Electron density distributions in substituted 2,3,4,5-tetraphenyl-1-germacyclopenta-2,4-dienes studied by NMR spectroscopy

The signals in the¹³C NMR spectra of 2,3,4,5-tetraphenyl-1-germacyclopenta-2,4-dienes (R¹=R²=H, Me,cyclo-C₃H₅, SiMe₃, SnMe₃, R¹=Me, R²=H, Cl) were completely assigned using 2D NMR spectroscopy. The pattern of the variation of the chemical shifts in the¹³C NMR spectra indicates that the effects of substituents R¹ and R² on the heterocycle and on the phenyl groups are of inductive rather than mesomeric origin and include the direct through-space polarization of bonds (field effect). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1962–1965, November, 1997.     

A dynamic nuclear magnetic resonance study of nitrogen inversion in N,N'-dimethylimidazolidine

The 251 MHz ¹H and the natural-abundance 63.1 MHz ¹³C NMR spectra of N,N'-dimethylimidazolidine have been measured from ?50 to ?170°C. Below about ?140°C. nitrogen inversion in the compound becomes slow on the NMR time scale and both the ¹H and the ¹³C spectra indicate that it exists in solution as a mixture of cis and trans conformations having nearly the same energies. The free-energy barrier (ΔG^≠) for nitrogen inversion in N,N'-dimethylimidazolidine is 6.4 kcal mol^?1, a value which is 1.5 kcal mol^?1 lower than that for N-methylpyrrolidine.     

Ajanolide A, a new germacranolide fromAjania fruticulosa

A new germacranolide, ajanolide A, was isolated from aerial parts ofAjania fruticulosa by means of extraction with CHCl₃ and adsorption chromatography. This compound was identified as (1(10)E,3S,4Z,6R,7S,11R)-3-acetoxygermacra-1(10),4-dien-12,6-olide ((1S,7S,10R,13R)-7-acetoxy-4,8,13-trimethyl-11-oxabicyclo[8.3.0]trideca-4(E),8(Z)-dien-12-one) by X-ray diffraction analysis. 2D¹H−¹H (COSY) and¹³C−¹H (COSY) NMR spectroscopy was used for assigning the¹H and¹³C NMR signals in the spectra of ajanolide A. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 167–170, January, 1998.     

13C NMR study of 2,3,4,5-tetraphenylsilole dilithium salt

The complete assignment of the signals in the¹³C NMR spectra of 2,3,4,5-tetraphenyl-1-R¹,R²-1-silacyclopenta-2,4-dienes (R¹=R²=H, Me) and of the dianion of lithium salt [(PhC)₄Si]²⁻·2Li⁺ was carried out by 2D NMR spectroscopy. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 216–218, January, 1999.     

13C NMR studies of conformational isomerism in thioureas: Signal assignments via chemical shift and population trends

The 22.63 MHz ¹³C NMR spectra of a series of alkylated thioureas are reported. Characteristic Z and E spectral regions were found for the ¹³C ? S resonances. The two regions were generally found to be non-overlapping for the series, with the region of the Z, Z resonances occurring more downfield than those of either the Z, E or E, Z conformers in the cases of 1,3-disubstitution. The Z, Z configuration became favored and the relative chemical shift difference (Rδ) increased linearly with increasing substituent size. At 217 K, hindered internal rotation caused a multiplicity of resonances which were normally single peaks in the broad band ¹H decoupled 62.86 MHz ¹³C spectrum of CH₃NHCSNH(CH₂)₂NHCSNHCH₃ (2MTE) at room temperature. The trends in chemical shifts and populations were employed to assign tentatively the resonances of five of the six possible configurational isomers contributing to the 2MTE spectra at 217 K. The isomer populations are given. The ¹³C NMR spectra reported here led to signal assignments of Z and E isomers which supported prior ¹H NMR results and contradicted more recent results of another ¹³C NMR study of N-methylthiourea. The major peak of the exchange doublet occurs at relatively high field strengths in both methanol-d₅.     

[4+2]-Cyclodimer of sabinone: formation,crystal structure,and NMR spectra

Oxidation of (+)-sabinol, (1S,3R,5S)-1-isopropyl-4-methylidenebicyclo[3.1.0]hexan-3-ol, by active MnO₂ afforded not the expected sabinone but only its [4+2]-cyclodimer. The molecular structure of the latter was established by X-ray diffraction analysis. The ¹H and ¹³C NMR spectra of this cyclodimer were interpreted using 2D NMR spectroscopy.     

