Study of the structure and stereochemistry of flavonoid O-arabinosides and xylosides with the aid of PMR spectroscopy

Summary A comparative analysis of the PMR spectra of a number of flavonoid O-arabinosides and O-xylosides and of their full acetates and TMS ethers has been made which has permitted the determination of the conformations of the pentoses and of the glycosidic bonds. The region of the signals of the protons at C-5 in the spectra of the full acetates is diagnostic for determining the size of the oxide ring of a pentose residue.Polystachoside and guaiaverin have been shown to be identical (quercetin 3-O--L-arabinopyranoside). Inversion of the conformation of the pyranose ring of the arabinose residue has been detected for the TMS ethers of guaiaverin and polystachoside.All-Union Scientific-Research Institute of Medicinal Plants, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 21–34, January–February, 1979.     

Proton magnetic resonance studies of compounds with bridgehead nitrogen atoms—XX: The NMR spectra and stereochemistry of some hexahydropyrido[2, 1-c] [1, 4] oxazin-3(4H)-ones and related compounds

A series of substituted hexahydropyrido [2,1-c] [1,4] oxazin-3(4H)-ones has been synthesised, and the configurations of these bicyclic lactones assigned utilising chemical and spectral data. All the compounds adopt trans-fused conformations and the conformation of the lactone ring is discussed with reference to the magnitude of the geminal coupling constant of the N? CH₂? C(O)? O protons, and the vicinal couplings between the angular proton and the methylene protons adjacent to the ring oxygen atom. The lactone ring conformation is shown to differ slightly from the half chair conformation described for some monocyclic δ -lactones. The synthesis and NMR spectra of some related compounds possessing the bridgehead N? CH₂? C(O)? O system are discussed and these compounds are also shown to adopt a trans-fused ring conformation.     

Conformationnal equilibrium of glucides on the level of liaisons sp 2 -sp 3 C-C. IV. Hybrid furans sp 2 in C(3)

The conformation of 34 branched-chain unsaturated sugars, prepared by reacting different Wittig reagents with a series of 1,2-O-isopropylidene-furannosul-3-oses, has been studied by PMR spectroscopy. The position of each compound in the flexible conformational cycle has been established by using the known stereo-dependence of the allylic coupling constants and checked with the other parameters of the spectra. An ‘isopropylidenic’ ⁴J_2,4 coupling constant, present in all compounds whose H—C(4) is endo and absent in their C(4)-epimers has proved useful for configurational assignment at C(4).     

Aromatic derivatives of 1H-2,3-dihydropyrazolo[4,5-b]-1,5-diazepine

Aromatic derivatives of 1H-2,3-dihydropyrazole[4,5-b]-1,5-diazepine were obtained by the reaction of 1-phenyl-3-methyl-4,5-diaminopyrazole with chalcones and acetylarenes, catalyzed by acetic or sulfuric acid. The seven-membered ring in these compounds has a conformation of the boat type. The IR, UV, PMR, and mass spectra of the compounds are discussed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 363–369, March, 1987.     

Studies in the field of aromatic heterocycles

An analysis of the PMR spectra of benzo- 2,1,3-selenadiazoles and also of compounds containing 2,1,3-oxadiazole and -thiadiazole nuclei has been carried out. It has been shown that the key atoms O, S, and Se ensure the possibility of the establishment of a ring current in the corresponding heterocycles. Aromaticity in the sense of a strong ring current in these compounds is somewhat weaker than in naphthalene. The estimates were obtained by calculating the difference _{0 ab} of the chemical shifts of the protons of the benzene ring linked to the heterocycle from the experimental spectra (A₂B₂ system) and the subsequent comparison of these data with the theoretical dependence of _{0 AB}on the distance separating the proton under study and the center of the contour of the ring current. In the case of the benzo-2, 1, 3-X-diazoles (X = O, S, Se), this contour is the heterocyclic ring. Direct measurements of the magnetic susceptibility of these compounds have confirmed the considerable equalization of the bonds in the heterocycle, i.e., the participation of the key atoms O, S, and Se in the formation of the ring system of delocalized electrons. It has been shown that the proposed structural scheme permits the prediction of the magnitudes of the chemical shifts and the PMR spectra of such compounds. The question has been raised of the evaluation of the real acceptor properties (electronegativity) of the key atoms completing the heterocycle.For part IV, see [6].     

