摘要:

Abstract1H,13C and19F NMR experiments for a series of 3‐alkyl‐2‐phenyl‐4,5,6,7‐tetrahydroindazoles revealed a six‐bond through‐space coupling between theortho‐fluorine and the hydrogen or fluorine atom of the position 3‐alkyl group. This was further supported by NOE experiments. Molecular mechanics calculations on a representative structure indicated that several low energy conformers met the fluorine‐carbon distance constraint suggested by the NMR data, and dynamic annealing experiments produced a conformer which was in complete agreement with the NMR data. This through‐space interaction is speculated to be a result of repulsion between N‐1 of the tetrahydroindazole and theortho‐flu

ISSN:0749-1581    

DOI:10.1002/mrc.1260310402

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe1H and13C NMR spectra of pentalongin, a physiologically active natural product, were completely assigned. Some previous assignments were revised. All13C1H coupling constants, both one‐bond and long‐range, were also mea

ISSN:0749-1581    

DOI:10.1002/mrc.1260310403

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractDoubly labelled 2,2,6,6‐tetramethylpiperidinyl‐1‐oxyl‐4‐yl 2′,3′,4′,6′‐tetra‐O‐acetyl‐β‐D‐glucopyranoside, 2,2,6,6‐tetramethylpiperidinyl‐1‐oxyl‐4‐yl β‐D‐glucopyranoside and 1′,2′‐O‐[1‐(2,2,6,6,‐tetramethyl‐4‐oxy‐1‐oxylpiperidinyl)ethylidene]‐3′,4′,6′‐tri‐O‐acetyl‐α‐D‐glucopyranose (13C and NO⋅) were used as model compounds to study the paramagnetic intramolecular contribution (1/T1e)intrato the13C longitudinal relaxation rate caused by the unpaired electron of the nitroxide radical. It is shown that measurement of (1/T1)intrais possible on the assumption that the electron–nucleus dipole–dipole interaction dominates the other terms contributing to 1/T1in the case of para

ISSN:0749-1581    

DOI:10.1002/mrc.1260310404

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe NMR parameters of (−)‐scopolamine free base in CDCl3determined by Sarazinet al.were used to reevaluate some of the stereochemical conclusions recently reported by the authors. Similarly to the case with the protonated salt, the major contributor to the fast exchange limit time‐averaged structure of the free base tropate ester moiety appears to be the rotamer in which the phenyl and hydroxyl groups are antiperiplanar. Long‐range heteronuclear coupling constants for the carboxyl carbon and also for the aromaticipsocarbon have been reassigned. Vicinal homonuclear coupling constants involving the methine proton and the two internally diastereotopic protons in the (S)‐CHCH2OH moiety have also been reassigned. Four conformational archetypes were calculated by molecular mechanics for the (−)‐(Ns,Cα‐S)‐scopolamine free base equatorialN‐methyl diastereomer based on x‐ray crystallographically determined scopolamine structural analogues. The degree of anisochronicity (and their relative shifts) exhibited by the internally diastereotopic oxiranyl protons and carbons and the observation of NOE intensity enhancement of oxiranyl protons upon {H‐aromatic} can be interpreted as arising from an interconverting profile of conformations at the fast exchange limit. While a conformation similar to that of the solid‐state hydrobromide sesquihydrate salt appears to be a major contributor to the time‐averaged structure, other conformatio

ISSN:0749-1581    

DOI:10.1002/mrc.1260310405

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThree xanthone derivatives, gambogic acid, isogambogic acid and isomorellinol, were isolated from the dried latex ofGarcinia hanburyi.Two of them, isogambogic acid and isomorellinol, are new. Determinations of the structures and stereochemistry were achieved independently by a series of NMR experiments including COSY, ROESY, HMQC, HMBC and selective INEPT. Isolation of isomorellinol fromG. hanburyiprovided important chemical evidence to link this species toG. morella.The presence of gambogic acid and isogambogic acid, however, demonstrated the difference between these two species. Cytotoxic evaluation of these isolates revealed that all three were active against KB and drug‐resistant KB‐V1 cell li

ISSN:0749-1581    

DOI:10.1002/mrc.1260310406

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe13C NMR spectra of a series of 2,3‐diaryl‐1,3‐thiazolidin‐4‐ones were measured and the resonance signals produced by the methylene, methine and carbonyl groups of the heterocyclic ring assigned. The variations in their chemical shifts were examined and the influence ofS‐oxidation was investigated. 3‐Benzyl‐2‐phenyl‐1,3‐thiazolidin‐4‐one, 3‐butyl‐2‐phenyl‐1,3‐thiazolidin‐4‐one and 2,3‐diphenyl‐5‐methyl‐1,3‐thiazolidin‐4‐one were also examined, the last compound being shown to exist as a mixture of isomers.T1relaxation times were determined for th

ISSN:0749-1581    

DOI:10.1002/mrc.1260310407

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe13C chemical‐shift tensor orientations in the OCH2CH2O moieties of poly(ethylene oxide) (PEO) and poly(ethylene terephthalate) (PET) were determined by means of1H13C separated‐local‐field NMR spectroscopy, and the literature values for the shift tensors in polyethylene (PE) and polyoxymethylene (POM) were improved. An angular resolution of ± 4° was attained on standard equipment with 90° pulse lengths exceeding 4 μs. Relatively high uniformity of the methylene tensor orientations, complying with local molecular symmetry

ISSN:0749-1581    

DOI:10.1002/mrc.1260310408

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractDetailed examination of the1H NMR spectra of acetylated β‐ether/β‐ether lignin model trimers, particularly those synthesized with partially defined stereochemistry, revealed that all of the eight chemically distinct isomers (sixteen optical isomers from four optical centers) are detectable. These compounds provide excellent models for both internal (etherified) and terminal (free‐phenolic) units in lignin and clearly delineate the chemical shifts for each isomer. High‐field two‐dimensional TOCSY or relayed coherence transfer NMR experiments allow correlation of all the side‐chain protons in each isomer, whileJ‐resolved experiments clearly show the presence of all expected isomers. The degree of complexity already encountered with these simple compounds points to the overwhelming improbability of finding even small crystalline re

ISSN:0749-1581    

DOI:10.1002/mrc.1260310409

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe1H NMR spectra of 26 nitrated aminopyridines were measured and interpreted. Chemical shift assignments were based on existing chemical shift rules for substituted pyridines and spectral comparison with compounds of similar structure. Someo‐aminonitropyridines were found to give a splitting of the amino signals due to intermolecular hydrogen bonding; steric hindrance is shown to influence this bondin

ISSN:0749-1581    

DOI:10.1002/mrc.1260310410

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractA new method and computer program for LIS calculations of highly flexible molecules was developed using molecular geometries obtained by randomly varied dihedral angles (up to ten angles rotating freely and independently from each other are possible) as start conformations for a STEEpest Descent (STEED) search algorithm to find minima characterized by theRfactor at the ‘LISRfactor hyper surface.’ This STEED–LIS method allows easy, accurate and rapid access to the probable global minimum of theRfactor depending on up to ten variable dihedral angles. This means that LIS investigations of highly flexible molecules with more than one freely rotating dihedral angle are no longer complicated and time consuming, but can be carried out automatically using the developed programs [JLLISSD (with interactive parameter dialogue) and LISSDEMU (using parameter input files prepared by LISSDINP)]. The results obtained are in very good agreement with known or reported preferred conformations of the investigated test compounds, propan‐1‐ol, butan‐1‐ol and ethylp‐m

ISSN:0749-1581    

DOI:10.1002/mrc.1260310411

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY