8,2'-O-环化-9β-D-腺嘌呤阿拉伯糖苷的NMR及结构研究

8,2'-O-环化-9β-D-腺呤阿拉伯糖苷是阿糖腺苷合成过程中的一个关键性中间体,该化合物的NMR数据只有¹H-NMR谱的糖环1'位质子化学位移和偶合常数的报道,¹³C-NMR谱未曾报道。本文用2D-NMR、NOE-DIF等技术,对它的¹H、¹³C-NMR谱进行归属,进一步明确了它的结构。     

奥拉西坦的核磁共振波谱特征与构象研究

奥拉西坦是一种改善脑功能药物. 该文研究了该药的核磁共振波谱特征和构象. 吡咯环的各项异性效应使侧链亚甲基的质子的磁不等价. 由于吡咯环内酰胺键的部分双键性质,观测到了远程耦合现象. 信封式构象为吡咯环的优势构象. 同时研究了环外侧链上的酰胺键的受阻旋转.     

The Assignment of Long-Range 13C-1H Coupling Constants in Pyrazoles 13C NMR,Selective Decoupling

Although quite a few papers have been devoted to the ¹³C NMR study of pyrazoles, there is one problem that has not yet been satisfactorily solved, namely the assignment of the ²JCH and ³JCH coupling constants. In general, the ³C chemical shifts and ¹JCH coupling constants can be easily attributed, but the assigment of the long-range ²JCH and ³JCH coupling constants is not straight forward. In N- substituted pyrazoles, where annular tautomerism is prevented, each carbon atom can have two long-range coupling constants with the ring protons.     

苯并二氮杂卓及其衍生物立体化学的1H NMR研究

应用¹H NMR技术对一些以[2+4]环加成方式合成出的苯并二氮杂卓的立体化学进行了研究.利用偶合常数和gNOESY的结果,证明了二氮杂卓七员环的扭船式溶液构象,并与其类似的苯并硫氮杂卓作了比较和讨论.     

Density-functional calculations of the nuclear magnetic shielding and indirect nuclear spin–spin coupling constants of three isomers of C20

The parameters of the nuclear magnetic resonance (NMR) spectrum – shielding constants and indirect spin–spin coupling constants – of three isomers of C₂₀ are studied using density-functional theory. The performance of different exchange–correlation functionals is analysed by optimising the geometry for the ring, bowl and cage isomers, followed by a computation of the NMR constants at the optimised structure. The results are analysed and rationalised by performing comparisons of the three isomers with one another and with related systems such as polyynes (for the ring), o-benzyne (for the bowl) and C₆₀ (for the cage). The shielding and spin–spin parameters calculated using the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional are sufficiently reliable to assist in future experimental NMR studies of C₂₀ and, in particular, the identification of its isomers.     

Microwave and Infrared Spectra,ab InitioCalculation, and Two-Dimensional Model of Amino Group Inversion and Ring Puckering in 2,5-Dihydropyrrole

The microwave spectra of 2,5-dihydropyrrole and 2,5-dihydropyrrole-1-d₁have been measured with Stark and Fourier transform spectrometers in the range 10–39 GHz. Rotational constants, centrifugal distortion constants, and¹⁴N quadrupole coupling constants have been determined from the observed transition frequencies for the ground vibrational state. In addition, two satellites of the normal species and one satellite of the deuterated species have been identified and measured. Splittings of the rotational transitions due to amino group inversion tunneling have been observed and analyzed. Infrared transitions of the amino group inversion mode have been measured in the range 490–720 cm⁻¹. The effect of ring puckering on the inversion motion of the amino group in 2,5-dihydropyrrole and 2,5-dihydropyrrole-N-d₁has been investigated byab initiocalculations and two-dimensional flexible model calculations from the results of microwave and infrared spectroscopy. The observed molecular properties have been reproduced by a model which involved adjustable parameters for the potential energy surface and the structural relaxation of the CCC valence angles. Additional parameters have been transferred from theab initiocalculations. The adjustment of the model to the experimental data has yielded an equatorial equilibrium conformation with slightly larger CCC valence angle than in the most stable axial conformation. Excitation of the first ring puckering state has been found to enhance the inversion tunnel splittings.     

Microwave spectrum,conformation, and quadrupole coupling constants of cyclopentyl chloride

The microwave spectra of the normal and two isotopic species of cyclopentyl chloride have been observed and analyzed. For the normal isotopic species the rotational constants (in MHz) are A = 4547.77 ± 0.01, B = 2290.22 ± 0.01, and C = 2073.34 ± 0.01. From the rotational constant data, it has been shown that the stable molecular conformation is the bent axial form. Quadrupole coupling constants have been measured for the ³⁵Cl nucleus, the values being (in MHz) χ_aa = ?23.70 ± 0.10, χ_bb = 32.33 ± 0.36, and χ_cc = ?8.63 ± 0.37. When transformed to the CCl bond axis system, the coupling constants confirm the axial structure. Extensive vibrational satellite structure, presumably arising from the pseudorotational ring mode with a fundamental frequency of 52 ± 5 cm^?1, has been observed and assigned. No spectral evidence has been observed for a second stable molecular conformer.     