Structures of the products of formylation of 1,3,5-triphenyl- and 1,5 -diphenyl-3-stryl-2-pyrazolines

It was established by means of IR and PMR spectra and chemical transformations that the products of the Vilsmeier formylation of 1,3,5-triphenyl- and 1,5-diphenyl-3-styryl-2-pyrazolines are the corresponding 1-(p-formylphenyl)-2-pyrazolines. The frequencies of the stretching vibrations of the carbonyl group and the chemical shift of the aldehyde proton of these compounds indicate considerable conjugation of the p electrons of the N₁ atom of the pyrazoline ring with the aldehyde group.     

(Anti)aromaticity of dehydroannulenes of various ring size proved by the ring current effect in 1H NMR spectra

The spatial magnetic properties (Through-Space NMR Shieldings—TSNMRS) of already synthesized dehydro[n]annulenes of various ring size (from C₁₂ to C₂₀) have been computed, visualized as Isochemical Shielding Surfaces (ICSS) of various size and direction, and were examined subject to present (anti)aromaticity. For this purpose the thus quantified ring current effect of the macro cycles on proximate protons in proton NMR spectra was employed.     

Conformation of substituted benzylidene and isopropylidene nucleosides

PMR and CD spectroscopy has been used to show that the phenyl grouping at C²″ of the dioxolane ring in each diastereomeric benzylidene nucleoside occupies the axial position, most probably due to electrostatic interaction with the heterocyclic base residue. The conformation of the ribose moiety of benzylidene nucleosides is somewhat different from that of isopropylidene analogues; the C_s-conformation of ribose is characteristic of trans-bezylidene uridines and of isopropylidene uridine and adenosine.     

Experimental and theoretical study of the induced paramagnetic ring-current in the 4-membered ring of biphenylene and related hydrocarbons

Ring current intensities and proton chemical shifts have been calculated for a series of condensed aromatic hydrocarbons including biphenylenes. The calculations show that an induced paramagnetic ring current occurs in the 4-membered ring of each of the biphenylenes studied and this effect accounts for the observed spectra. PMR measurements on 1-mono-, 2-mono- and 2,3,6,7-tetra-deuteriated biphenylene prove that the chemical shifts of the 1- and 2-protons are at δ_CDCl3 6.60 and 6.70 respectively and not vice versa as assumed by previous authors.     

1H-NMR investigations of the conformation of aryl-(hydroxynaphthyl)-methylpiperidines. Intramolecular interactions,IV

Summary The¹H-NMR spectra of aryl-(hydroxynaphthyl)-methylpiperidines, which are model compounds for intramolecular hydrogen bonding, have been analyzed in order to investigate their conformations in solution. As dynamic phenomena can be assumed from line broadening, low temperature spectra have been measured to evaluate the coalescence temperatures and the energy barriers. The latter have been discussed with respect to the size and position of selected substituents. It can be shown that the molecules exist in one energetically favorable conformation with the aryl ring perpendicular to the plane of the naphthol ring system. The interaction between the naphthol ring and the aryl ring influences the conformation at the piperidine ring moiety. This effect leads to an increase of the inversion barrier of the piperidine residue.On Sabbatical Leave from Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan     

Stannoboroxanes : III. Synthesis and reactions of 1,3-bis(boryloxy)tetraalkyldistannoxanes and related compounds

1,3-Bis(boryloxy)tetraalkyldistannoxanes,

, R = Me, Bu; G = CMe₂CH₂CHMe—, —CMe₂ —CMe₂ — and —CHMe—CH₂ — have been prepared from dialkyltin oxide and pyroborate, or directly from dialkyltin oxide, boric acid and glycol. A dimeric structure for these compounds is proposed on the basis of molecular weight, IR and PMR studies. The reactivity of the SnOB bond towards PhNCO and Me₃SiCl has been demonstrated.     