Study of spontaneous E/Z isomerization of bis[(Z)‐cyanomethylidene]‐diazapentacyclodienedicarboxylates by 1H, 13C,and 15N NMR spectroscopy,X‐ray,and quantum chemical calculation data

X‐ray data show that the diethyl 6,13‐bis[(Z)‐cyanomethylidene]‐5,5,14,14‐tetramethyl‐4,15‐dioxa‐7,12‐diazapentacyclo[9.5.2.0^2,10.0^3,7.0^12,16]octadeca‐8,17‐diene‐10,17‐dicarboxylate is formed as the ZZ isomer and diastereomer with the (1R*,2R*,3R*,10S*,11R*,12R*,16R*) configuration. The ¹H, ¹³C, and ¹⁵N NMR data exhibit that on standing in chloroform‐d solution, there is a spontaneous isomerization of this compound resulting in a thermodynamically stable mixture of the ZZ, ZE, EE, and EZ isomers with the same backbone. Using the 2D [¹H–¹H] COSY, [¹H–¹³C] HSQC, and [¹H–¹³C, ¹H–¹⁵N] HMBC NMR techniques and quantum chemical calculations makes it possible a complete assignment of signals in the ¹H, ¹³C, and ¹⁵N NMR spectra of each of the isomers. Such isomerization does not occur for similar compounds with the more bulky substituents at the 1,3‐oxazolidine rings. Copyright © 2016 John Wiley & Sons, Ltd.     

Slowly Interconverting Conformers of The Briarane Diterpenoids Verecynarmin B,C, and D,Isolated from the nudibranch mollusc Armina maculata and the pennatulacean octocoral Veretillum cynomorium of East Pyrenean waters

The novel briarane diterpenoids verecynarmin B(=(?)-(1R*, 10S*, 11R*, 4E, 12Z)-briara-4,7,12,17,-tetraen-14-one; (?)- 4 ); verecynarmin C (=(?)-(1R*, 10R*, 11S*, 4E, 12Z)?11-hydroxybriara-4,7,12,17-tetraen-14-one; (?)- 5 ); and verecynarmin D (=(?)-(1R*, 10R*, 11R*, 4E, 12E)-13-chloro-11-hydroxybriara-4,7,12,17,-tetraen-14-one; (?)- 6 ) are reported here as constituents of both the Mediterranean nudibranch mollusc Armina maculata (Rafinesque) and its prey, the pennatulacean octocoral Veretillum cynomorium (PALLAS ). The structural assignments rest mainly on (i) establishing that these briaranes occur in solution as two stable conformers which interconvert slowly (ca. 10 time per second at r.t. according to dynamic NMR) by 180° flipping of the C(4)?C(5) group in the ten-membered ring (Scheme 1), (ii) deriving, for both conformers, ¹H, ¹H coupling constants from 1D spectra, as well as ¹H, ¹H, ¹³C, ¹H, and ¹³C, ¹H, and ¹³C, ¹³C, correlations from 2D experiments; (iii) subjecting the briaranes to SeO₂ oxidation at the C(16) methyl group with isomerization at the C(4)=C(5) bond to give, in each case, only one observable molecular species as shown by NMR spectroscopy (Scheme 2).     

QSAR studies of phenylhydrazine-substituted tetronic acid derivatives based on the 1H NMR and 13C NMR chemical shifts

The quantitative structure–activity relationship models of 40 phenylhydrazine-substituted tetronic acid derivatives were established between the ¹H nuclear magnetic resonance (NMR) and ¹³C NMR chemical shifts and the antifungal activity against Fusarium graminearum, Botrytis cinerea, Rhizoctonia cerealis, and Colletotrichum capsici. The models were validated by R, R², R_A², variance inflation factor, F, and P values testing and residual analysis. It was concluded from the models that the ¹³C NMR chemical shifts of C8, C10, C7, and the ¹H NMR chemical shifts of Ha contributed positively to the activity against Fusarium graminearum, Botrytis cinerea, Colletotrichum capsici, and Rhizoctonia cerealis, respectively. The models indicated that decreasing the election cloud density of specific nucleuses in compounds, for example, by the substituting of electron withdrawing groups, would improve the antifungal activity. These models demonstrated the practical application meaning of chemical shifts in the quantitative structure–activity relationship study. Furthermore, a practical guide was provided for further structural optimization of the antifungal phenylhydrazine-substituted tetronic acid derivatives based on the ¹H NMR and ¹³C NMR chemical shifts.     

Synthesis and structural elucidation of a phthalide analog using NMR analysis and DFT calculations

Phtalides are secondary metabolites found in several fungi with a wide range of biological activities. A novel phthalide analog was synthesized by Diels–Alder reaction between cyclopentadiene and 3,4-dichlorofuran-2(5H)-one. Quantum mechanical calculations were used in conjunction with the spectrometric methods to determine the structure of the title compound. The calculated NMR chemical shifts for eight candidate pairs of enantiomers were compared with the experimental NMR chemical shifts applying the DP4 probability and mean absolute errors methodology. DP4 analysis using ¹H and ¹³C NMR chemical shifts without assignment of the signals presented 100% probability for the correct candidate structure 3d , proving the consistency of the method even without spectra interpretation. Results from theoretical calculation and NMR spectra interpretation were in agreement to the structure of rac-(3aR,4S,4aS,5R,8S,8aR,9R,9aS)-3a,9a-dichloro-3a,4,4a,5,8,8a,9,9a-octahydro-4,9:5,8-dimethanonaphtho[2,3-c]furan-1(3H)-one.     

