide-d ₆ have been assigned completely by two-dimensional, proton chemical shift correlated NMR spectroscopy and spin decoupling at 400 MHz. Analysis of resolution enhanced, one-dimensional ¹H NMR spectra yielded an almost complete set of CH and OH proton-proton coupling constants for the four ring forms. The free aldehydo form of d-digitoxose in dimethylsulfoxide-d ₆ solution has been detected by means of its characteristic H-l quartet at 6 9.687. Quantitative analysis of the equilibrated mixture of the five forms of d-digitoxose gave the composition:- α-pyranose, β-pyranose, α-furanose, β-furanose, aldehydo form, 11.2, 67.3, 8.4, 13.0, and 0.1%, respectively. The ⁴ C ₁ chair conformations have been assigned to the α- and β-pyranose forms by analysis of the coupling constants and are discussed qualitatively in terms of their relative stabilities.     

Complete assignment of the 13C n.m.r. spectrum of mutarotated D-ribose by integration and specific deuteration

The ¹³C n.m.r. spectrum of mutarotated D -ribose is completely assigned by using 2- and 3-deuterio-D -ribose as reference compounds. The signal intensities of the largely relaxed ¹³C n.m.r. spectrum are evaluated, and the equilibrium mixture of mutarotated D -ribose is found to consist of 6·1% α-furanose, 11·6% β-furanose, 20·3% α-pyranose and 62% β-pyranose.     

Synthesis of homochiral 2-C-perfluoroalkyl substituted d- and l-riboses

Homochiral 2-C-perfluoroalkyl substituted d- and l-riboses were synthesized via Barbier, Grignard and Ruppert type reactions. The influence of the size of the perfluoroalkyl groups, attached to C-2, on the furanose/pyranose as well as on the α-furanose/β-furanose and α-pyranose/β-pyranose ratio in solution was studied.     

The Composition and Conformation of Sugars in Solution

The conformations of the aldoses in solution, and the compositions of the equilibrium mixtures formed from aldoses after dissolution in water, have been determined by NMR spectroscopy. Calculations based on non-bonded interactions and on the anomeric effect account for the observed conformations, and allow an approximate prediction of the α:β ratio of pyranoses in equilibrium and of the extent of anhydride formation from sugars in acid solution. In certain circumstances the furanose forms are more stable than the pyranose forms.     

A new hybrid (experimental–theoretical) quantitative method for detection of relative anomer concentrations in water

A novel and inexpensive hybrid (combined experimental and theoretical) approach was used to quantitatively identify anomer proportions of d-glucose, d-galactose and d-mannose in water. The study involves three parts: recording of experimental FT-IR spectra of monosaccharides in water, calculation of vibrational frequencies of all stable anomers of monosaccharides and regression analysis of theoretical and experimental intensities. No expensive experimental processes and high-level calculations were needed during the study. The results were compared with the data from pure experimental and molecular dynamic studies. It has been concluded that in water while d-glucose and d-mannose have two stable anomers, α-pyranose and β-pyranose, d-galactose has four stable anomers, α-pyranose, β-pyranose, α-furanose and β-furanose. Comparison of detected results with the literature data showed that the developed method is working for the quantitative detection of anomer proportions of monosaccharides in water.     

PMR investigation into the structure of some n-substituted 1-amino-1-deoxy-D-fructoses (amadori rearrangement products) : Evidence for a preferential conformation in solution

The PMR spectra at 220 MHz of some Amadori rearrangement products deduced from D-glucose with p-toluidine (1), N-methylphenylamine (2), di-butylamine (4), piperdine (5), and morpholine (6) have been studied in detail.Compounds 1-6 appear to exist in solution predominantly as an equilibrium mixture of the furanose and pyranose ring. The pyranose ring occurs exclusively in the β(D)-²C₅-conformation (corresponds to Reeves 1C-conformation). The furanose ring probably exists as a mixture of both the β- and α-anomer, in which the β-anomer is favoured.     

枸杞子糖缀合物LbGp4糖链的结构及其免疫活性的研究

枸杞子糖缀合物LbGp4经β--消除、柱层析分离后得到一条分子量高达180800的分支多糖化合物LbGp4-OL.LbGp4-OL糖基组成为n(Rha):n(Ara):n(Gal)=0.05:1.33:1.根据甲基化、部分酸水解及¹HNMR、¹³CNMR分析,确定了其主要的结构,并对LbGp4和LbGp4-OL的免疫活性进行了研究.     