Conformational and configurational studies on 3-azabicyclo[3.3.1]nonane (3-abn) derivatives and related systems employing carbon-13 NMR spectroscopy

The ¹³C nmr spectra of 4 cis-2,4-diphenyl-3-azabicyclo[3.3.1]nonanes, 11 cis-2,4-diaryl-3-azabicyclo[3.3.1]-nonan-9-ones, 26 cis-2,4-diaryl-3-azabicyclo[3.3.1]-nonan-9-ols or acetates thereof, 5 cis-2,4-diaryl-3-azabi-cyclo[4.3.1]decan-10-ones or -10-ols and 5 cis-2,4-diphenyl-3-aza-7-thiabicyclo[3.3.1]nonan-9-ones, -9-ols or 9-yl acetates have been recorded. Except for the 7-thia compounds, which appear to exist mainly in the configuration and conformation with the nitrogen-containing ring in the boat form, these compounds seem to exist overwhelmingly in chair-chair conformations. The configuration of the 9-ols and their acetates (syn or anti to the nitrogen-containing ring) has been deduced from the spectra. In a number of cases, the structures assigned differ from those earlier postulated. Broadening of one set of aryl signals (probably those due to the ortho carbons) in the case of N-methyl (but not N-H) compounds without ortho substituents is ascribed to restricted phenyl rotation.     

DNA fragment conformations. I—methods for the assignment of DNA fragment proton resonances. 1H NMR spectra of dApTpGpT and dApCpApTpGpT

A general method for the assignment of DNA fragment proton resonances, especially for the sugar protons, has been presented and used to interpret the 400 MHz proton spectra of dApTpGpT and dApCpApTpGpT in neutral aqueous solution. Only fine splittings of about 3 Hz are observed in the H-2″ resonances, and the total splitting is larger for the H-2′ (≈29 Hz) than for the H-2″ (22–23 Hz) multiplets. The purine and pyrimidine resonances can be distinguished on the basis of the H-2″ and H-2″ chemical shifts. The resonances of the H-2′ and H-2″ protons (above and below the sugar plane, respectively) of dA and dG exhibit chemical shifts of 2.65—2.80 ppm, while those of dC and dT residues are located at higher fields between 1.95 and 2.40 ppm. At high temperature (≥60°C), δH-2′>YδH-2″ for the purine family, while δH-2′ « δH-2″ in the case of the pyrimidine family. Except for the terminal residue, the H-3′ resonances of dA and dG are located at lower fields compared with those of the dC and dT residues. The same is true for the H-4′ resonances. In general δA_1′>δG_1′ and in the case of self complementary duplexes the H-1′ and H-2′ chemical shift variations versus temperature are found to be larger for the dC than for the dT residues.     

A study of nitrogen- and sulfur-containing heterocycles. 41. Synthesis and spatial structure of 3α-alkyl-4,7-dichloro-1,2-dioxooxazolidino[3,2-f]pyrido[2,3-b][1,4]thiazines

The structure of the products of the chlorination of oxazolidino[3,2-f]pyrido-[2,3-b][1,4]-thiazines has been studied with the aid of spectral methods. It has been shown that the chlorine atom substitutes position 4 of the tricyclic system (in the thiazine ring). It has been established on the basis of PMR and ¹³C NMR spectra that in solution the compounds obtained exist in the form of mixtures of two diasteromers with the cis and trans orientations of the substituents at the C₍₃₎ and C₍₄₎ carbon atoms of the tricyclic system in each case. The configurations of the diastereomers have been determined from the chemical shifts in the ¹³C NMR spectra and from the relaxation times in the PMR spectra. It has been shown that the isomer with the transoid arrangement of the substituents on the C₍₃₎ and C₍₄₎ atoms of the tricyclic system, which are included in a thiazine ring present in the half-chair conformation, is energetically the more favorable.For communication 40, see [1].Translated from Khimiya Geterotsiklicheskikh Khimiya, No. 9, pp. 1191–1197, September, 1984.     

