Home About us Contact
|
Home About us Contact | ||
|
|
||
R Configuration (r + configuration)
Selected AbstractsNovel Asymmetric Wittig Reaction: Synthesis of Chiral Allenic EstersEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 23 2004Teresa M. V. D. Pinho e Melo Abstract Wittig reactions between 10-(phenylsulfonyl)isobornyl (triphenylphosphoranylidene)acetates (1 and 6) and ketenes resulted in asymmetric induction, with the selective synthesis of allenes with axial chirality. Use of the (1R)-(,)-10-(phenylsulfonyl)isoborneol unit allows the synthesis of allenes with S configuration, whereas use of the (1S)-(+)-10-(phenylsulfonyl)isoborneol unit produces allenes with R configuration. The structure of (1R)-(,)-10-(phenylsulfonyl)isobornyl (S)-5,5-dimethylhexa-2,3-dienoate (2e) was determined by X-ray crystallography. Chirooptical studies of the allenic esters were carried out, confirming that two sets of enantiomeric derivatives were obtained. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Macromolecular helicity inversion of poly(phenylacetylene) derivatives induced by various external stimuliMACROMOLECULAR SYMPOSIA, Issue 1 2003Katsuhiro Maeda Abstract Unique macromolecular helicity inversion of stereoregular, optically active poly(phenylacetylene) derivatives induced by external achiral and chiral stimuli is briefly reviewed. Stereoregular, cis-transoidal poly(phenylacetylene)s bearing an optically active substituent, such as (1R,2S)-norephedrine (poly- 1) and , -cyclodextrin residues (poly- 2), show an induced circular dichroism (ICD) in the UV-visible region of the polymer backbone in solution due to a predominantly one-handed helical conformation of the polymers. However, poly- 1 undergoes a helix-helix transition upon complexation with chiral acids having an R configuration, and the complexes exhibit a dramatic change in the ICD of poly- 1. Poly- 2 also shows the inversion of macromolecular helicity responding to molecular and chiral recognition events that occurred at the remote cyclodextrin residues from the polymer backbone; the helicity inversion is accompanied by a visible color change. A similar helix-helix transition of poly((R)- or (S)-(4-((1-(1-naphthyl)ethyl)carbamoyl)phenyl)acetylene) is also briefly described. [source] Computational Study of the Lipase-Mediated Desymmetrisation of 2-Substituted-Propane-1,3-DiaminesCHEMBIOCHEM, Issue 18 2009Eduardo García-Urdiales Dr. Abstract The enantioselectivity displayed by the lipase from Pseudomonas cepacia towards a wide range of prochiral 2-substituted-propane-1,3-diamines was studied by means of molecular dynamics simulations (MDS). In all cases the enzyme allows the recovery of the corresponding amino carbamates of R configuration. However, the enantioselectivity is only synthetically useful if no ortho substituent is present and the aromatic ring is directly bonded to the 2-carbon of the 1,3-diamine core. Analysis of the MDS trajectories revealed that the homologation of 2-aryl substituents by means of a methylene group lowers enantioselectivity by alleviating the conformational tension of the slow-reacting orientations due to unfavourable intramolecular contacts between the ortho carbons of the aryl group and the nucleophilic nitrogen, as well as between the chiral carbon and the oxyanion. Additionally, the relative solvent accessible surfaces of the atoms of the aryl ring nicely correlate with the effect of the location of the substituent on enantioselectivity. [source] On the Mechanism and Stereochemistry of Chiral Lithium-Carbenoid-Promoted Cyclopropanation ReactionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 23 2007Zhuofeng Ke Abstract An investigation into the mechanism and stereochemistry of chiral lithium-carbenoid-promoted cyclopropanation reactions by using density functional theory (DFT) methods is reported. Previous work suggested that this type of cyclopropanation reaction may proceed by competition between a methylene-transfer mechanism and a carbometalation mechanism. In this paper, it is demonstrated that the intramolecular cyclopropanation reactions promoted by chiral carbenoids 1 and 2 proceed by the methylene-transfer mechanism. The carbometalation mechanism was found to have a much higher reaction barrier and does not appear to compete with the methylene-transfer mechanism. The Lewis base group does not enhance the carbometalation pathway enough to compete with the methylene-transfer pathway. The present computational results are consistent with experimental observations for these cyclopropanation reactions. The factors governing the stereochemistry of the intramolecular cyclopropanation reaction by the methylene-transfer mechanism were examined to help elucidate the origin of the stereoselectivity observed in experiments. Both the directing group and the configuration at the C1 centre were found to play a key role in the stereochemistry. Carbenoid 1 has a chiral C1 centre of R configuration. The Lewis base group directs the cyclization of carbenoid 1 to form a single product. In contrast, the Lewis base group cannot direct the cyclization of carbenoid 2 to furnish a stereoselective product due to the S configuration of the chiral C1 centre in carbenoid 2. This relationship of the stereochemistry to the chiral character of the carbenoid has implications for the design of new efficient carbenoid reagents for stereoselective cyclopropanation. [source] Bis-Chelated Imine-Alkoxytitanium Complexes: Novel Chiral Dopants with High Helical Twisting Power in Liquid CrystalsCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2005Manfred Braun Prof. Dr. Abstract Enantiomerically and diastereomerically pure bis-chelated imine-alkoxytitanium complexes 6 and 7 have been synthesized and used as chiral dopants for converting nematic into cholesteric phases. The dopants were tested in mainly commercially available nematic liquid crystalline compounds or mixtures: LC1 (BASF), ZLI-1695 and ZLI-1840 (Merck), as well as N -(4-methoxybenzylidene)-4,-butylaniline (MBBA). The values of the helical twisting power (HTP) were determined by the Grandjean,Cano method. Exceptionally high helical twisting powers were obtained. Thus, the titanium complex 6,h displayed a HTP value of 740 ,m,1 in MBBA, the highest HTP value reported. The helical twisting power has been found to depend strongly on the structure of the nematic phase and the substitution pattern of the chiral ligand in the titanium complexes 6 and 7. Crystal structure analysis of 6,f confirmed the A,R,R configuration of the metal complex. The chiral imine ligands 4 and 5 were derived from the regioisomeric amino alcohols 1 and 2. [source] Straightforward Synthesis of (R)-(,)-KjellmanianoneCHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2004Jens Christoffers Prof. Dr. Abstract A direct route to enantiomerically pure (,)-kjellmanianone is reported. The synthesis involves a cerium-catalyzed ,-hydroxylation and an enzyme-catalyzed procedure to resolve tertiary alcohols at key stages. The intermediate ,-oxo ester was ,-hydroxylated to give good yields of racemic kjellmanianone. The resolution of the racemic material was achieved by enzymatic saponification, followed by a chemical decarboxylation sequence to give enantiopure (,)-kjellmanianone with 99,% ee. Bromination then afforded the (,)-bromo derivative, whose X-ray structure provided evidence for the R configuration of (,)-kjellmanianone. [source] Metallocyclo- and Polyphosphazenes Containing Gold or Silver: Thermolytic Transformation into Nanostructured MaterialsCHEMISTRY - A EUROPEAN JOURNAL, Issue 48 2009Josefina Jiménez Dr. Abstract A cyclotriphosphazene bearing two 4-oxypyridine groups on the same phosphorus atom, gem -[N3P3(O2C12H8)2(OC5H4N-4)2] (I), and its analogous polymer [{NP(O2C12H8)}0.7{NP(OC5H4N-4)2}0.3]n (II), have been used to prepare gold or silver, cyclic and polymeric, metallophosphazenes. The following complexes, gem -[N3P3(O2C12H8)2(OC5H4N-4{ML})2] (ML=Au(C6F5) (1) or Au(C6F5)3 (2)), [N3P3(O2C12H8)2(OC5H4N-4{AuPPh3})2][NO3]2 (3), and [N3P3(O2C12H8)2(OC5H4N-4{AgPPh2R})2][SO3CF3]2 (R=Ph (4) or Me (5)) have been obtained. Complexes 1 and 4 are excellent models for the preparation of the analogous polymers [{NP(O2C12H8)}0.7{NP(OC5H4N-4{ML})2}0.3]n (ML=Au(C6F5) (P1), Ag(OSO2CF3)PPh3 (P2)). All complexes have been characterized by elemental analysis, various spectroscopic methods, and mass spectrometry. The polymers were further investigated by thermochemical methods (thermogravimetric analysis) and differential scanning calorimetry. For compounds 1,5 and for the starting phosphazene I, a mixture of different stereoisomers may be expected. The stereochemistry in solution has been studied by variable-temperature NMR spectroscopy studies, which provided evidence for interconversion processes that involve changes in the chirality of a 2,2,-dioxybiphenyl group. A single-crystal X-ray analysis of the gold complex 2 confirmed not only the