Search results for "Configuration"
showing 10 items of 601 documents
Gold(iii) tetraarylporphyrin amino acid derivatives: ligand or metal centred redox chemistry?
2015
EPR spectroscopy and DFT calculations show that the site of reduction of porphyrinato gold(iii) complexes depends on the counterions X, the meso substituents R and the solvent.
Oxotris(oxalato)niobate(V) as counterion in cobalt(II) spin-crossover systems
2016
Abstract This work is devoted to the investigation of the thermally induced spin-crossover behavior from a high-spin state (HS, S = 3/2) at higher temperatures to a low-spin phase (LS, S = 1/2) at lower temperatures of the six-coordinate cobalt(II) complex in the compound [Co(terpy)2]3[NbO(C2O4)3]2·3CH3OH·4H2O (2). The crystal structure of 2 together with that of its counterion as tetraphenylarsonium(V) salt (AsPh4)3[NbO(C2O4)3]·9H2O (1) are also included. The spin-crossover process was followed by the thermal variation of the χMT product between 2.0 and 400 K under the warming mode, with the LS configuration being achieved at T ⩽ 200 K and the LS → HS interconversion being incomplete at 4…
Rollimembrin, a novel acetogenin inhibitor of mammalian mitochondrial complex I
1997
Abstract Rollimembrin (3), is a new adjacent bis-tetrahydrofuranic acetogenin with a scarce relative configuration, threo/cis/threo/cis/erythro, isolated from Rollinia membranacea seeds. The mechanism of cytotoxic activity, determined by NADH-oxidase experiments, establish that rollimembrin (3) is the most potent inhibitor of mammalian mitochondrial complex I.
Arabinosylamine in Asymmetric Syntheses of Chiral Piperidine Alkaloids.
2004
The stereodifferentiating potential of arabinosyl aldimines was utilized in stereoselective syntheses of 2-substituted dehydropiperidinones and their further transformation to 2,6-cis-substituted piperidinones. The absolute configuration was proven by X-ray analysis and by the synthesis of the enantiomerically pure alkaloid (+)-dihydropinidine. The presented method offers the possibility to synthesize piperidine derivatives enantiomeric to those obtained by the application of the corresponding galactosylamine auxiliary.
Rearrangement of Germacranolides. Synthesis and Absolute Configuration of Elemane and Heliangolane Derivatives from Cnicin
2003
A study of the Cope rearrangement of 15-oxo-germacranolides to 15-oxo-elemanolides has been carried out. The synthesis of two natural elemanolides, isolated from Centaurea paui, and an efficent isomerization of the C-4/C-5 double bond of 15-oxo-germacranolides to form heliangolides are reported. The absolute configuration of all the compounds has been ascertained. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)
Effect of addition of Lewis/Brönsted acids in the asymmetric aldol condensation catalyzed by trifluoroacetate salts of proline-based dipeptides
2012
Abstract Proline-based dipeptides catalyze aldol condensations with good yield and stereoselectivity after addition of zinc or sodium acetate to the trifluoroacetate peptide. The chirality of the N-terminal proline in the catalyst determines the absolute configuration of the aldol product, but stereoselectivity depends on the configuration of both amino acids, and is higher for the enantiomeric pair R–S, S–R. Regarding the nature of the second component, optimal results in both yield and stereoselectivity are obtained when neutral unbranched primary amino acids are used.
The structure and stereochemistry of atractyligenin
1966
Abstract The structure of atractyligenin C19H28O4, the nor-diterpenoidic aglycone of atractyloside C30H44O16S2K2 has been substantiated as Ia. The absolute configuration shown in Ia proves it to be a derivative of (−)kaurene.
Structurally diverse second-generation [2.2]paracyclophane ketimines with planar and central chirality: syntheses, structural determination, and eval…
2005
A set of 20 novel [2.2]paracyclophane ketimines with planar and central chirality has been synthesized from enantiomerically pure and racemic 5-acyl-4-hydroxy[2.2]paracyclophane and alpha-branched chiral amines. Their X-ray structures were determined to elucidate the three-dimensional structures and the absolute configuration. The ketimines were used as catalysts in the asymmetric 1,2-addition reactions of diethylzinc with substituted benzaldehydes to furnish chiral alcohols in up to 95 % ee.
Absolute Configuration Determination of 2,3-Dihydro-1H,5H-pyrazolo[1,2-a]pyrazoles Using Chiroptical Methods at Different Wavelengths
2016
A correlation between the absolute configuration and chiroptical properties of nonracemic 1,6,7-trisubstituted 2,3-dihydro-1H,5H-pyrazolo[1,2-a]pyrazoles was studied. A series of 16 novel representatives were prepared by Cu-catalyzed [3 + 2] cycloadditions of racemic (Z)-2-benzylidene-5-oxopyrazolidin-2-ium-1-ides to tert-butyl (S)-(3-oxopent-4-yn-2-yl)carbamate, and their structures were determined by NMR, VCD, ECD, and X-ray diffraction. A clear correlation between the sign of specific rotation and configuration at position C(1) allows for easy determination of the absolute configuration of 1,6,7-trisubstituted 2,3-dihydro-1H,5H-pyrazolo[1,2-a]pyrazoles by ECD and NMR. While VCD, requirin…
Revision of Na 2 A1Σ+u state molecular constants by polarization labeling spectroscopy
1997
ABSTRACT. This paper contains the analysis of the A1 of Na2 based on the data obtained from thepolarization labeling spectroscopy experiment on the A' —X' transition. A set of Dunham coefficients is derived, which describes the A state in the wide range of v and J 126) quantum numbers and reproduces the positions of unperturbed rotational lines in the A-X band system towithinO.1 cm'.1. INTRODUCTION.We report' new analysis of the A' state based on the data obtained from polarization labelingspectroscopy (PLS) experiment"2, based on V-type optical—optical double resonance scheme, on the 1 : ' ; band system of sodium dimer. The diatomic alkali molecules, with their simple electronic configurat…