Comprehensive structural studies of 2',3'-difluorinated nucleosides: comparison of theory, solution, and solid state

The conformations of three 2',3'-difluoro uridine nucleosides were studied by X-ray crystallography, NMR spectroscopy, and ab initio calculations in an attempt to define the roles that the two vicinal fluorine atoms play in the puckering preferences of the furanose ring. Two of the compounds examined contained fluorine atoms in either the arabino or xylo dispositions at C2' and C3' of a 2',3'-dideoxyuridine system. The third compound also incorporated fluorine atoms in the xylo configuration on the furanose ring but was substituted with a 6-azauracil base in place of uracil. A battery of NMR experiments in D 2O solution was used to identify conformational preferences primarily from coupling constant and NOE data. Both (1)H and (19)F NMR data were used to ascertain the preferred sugar pucker of the furanose ring through the use of the program PSEUROT. Compound-dependent parameters used in the PSEUROT calculations were newly derived from complete sets of conformations calculated from high-level ab initio methods. The solution and theoretical data were compared to the conformations of each molecule in the solid state. It was shown that both gauche and antiperiplanar effects may be operative to maintain a pseudodiaxial arrangement of the C2' and C3' vicinal fluorine atoms. These data, along with previously reported data by us and others concerning monofluorinated nucleoside conformations, were used to propose a model of how fluorine influences different aspects of nucleoside conformations.     

NMR studies of conformations and molecular recognition of pyridinio-appended and nicotinamide-appended β-cyclodextrin

Abstract

The conformations of pyridinio-appended β-cyclodextrin (CDP⁺) and nicotinamide-appended β-cyclodextrin (CDNA⁺) were studied by NMR spectroscopy. The orientations of the pyridine residue of CDP⁺ and the nicotinamide residue of CDNA⁺ were determined by using a combination of NMR spectroscopic techniques. NMR spectra indicate that the shapes of the cavities of CDP⁺ and CDNA⁺ were changed after forming complexes. This change depended on the shape of the guest. CDNA⁺ could separate the ¹H resonances at the Cβ position of racemic tryptophan into two sets of resonances for each enantiomer.     

Conformational Studies of Marine Polyhalogenated α-Chamigrenes Using Temperature-Dependent NMR Spectra. Cyclohexene-ring flipping and rigid-chair cyclohexane ring in the presence of equatorial halogen atoms at C(8) and C(9)

Temperature-dependent NMR spectra indicate that the α-chamigren-3-ones (?) -11 , (+) -12 , (+) -14 (?) -15 , (+) -16, 18 , and 19 bearing equatorial halogen atoms at C(8) and C(9) undergo slow conformational flipping of the envelope-shaped enone ring, while the cyclohexane ring is maintained in the chair conformation. The α-chamigren-3-ols (+) -20 and (+) -21 , obtained by hydride reduction of (+) -12 , behave similarly, with slow half-chair inversion of the cyclohexenol ring. In each case, both conformers are about equally populated and detectable by NMR, except in the case of (+) -15 , where repulsive interactions between Br? C(2) and H_eq?C(7) make the population of the conformer 15b with Me—C(5) faced to H_ax?C(10) so low that it escapes direct ¹H-NMR detection. The energy barriers to these conformational motions are viewed to arise mainly from repulsive interactions between Me—C(5) and the axial H-atoms at C(8) and C(10), while, contrary to previous beliefs, no twist-boat conformations of the cyclohexane ring intervene. Similar conclusions hold for the 4,5-epoxides of both (?) -6 and (+) -7 . Clean Jones oxidatio of (?) -2 to 17 , where the CH₂?C(5) bond is maintained, and acid dehydration-isomerization of the α-chamigrene (+) -21 to the β-chamigrene (+) -24 , reflect the special stability of β-chamigrenes, providing a reason for their frequent occurrence in nature.     