proposed structure, but also S,S and R,R configurations at the two biphenoxy-substituted phosphorus centers, in contrast to those observed for the precursor I. Pyrolysis of these new metallophosphazenes was also studied. Notably, pyrolysis of the gold derivatives gave macroporous metallic gold sponges without the requirement of either an external reducing agent or a porous support. These materials were all characterized by XRD, TEM, SEM, and energy-dispersive X-ray spectroscopy. En este trabajo se ha usado el ciclotrifosfazeno que tiene dos grupos 4-oxipiridina en el mismo átomo de fósforo, gem -[N3P3(O2C12H8)2(OC5H4N-4)2] (I), y su polímero análogo, [{NP(O2C12H8)}0.7{NP(OC5H4N-4)2}0.3]n(II), para preparar nuevos compuestos de oro o plata, cíclicos o polímeros. Se han obtenido los siguientes complejos, gem -[N3P3(O2C12H8)2(OC5H4N-4{ML})2] [ML=Au(C6F5) (1), Au(C6F5)3 (2)], [N3P3(O2C12H8)2(OC5H4N-4{AuPPh3})2][NO3]2 (3) and [N3P3(O2C12H8)2(OC5H4N-4{AgPPh2R})2][SO3CF3]2 [R=Ph (4) or Me (5)], que a su vez han resultado ser excelentes modelos para preparar los polímeros análogos de oro o plata, [{NP(O2C12H8)}0.7{NP(OC5H4N-4{ML})2}0.3]n[ML=Au(C6F5)(P1), Ag(OSO2CF3)PPh3 (P2)]. Todos los complejos, cíclicos o polímeros, se han caracterizado por análisis elemental, por métodos espectroscópicos y por espectrometría de masas. Los polímeros, además, se han caracterizado por métodos termoquímicos (TGA y DSC). Para 1,5 y para el fosfazeno de partida (I) puede esperarse una mezcla de varios estereoisómeros. Se ha estudiado su estereoquímica en disolución por RMN a temperatura variable, lo que ha indicado la presencia de un proceso de interconversión que implica cambios de quiralidad del grupo 2,2,-dioxibifenilo. La resolución de la estructura cristalina del complejo 2, por difracción de Rayos X, no sólo ha confirmado la estructura propuesta sino que, además, indica una configuración (S,S)- y (R,R)- , a diferencia de lo observado para el precursor I. Se ha estudiado también la pirólisis de estos nuevos metalofosfazenos. Cabe destacar que la pirólisis de los derivados de oro, trímero (1) y polímero (P1), dio esponjas macroporosas de oro metálico sin utilizar un agente reductor externo ni un soporte poroso. Todos estos materiales se han caracterizado por XRD, TEM, SEM y EDAX. [source] Spontaneous absolute asymmetric synthesis in the presence of achiral silica gel in conjunction with asymmetric autocatalysisCHIRALITY, Issue 7 2006Tsuneomi Kawasaki Abstract An enantiomerically enriched pyrimidyl alkanol with either S or R configurations was obtained stochastically from the reaction between pyrimidine-5-carbaldehyde and diisopropylzinc in the presence of achiral silica gel in conjunction with asymmetric autocatalysis with amplification of chirality. Chirality 18:479,482, 2006. © 2006 Wiley-Liss, Inc. [source] Conformational Analysis of R207910, a New Drug Candidate for the Treatment of Tuberculosis, by a Combined NMR and Molecular Modeling ApproachCHEMICAL BIOLOGY & DRUG DESIGN, Issue 2 2006Sandrine Gaurrand R207910 is an enantiomeric compound from a new class of antimycobacterial agents, the diarylquinolines [Science; 307:223 (2005)]. As enantiospecific interaction is required for biologic activity, we have undertaken a combined nuclear magnetic resonance and molecular modeling study to gain new insights into its conformation in solution and its absolute configuration. A conformational analysis using a Monte-Carlo method has been performed on each of the four possible stereomers of this compound leading to the identification of their most stable conformation. Additional ab initio calculation was performed with emphasis on the strength of the observed intramolecular hydrogen bond. Simultaneously, a complete structural identification has been carried out by a set of monodimensional and bidimensional 1H- 13C-NMR experiments. Determination of inter-proton distances has been achieved by a series of 1H- 1H ROESY NMR experiments with different mixing times followed by a volume quantification of the correlations peaks. These experimental data were compared with the theoretical distances obtained from the conformational analysis. The remarkable match shows that R207910 adopts one of the low-energy conformations predicted by molecular modeling and belongs to the (RS, SR) couple of diastereoisomers. A posteriori validation of our approach has been performed by X-ray structure determination that concluded for the RS configuration. [source] | ||||||||||