1H- and 13C-NMR Investigations of Heptafulvenes

¹H- and ¹³C-NMR spectra of a series of 8-R¹-substituted as well as of 8,8-R¹, R²-disubstituted heptafulvenes, varying from inversely polarized ( 3l ) to unpolar ( 3h ) and polar heptafulvenes with electron-withdrawing groups ( 3d , e , f ), have been analyzed and compared with those of methoxytropylium salt 5a . The results concerning ³J (H,H) values and ¹³C-chemical shifts are shown in Figs. 5 and 6. It turns out that all the NMR parameters are strongly influenced by substituents R¹, R², but contrary to planar pentafulvenes, no linear correlations of the NMR parameter vs. Hammett substituent constant σ⁺ are obtained in the series 3l → 3d . ³J coupling constants J(2,3)/J(4,5) and J(3,4) are not much influenced by substituent changes in the series 3l → 3h , but are approaching in the row 3h → 3d . Similarly, signals of the ¹³C-atoms undergo a moderate shift to higher frequencies in the row 3l → 3h , but are strongly influenced by ? M groups, whereby the sensitivity is decreasing in the series C(7) > C(2)/C(5) > C(3)/C(4) > C(1)/C(6). These results are essentially explained by a boat conformation of inversely polarized heptafulvenes of the type 3l and an increasing planarization of the ring on going to polar heptafulvenes of type 3d .     

Crystal and molecular structure of subchrysine (3-O-acetylridentine), a new germacranolide fromArtemisia subchrysolepis

A new sesquiterpenoid lactone was isolated from terrestrial parts ofArtemisia subchrysolepis Filat. and studied by X-ray diffraction analysis and CD spectroscopy. This compound was called subchrysine and identified as (1R,3S,4E,6R,7S)-3-acetoxy-1-hydroxygernmacra-4,10(14), 11(13)-trien-12,6-olide. Pronounced temperature dependence of the¹H and¹³C NMR spectra of subchrysine was observed. The results of conformational analysis of subchrysine by quantum-chemical (PM3) and molecular mechanics (MMX) methods are also presented. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1429–1433, July, 1998.     

Synthesis and Structure of Tetrahydro-4,7-ethanoisobenzofuran-1,3-dione Derivative

(1R,2R,3S,4R,7R)‐7‐Isopropyl‐6‐methylbicyclo[2.2.2]oct‐5‐ene‐2,3‐dicarboxylic acid anhydride (tetrahydro‐4,7‐ethanoisobenzofuran‐1,3‐dione derivative) adduct 2 was prepared via the isomerization of α‐pinene and β‐pinene in turpentine followed by the Diels‐Alder cycloaddition with maleic anhydride in the presence of phosphoric acid/iodine catalysis. The molecular structure of adduct 2 was characterized by IR, ¹H NMR, ¹³C NMR, ¹H‐¹H COSY, DEPT, HSQC, HMBC, 2D NOESY and MS spectra. The single crystal X‐ray crystallographic analysis of adduct 2 was performed, and the X‐ray powder diffractive spectrum of the sample adduct 2 is consistent with the diffractive spectrum calculated from the single crystal data. Therefore the structure and stereochemistry of adduct 2 was established based on extensive spectral data and single crystal X‐ray analysis.     

Studies on mononuclear chelates derived from substituted Schiff-base ligands (part 2): synthesis and characterization of a new 5-bromosalicyliden- p -aminoacetophenoneoxime and its complexes with Co(II), Ni(II), Cu(II) and Zn(II)

In this study, 5-bromosalicyliden-p-aminoacetophenoneoxime (LH) was synthesized starting from p-aminoacetophenoneoxime and 5-bromosalicylaldehyde. Complexes of this ligand with Co^+?2, Ni^+?2, Cu^+?2 and Zn^+?2 were prepared with a metal?:?ligand ratio of 1?:?2. Their structures have been elucidated on the basis of elemental analyses, IR, ¹H and ¹³C NMR spectra, electronic spectra, magnetic susceptibility measurements, molar conductivity and thermogravimetric analyses (TGA).     