Conformational Analysis of Baclofen Analogues by 1H and 13C NMR: Phaclofen,Saclofen, and Hydroxy-Saclofen. Influence of the Anionic Moiety

The conformations of three analogues of baclofen 1: phaclofen, saclofen, and hydroxy-saclofen 2–4, potent GABA_B antagonists, in solution (D₂O) are estimated from high-resolution (300 MHz) H NMR coupling data. Conformations and populations of conformers are calculated by means of a modified Karplus-like relationship for the vicinal coupling constants. H NMR spectral analysis evidences how 1–3 keep in solution the preferred a conformation around C3-C4 bond. A partial rotation is set up around C2–C3 bond (the conformations about C2–C3 are all highly populated in solution) particularly for 2 and 3 while 1 shows a relative preferred a conformation. This evidences the influence of the anionic moiety.     

Conformational Studies on Pentagastrin by 2D NMR and Restrained Molecular Dynamics

The present study uses 2D NMR to assign all resonance lines of pentagastrin completely. From 2D NOESY and coupling constants J_HNα, interproton distance and torsion angle Φ restraints were obtained. Based on them, the model of conformation for pentagastrin was proposed by restrained molecular dynamics calculation. It is shown that the C-terminal part of pentagastrin forms a Δ turn with a weak H-bond between the CO of Met and the NH of Phe. A strong H-bond between indole NH of Trp and β-carboxyl group of Asp stablizes the folded form in DMSO, which may result in a significant increase in the population of a favorable conformation for interaction with receptor.     

一个A环开环型芳基萘内酯类木脂素2D NMR全归属

采用二维核磁共振技术(¹H-¹H COSY, DEPT, HSQC, HMBC, NOESY)和CD光谱,确定了A环开环型芳基萘内酯类木脂素3′, 4′-O, O-demethylene-4-O- demethylpodophyllotoxin (1)的结构. 首次对化合物1的¹³C NMR数据进行了全归属.     

Shielding and spin-spin coupling constants from gas-phase NMR

Intermolecular interactions modify nuclear magnetic resonance (NMR) chemical shifts and spin-spin coupling constants. The intermolecular effects can be determined if the NMR parameters for an isolated molecule are known. Gas-phase NMR spectroscopy offers such methods which allow one to measure the shielding and spin-spin coupling constants at the zero-density limit where the NMR parameters are free from intermolecular contributions. It is also shown that at present the multinuclear NMR spectra can easily be obtained for gaseous samples containing several micrograms of a solute compound.     

Nuclear Overhauser Effect Spectroscopy (NOESY) and 3JHH Coupling Measurements in the Determination of the Conformation of the Sesquiterpene Antimalarial Arteether in Solution

The twisting of the A-, B-, and D- rings of β-arteether were determined from dihedral angles derived from ³J_HH coupling constants and a Karplus-type equation that takes into account heteroatom substitution. The angles obtained by NMR were also compared to the angles obtained by molecular modeling and to those obtained from the methyoxyl analog (artemether) using x-ray crystallography. The NMR results showed that the A-ring was a relatively undistorted chair conformation with the 14-methyl group in an equatorial position. The NMR data indicated that the B-ring was very distorted and the D-ring was twisted outward because of the steric interactions with the ethoxy group. Two-dimensional NOESY data was used to measure selected proton-proton distances for arteether in solution and these distances were also compared to those obtained by molecular modeling and from the x-ray data for artemether. The NOESY data also indicated that the conformation of the A-ring was fairly normal, the B-ring was twisted, and the ethoxy group was in an extended conformation (C₁₁-C₁₂-O-C₁₆=+170°, and C₁₂-O-C₁₆-C₁₇=+169°)     

Conformational Analysis of (S)-4,5,6,7-Tetrahydro-5-Methylimidazo [4,5,1-jk][1,4]-Benzodiazepin-2(1H)-one (R78362)

The solution and solid state conformation of (S)-4,5,6,7-tetrahydro-5-methylimidazo [4,5,1-jk][1,4]-benzodiazepin-2(1H)-one (R78362) have been investigated by low temperature NMR and x-ray diffraction studies. The ¹H NMR spectrum of R78362 shows no evidence of the presence of multiple conformers in the temperature range 340K - 177K. Molecular mechanisms and semiempirical molecular orbital calculations suggest that the nitrogen and ring inversion barriers of R78362 are small and thus a time-averaged ¹H NMR spectrum is probably occurring at 298K. The x-ray diffraction data indicated that there were two independent molecules in the asymmetric unit of the crystal. The two molecules had similar conformations with the benzoimidazole ring being planar and the diazepine ring in a “half-chair” conformation.     