Studies on mechanism of reaction and density behavior during anhydrous D-fructose mutarotation in the supercooled liquid state

Recently, we have studied the mutarotation kinetics in D-fructose by means of dielectric spectroscopy. In the present work we investigate density behavior of D-fructose during mutarotation process. By performing volume measurements at temperature T = 303 K and pressure p = 10 MPa we are able to monitor kinetics of this process. As a result we found nearly the same value of the rate constant as previously determined from dielectric measurements. However, these two experimental methods monitor different molecular aspects of mutarotation phenomenon in D-fructose. Dielectric spectroscopy is sensitive to the decay of former ring as well as to the forming of another, while specific volume measurements are sensitive to the forming of new tautomers only. Calculations of activation energy of mutarotation in D-fructose led us to the conclusion, that mechanism of this reaction in amorphous phase could be based on internal proton transfer. Moreover it was found that the main mutarotation path in quenched D-fructose melt is transformation of α,β-furanose to β-pyranose.     

Synthesis and High-Field NMR of α-D-Arabinofuranosyl-(1→6)-2-AcetamidO-2-Deoxy-O-D-Glucopyranosyl-(1→3)-6-Deoxy-L-Talose,the Repeating Unit of an O-Specific Lipopolysaccharide from Pseudomonas Maltophilia N.C.I.B. 9204

Abstract

Starting from L-rhamnose, benzyl 2,4-di-O-benzyl-6-deoxy-α-L-talopyranoside (7) was prepared by hydrogenolysis of a dioxolane-type benzylidene acetal and used as the aglycon to prepare 2-acetamido-2-deoxy-6-D-glucopyranosyl-(1–3)-6-deoxy-L-talose (13) and the title trisacchafide (20). Due to fast interconvirsion between the α-, β- -pyranose/furanose forms at the reducing end of the molecule in aqueous solutions, the di- and trisaccharides occur as mixtures of four isomers all in significant concentration. By two-dimensional (20) methods, the proton (400) and carbon (100 MHz) NMR spectra of the individual tri-saccharide isomers were completely assigned and interpreted in terms of stereochemistry of the 6-deoxy-L-talose residue.     

Conformational composition of the 2,4,6-trimethyl-1,3,2-dioxaborinane stereoisomers

A study of conformational transformations of 2,4,6-trimethyl-1,3,2-dioxaborinane cis- and trans-isomers in non-empirical quantum-chemical HF/6-31G (d) and PBE/3z approximations showed that the transformation routes include the sofa conformers with different orientations of the substituents at the ring atoms C⁴ and C⁶ through the transition states corresponding to diequatorial, axial-equatorial, and dixial conformations of the 2,5-twist form. The calculation and ¹H NMR data show that the conformational equilibrium of cis-isomer is almost entirely shifted to diequatorial sofa, and the trans-form is characterized by the interconversion between the two axial-equatorial sofa conformers.     

13C and 1H NMR spectra of monosubstituted ferrocenes containing a chiral centre in the substituent

The ¹³C-{¹H} and ¹H NMR spectra of ferrocenyl chalkones (β-benzoylvinylferrocene add cinnamoylferrocene) and their iron carbonyl complexes were recorded. Splitting of the resonances of both α- and β-substituted cyclopentadienyl ring atoms was found in ¹³C-{¹H} NMR spectra of all complexes. In ¹H NMR spectra splitting of the resonances of the substituted cyclopentadienyl ring α-protons was detected only in the case of (cinnamoylferrocene)iron tricarbonyl. The splitting effect mentioned can be due to thesochsochronism (magnetic nonequivalence) of all carbon and hydrogen nuclei because of the iron carbonyl fragment coordination oauses the substituent to become the chiral group. The coordination of iron carbonyl group results in a reduction of the conjugation in the α,β-unsaturated ketone system. This phenomenon is discussed on the basis of ¹³C-{¹H} and ¹H NMR data.     

13C-{1H} NMR spectra of episulphide copolymers

Ethylene sulphide-isobutylene sulphide and propylene sulphide-isobutylene sulphide copolymers have been prepared using anionic catalysts and investigated by ¹³C-{¹H} NMR spectroscopy. The carbon-13 NMR spectra are assigned in terms of diad and triad sequences. There is discussion of the effects of mono- or dimethyl substitution in the α, β, γ or δ positions on the chemical shift of the main chain carbon atoms. It has also been shown that for isobutylene sulphide, as for propylene sulphide, under the influence of an anionic catalyst, there is a normal ring opening only at the primary carbon atom.     