Application of two-dimensional NMR spectroscopy to the complete analysis of the 1H and 13C NMR spectra of d-biotin in aqueous solution

The ¹H and ¹³C NMR spectra of d-biotin were observed at 400 and 100 MHz, respectively. Various types of two-dimensional NMR spectroscopy were performed to assign the spectra. The previous assignment of ¹³C NMR spectrum of d-biotin reported by Bradbury and Johnson was modified, and the dihedral angles between the C? H bonds of the ring were determined. The populations of the conformers produced by internal rotation around the C-2? C-δ bond were estimated.     

Ascorbigen A—NMR identification

The connectivities of all atoms in ascorbigen A, an important metabolite, were determined unambiguously for the first time. The connectivity between carbon atoms was established by 2D INADEQUATE, and one-bond ¹³C–¹³C coupling constants were determined for all pairs of directly connected carbon atoms except for two strongly coupled carbon pairs. The ¹³C–¹³C coupling in one of the pairs was proved by a modification of standard INADEQUATE; however, the signals from the other pair were too weak to be observed. The connectivity within the two strongly coupled C–C pairs was confirmed by a combination of COSY and gHSQC; the latter experiment also identified all C–H bonds. The proton nuclear magnetic resonance (¹H NMR) spectra in dry dimethyl sulfoxide allowed identification and assignment of the signals due to NH and OH protons. The derived structure, 3-((1H-indol-3-yl)methyl)-3,3a,6-trihydroxytetrahydrofuro[3,2-b]furan-2(5H)-one, agrees with the structure suggested for ascorbigen A in 1966. The density functional theory (DFT) calculations showed that among 16 possible stereoisomers, only two complied with the almost zero value of the measured ³J(H6–H6a). Of the two stereoisomers, 3S,3aS,6S,6aR and 3R,3aR,6R,6aS, the latter was excluded on synthetic grounds. The nuclear Overhauser effect measurements unveiled close proximity between H2′ proton of the indole and the H6a proton of the tetrahydrofuro[3,2-b]furan part. Detailed structural interpretation of the measured NMR parameters by means of DFT NMR was hampered by rotational flexibility of the indole and tetrahydrofuro[3,2-b]furan parts and inadequacy of Polarizable Continuum Model (PCM) solvent model.     

多羟基甾醇25(R)-异螺甾环-5-烯-2β,3α,19-三醇的合成

以薯蓣皂素为原料, 经过磺酰酯化、N-溴代丁二酰亚胺(NBS)氧化加成、Pb(OAc)₄远程氧化关环、消去反应、间氯过氧苯甲酸(mCPBA)氧化、高氯酸开环及锌粉还原7步反应, 合成多羟基甾体25(R)-异螺甾环-5-烯-2β,3α,19-三醇. 并用IR, ¹H NMR, ¹³C NMR, MS及元素分析对各中间体及目标化合物进行了表征.     

Total Synthesis of Plakortone B

Plakortone B is a naturally occurring bicyclic[3.3.0]furanolactone compound with attractive bioactivities. Although the relative configuration of plakortone B’s central core had been established by NMR spectroscopic methods, the absolute configuration of its four stereocenters remained unknown. In the present paper, all four possible isomers of plakortone B were synthesized and one of these molecules was found to be identical with the natural plakortone B on the basis of ¹H, ¹³C NMR spectra and specific rotation comparisons. Thus, the absolute configuration of the natural plakortone B was determined to be (3S,4S,6R,10R).     

Conformational Analysis,Thermal Rearrangement,and EI‐MS Fragmentation Mechanism of (1(10)E,4E,6S,7R)‐Germacradien‐6‐ol by 13C‐Labeling Experiments

An uncharacterized terpene cyclase from Streptomyces pratensis was identified as (+)‐(1(10)E,4E,6S,7R)‐germacradien‐6‐ol synthase. The enzyme product exists as two interconvertible conformers, resulting in complex NMR spectra. For the complete assignment of NMR data, all fifteen (¹³C₁)FPP isotopomers (FPP=farnesyl diphosphate) and (¹³C₁₅)FPP were synthesized and enzymatically converted. The products were analyzed using various NMR techniques, including ¹³C, ¹³C COSY experiments. The (¹³C)FPP isotopomers were also used to investigate the thermal rearrangement and EI fragmentation of the enzyme product.     

Synthesis and characterization of a new pillar[5]arene-based [1]rotaxane

In this study, we reported the synthesis of three kinds of mono-functional pillar[5]arene derivatives PRI, PRII and R and their structures were studied by 1D and 2D NMR spectra and mass spectra. The 2D NMR spectra including ¹H-¹³C HSQC, ¹H-¹H COSY and NOESY spectra indicated that PRI and PRII are both stable self-included pseudo[1]rotaxanes in CDCl₃. These original structures are promising compounds for the design of pillar[5]-based [1]rotaxane. And the results showed that R could exist stable in CDCl₃ and DMSO because of the coordination of N-H?O hydrogen bonding interaction and C-H?π interaction.