Conformational studies of cyclo-gly-pro(S) by X-ray diffraction and proton magnetic resonance

The X-ray crystal structure and the PMR spectrum in ²H₂O and DMSO-d₆ of cyclo-glycyl-4-thiaprolyl has been determined. In the crystal, the diketopiperazine ring of the molecule adopts a boat conformation and the thiazolidine ring an envelope conformation, very similar to the analogous compound cyclo-glycyl-prolyl. Comparison of proton-proton dihedral angles derived from the crystal structure and from vicinal coupling constants in solution indicates that the conformation of cyclo-glycyl-4-thiaprolyl is nearly the same in crystal and in solution in the limits of applied mehods.     

Accurate assignments in PMR and <Superscript>13</Superscript>C NMR spectra of anhydrolycoctonine using 2D spectroscopy

Resonances in PMR and ¹³C NMR spectra of anhydrolycoctonine were fully assigned based on a series of 1D and 2D NMR experiments. The conformation of ring A was concluded to be a distorted boat with H-1β from a comprehensive analysis of chemical shifts, SSCC, and the NOE for two possible conformations. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 267–269, May–June, 2008.     

A proton magnetic resonance study of the conformation of the seven-membered ring in benzocycloheptene

The computer analysis of the PMR spectra of several partially deuterated benzocycloheptene derivatives at ?120° provides values for all the coupling constants about the C₃? C₄ bond. An interpretaion using the Karplus equation shows unambiguously that the seven-membered ring exists as a chair conformation. The relationship between the coupling constants determined shows that the form of the Karplus equation established empirically for cyclohexane is applicable to this seven-membered ring. A comparison of coupling constants shows that the benzocycloheptene chair is more puckered than the cyclohexane chair.     

Conformational analysis of aromatic derivatives of 2,3-dihydro-1H-1,5-benzodiazepine

The PMR spectra and dipole moments of some 2,4-disubstituted 2,3-dihydro-1H-1,5-benzodiazepines have been examined. It is shown that seven-membered rings with an aromatic or heterocyclic substituent in the 2-position have the boat configuration, the substitutent preferentially being equatorially oriented. It has been found that the changes in the chemical shifts and spin coupling constants for the protons of the CH-CH₂ fragment in the dihydrobenzodiazepine ring are mainly due to differences in the electronegativity of the substituents in the 2- and 4-positions. Some dipole moment vectors have been calculated for the dihydrobenzodiazepine ring.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1662–1666, December, 1984.     

PMR spectra and structures of 2H,6H-2,6-dimethyl-4-amino-1,3,5-dithiazine and its derivatives

An analysis of the PMR spectra of 2H, 6H-2, 6-dimethyl-4-amino-1,3,5-dithiazine, its acyl derivatives, and 2H, 3H, 6H-2, 6-dimethyl-4-amino-1,3,5-thiadiazine in various solvents and in the presence of Eu(FOD)₃ [tris(1,1,1,2,2,3,3-heptafluoro-7,7-dimettyl-4,6-octanedionato)₃ europium] confirms the structure assigned to it and makes it possible to prefer a cis orientation of the methyl substituents in the half-chair conformation.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 485–487, April, 1981.     

Conformations of the Nine‐Membered Ring in the B‐Nor‐5,10‐secosteroids. X‐Ray and NMR Analysis

The conformations of (Z)‐ and (E)‐5‐oxo‐B‐nor‐5,10‐secocholest‐1(10)‐en‐3β‐yl acetates ( 2 and 3 , resp.) were examined by a combination of X‐ray crystallographic analysis and NMR spectroscopy, with emphasis on the geometry of the cyclononenone moiety. The ¹H‐ and ¹³C‐NMR spectra showed that the unsaturated nine‐membered ring of (E)‐isomer 3 in C₆D₆ and (D₆)acetone solution exists in a sole conformation of type B ₁ , which is similar to its solid‐state conformation. The (Z)‐isomer 2 in C₆D₆, CDCl₃, and (D₆)acetone solution, however, exists in two conformational forms of different families, with different orientation of the carbonyl group, the predominant form (85%) corresponding to the conformation of type A ₁ and the minor (15%) to the conformation A ₂ present also in the crystalline state. In this solid‐state conformations of the nine‐membered ring of both compounds, the 19‐Me and 5‐oxo groups are ‘β’‐oriented. The NMR analysis suggests that the nine‐membered ring of 4 has a conformation of type C ₁ in CDCl₃ solution.