13C NMR study of the enol forms of β-diketones

Carbon-13 NMR spectra of a series of β-diketones in the enol form with various β-substituents have been studied. An additive influence of the β-substituents on the chemical shifts of the carbon atoms in the hydrogen bonded chelate ring has been found. It is shown that the α- and β-carbon chemical shifts can be calculated by means of a set of increments for arbitrary combinations of the X and Y substituents. Analysis of the experimental data enables the conclusions to be drawn that enol–enolic tautomerism with different populations of the forms (A) and (B) is absent in β-diketones and that the carbon chemical shift changes are caused by electron density redistribution in the hydrogen bonded chelate ring (C).     

Solid-state nuclear magnetic resonance spectroscopy studies of furanose ring dynamics in the DNA HhaI binding site

The dynamics of the furanose rings in the GCGC moiety of the DNA oligomer [d(G 1A 2T 3A 4 G 5 C 6 G 7 C 8T 9A 10T 11C 12)] 2 are studied by using deuterium solid-state NMR (SSNMR). SSNMR spectra obtained from DNAs selectively deuterated on the furanose rings of nucleotides within the 5'-GCGC-3' moiety indicated that all of these positions are structurally flexible. The furanose ring within the deoxycytidine that is the methylation target displays the largest-amplitude structural changes according to the observed deuterium NMR line shapes, whereas the furanose rings of nucleotides more remote from the methylation site have less-mobile furanose rings (i.e., with puckering amplitudes < 0.3 A). Previous work has shown that methylation reduces the amplitude of motion in the phosphodiester backbone of the same DNA, and our observations indicate that methylation perturbs backbone dynamics through the furanose ring. These NMR data indicate that the 5'-GCGC-3' is dynamic, with the largest-amplitude motions occurring nearest the methylation site. The inherent flexibility of this moiety in DNA makes the molecule more amenable to the large-amplitude structural rearrangements that must occur when the DNA binds to the HhaI methyltransferase.     

MM3 MODELING OF ALDOPENTOSE PYRANOSE RINGS

MM3 (version 1992, ?=3.0) was used to study the ring conformations of d-xylopyranose, d-lyxopyranose and d-arabinopyranose. The energy surfaces exhibit low-energy regions corresponding to chair and skew forms with high-energy barriers between these regions corresponding to envelope and half-chair forms. The lowest energy conformer is ⁴ C ₁ for α- and β-xylopyranose and α- and β-lyxopyranose, and the lowest energy conformer is ¹ C ₄ for α- and β-arabinopyranose. Only α-lyxopyranose exhibits a secondary low-energy region (¹ C ₄) within 1 kcal/mol of its global minimum. Overall, the results are in good agreement with NMR and crystallographic results. For many of these molecules, skew conformations are found with relatively low energies (2.5 to 4 kcal/mol above lowest energy chair form). The ² S _O and ¹ C ₄ conformers of crystalline benzoyl derivatives of xylopyranose are in secondary low-energy regions on the β-xylopyranose surface, within 3.8 kcal/mol of the global ⁴ C ₁ minimum.     

Diazepines—XXII : 13C NMR spectra of 2,3-dihydro-1,4-diazepinium salts

The ¹³C NMR spectra of a number of 2,3-dihdyro-1,4-diazepinium salts, like ¹H NMR spectra, confirm shape, and charge distribution for these compounds. The ring has a half-chair shape which is inverting rapidly at room temperature. The atoms in the conjugated portion of the ring show alternating polarity, and phenyl groups attached to these positions act, respectively, as electron-donors or electron-acceptors. A methyl substituent in the saturated portion of the ring shows equal preference for quasi-equatorial and quasi-axial conformations. Some comparisons are drawn between present results and similar results obtained with related benzene derivatives.     

Preferred conformations of pilocarpine,pilocarpine.HCl and isopilocarpine.HCl in solution,deduced from 200 MHz 1H NMR spectra

The 200 MHz ¹H NMR spectra of pilocarpine, pilocarpine.HCl and isopilocarpine.HCl have been studied. Evidence is presented for the preferred conformations of the compounds in aqueous solution, with the methylimidazolyl moiety antiperiplanar with respect to the α-carbon of the butanolide ring.