Quantum cluster equilibrium theory of liquids: temperature dependent chemical shifts,quadrupole coupling constants and vibrational frequencies in liquid ethanol

Temperature dependent NMR chemical shifts for hydrogen and oxygen, NMR quadrupole coupling constants for deuterium and vibrational frequencies for the OH bond of liquid ethanol are calculated for the temperature range 250 to 350 K using the ab initio quantum cluster equilibrium method and compared with experimental measurements. The calculated NMR chemical shifts, quadrupole coupling constants and infrared vibrational frequencies are in good agreement with experimental measurements. The results indicate that cyclic tetramer and pentamer structures rather than linear structures are the principal components of liquid ethanol.     

Prediction of the 1H NMR spectra of epoxy-fused cyclopentane derivatives by calculations of chemical shifts and spin–spin coupling constants

¹H NMR spectra of epoxy-fused cyclopentane derivatives have been computationally investigated with density functional calculations in order to unravel the shielding effect of the epoxy ring on the ¹H NMR chemical shifts of N-substituted epoxy-fused cyclopentane-3, 5-diol derivatives. Both ¹H NMR chemical shifts and spin–spin coupling constants have been calculated with the WP04/cc-pVTZ level of theory in solution. The WP04/cc-pVTZ// B3LYP/6-31+G(d) methodology has been found to reproduce the best experimental results on epoxy-fused cyclopentane derivatives. This study is expected to lead experimentalists in their endeavour to characterize epoxy-fused cyclic systems with ease.     

Accurate measurements of small J coupling constants under inhomogeneous fields via intermolecular multiple-quantum coherences

Two new NMR pulse sequences, based on intermolecular multiple-quantum coherences (iMQCs), were developed to obtain apparent J coupling constants with a scaling factor from one to infinity relative to the conventional J coupling constants. Here the apparent J coupling constants were defined as apparent peak separations in unit of Hz in a reconstructed spectrum for a coupled spin system. Except for the adjustable scaling factor for apparent J coupling constants, the sequences hold the advantage of high acquisition efficiency, and retain the spectral information such as chemical shifts, multiplet patterns, and relative peak areas under inhomogeneous fields. For spin systems with small scalar coupling constants, well-resolved J-spectra can be achieved by selecting a proper scaling factor. Theoretical predictions are in good agreement with simulation results and experimental observations.     

几种叔丁墓酚化合物的核磁共振研究

研究了11种叔丁基酚化合物的化学位移和偶合常数,并讨论了影响因素.     

氮杂环化合物中1JCH与电荷密度的关系

本文用EHMO程序计算了氮杂环化合物中N-CH₂-R基团上C和H原子的S电子密度S_C^(o)和S_H^(o),对于吡啶盐系列S_C^(o)S_H^(o)等于1.172±0.006,对于咔唑系列S_C^(o)S_H^(o)等于1.162±0.015。实验指出,这两个系列化合物的NMR耦合常数¹J_CH分别为144±2Hz_o和135±2Hz对比计算结果和实验数据可以很好地解释¹J_CH不随取代基的改变而变化。     

无规立构聚乙烯醇缩酮环结构13C NMR研究

报道了无规立构聚乙烯醇与丙酮、丁酮缩酮化产物的¹³C NMR研究结果,发现丙酮的产物(PVKA)主要生成六元环.而丁酮产物(PVKB)除六元环外还有八元、十元环存在,且这3种环上的CH₃均是以轴立式构象为主.由¹³C NMR求得PVKB中相应的3种环的比例分别为36.3%,14.4%,5.8%.反应的平衡常数分别为1.8×10^-2、2.03×10^-3、2.8×10^-4,并讨论了它们对结晶等性能的影响.     

Microwave spectra and structure of an isoxazole-water complex

The rotational spectrum of a hydrogen-bonded isoxazole-water complex has been measured between 6–18 GHz with a pulsed nozzle Fourier transform microwave spectrometer. In addition to isoxazole-H₂O, the complexes with HDO and D₂O as well as isoxazole-¹⁵ N-H₂O have been investigated in order to determine the structure of the complex. Rotational constants, quartic centrifugal distortion constants and quadrupole coupling constants, where applicable, have been fitted to the measured transition frequencies of the isotopomers. Structural data, which have been deduced from the planar moments of inertia and the quadrupole coupling constants of the isotopomers, have established conclusively that water binds to nitrogen in the ring plane of isoxazole. Ab initio calculations have revealed that complexes with a hydrogen-bond to nitrogen or to oxygen are both stable. The complex with water attached to nitrogen has been found to be more strongly bound than that with water attached to oxygen. Small splittings of the rotational transitions of the two complexes with H₂O have been interpreted as being the result of an internal rotation of water with respect to isoxazole.