Home About us Contact
|
Home About us Contact | ||
|
|
||
Chiral Centers (chiral + center)
Selected AbstractsHighly Enantioselective Construction of the ,-Chiral Center of Amides via Iridium-Catalyzed Hydrogenation of ,,,-Unsaturated AmidesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2009Wei-Jing Lu Abstract The chiral center at the ,-position of amides is installed in excellent enantioselectivity via the iridium-catalyzed asymmetric hydrogenation of ,,,-unsaturated amides under mild conditions. Even aliphatic amides are suitable substrates. The presence of a hydrogen atom on the nitrogen of the amide is important for the enantioselectivity of the reaction. [source] ChemInform Abstract: Enantioselective Michael Addition of Nitroolefins and Cyclohexanones Catalyzed by Novel Pyrrolidine-thiourea Bifunctional Catalysts with Two Chiral Centers.CHEMINFORM, Issue 44 2008Dongping Cheng Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Catalytic Enantioselective Alkylative Aldol Reaction: Efficient Multicomponent Assembly of Dialkylzincs, Allenic Esters, and Ketones Toward Highly Functionalized ,-Lactones with Tetrasubstituted Chiral Centers.CHEMINFORM, Issue 42 2007Kounosuke Oisaki Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] ChemInform Abstract: Use of a Modified Ring-Switching Strategy to Synthesize the Glutamate Antagonist (2S)-2-Amino-3-(2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yl)propionate and Related Compounds with Two Chiral Centers.CHEMINFORM, Issue 21 2002Andrew Dinsmore Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Chiral separation of N -imidazole derivatives, aromatase inhibitors, by cyclodextrin-capillary zone electrophoresis.ELECTROPHORESIS, Issue 16 2004Mechanism of enantioselective recognition Abstract Baseline separation of ten new, substituted [1-(imidazo-1-yl)-1-phenylmethyl)] benzothiazolinone and benzoxazolinone derivatives with one chiral center was achieved using cyclodextrin-capillary zone electrophoresis (CD-CZE). A method for the enantiomeric resolution of these compounds was developed using neutral CDs (native ,-, ,-, ,-CDs or ,-, ,-, ,-hydroxypropyl (HP)-CDs) as chiral selectors. Operational parameters including the nature and concentration of the chiral selectors, pH, ionic strength, organic modifiers, temperature, and applied voltage were investigated. The use of neutral CDs provides enantiomeric resolution by inclusion of compounds in the CD cavity. The HP-,-CD and HP-,-CD were found to be the most effective complexing agents and allowed efficient enantiomeric resolutions. Optimal separation of N -imidazole derivatives was obtained using 50 mM phosphate buffer at pH 2.5 containing either HP-,-CD or HP-,-CD (7.5,12.5 mM) at 25°C, with an applied field of 0.50 kV·cm,1 giving resolution factors Rs superior to 1.70 with migration times of the second enantiomer less than 13 min. The same enantiomer migration order observed for all molecules can be related to a close interaction mechanism with CDs. The influence of structural features of the solutes on Rs and tm was studied. The lipophilic character (log kw) of the solutes and the apparent and averaged association constants of inclusion complexes for four compounds with the six different CDs led us to rationalize the enantioseparation mechanisms. The conclusions were corroborated with reversed-phase high-performance liquid chromatography (HPLC) on chiral stationary phases (CSPs) based on CDs. [source] Permanganate Oxidation Revisited: Synthesis of 3-Deoxy-2-uloses via Indium-Mediated Chain Elongation of CarbohydratesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 25 2010Christoph Schmölzer Abstract Application of the Barbier-type indium-mediated allylation method to suitable substrates offers access to carbohydrates bearing a terminal olefin moiety. The C,C bond forming reaction generates a defined stereochemistry of the new chiral center and tolerates a wide variety of starting aldehydes thus allowing modifications in the carbohydrate backbone. Further transformations of the alkene moiety via an environmentally benign and subtle controlled protocol using potassium permanganate gives rise to the structural motif of 3-deoxy-2-uloses in good yields. The final part of the reaction sequence focuses on the deprotection of the acetyl groups essential for the success of the oxidation step. The acidic and labile 3-deoxy position of the target molecule is prone to elimination applying standard deacetylation conditions and therefore demands derivatisation of the molecule. The introduction of a thioketal moiety using microwave conditions shows promising results and subsequent standard transformations are applicable leading to the desired products. [source] A Highly Enantioselective Receptor for Carbamoyl Lactic Acid,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 31 2009Francisco M. Muñiz Abstract A new receptor based on a 9,9-dimethylxanthene framework was synthesized. Owing to its suitable oxyanion hole structure, this receptor is able to associate carboxylic acids and anions. The introduction of a chiral center provides enantioselective properties to this receptor as a result of its different interactions with both enantiomers of the substrate. The combination of this skeleton with a fluorescent unit such as dansyl allows the detection of small amounts of carboxylic acids by making use of fluorescent techniques.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Efficient, Enantioselective Organocatalytic Synthesis of Trichostatin AADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 10-11 2006Shilei Zhang Abstract An efficient, highly stereocontrolled total synthesis of trichostatin A (1) has been achieved in 9 steps with 17.4,% overall yield and >99,% optical purity from readily available achiral starting materials. The key features of this synthesis include the L -proline-promoted, highly enantioselective cross-aldol reaction as a crucial step for the construction of the C-6 chiral center and the minimization of racemization by final step oxidation of the OH group to a ketone at position 7. [source] Synthesis of 14C-labeled and tritiated AMPA potentiator LY450108JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 2 2005Boris A. Czeskis Abstract Asymmetric synthesis of AMPA potentiator LY450108-[14C] containing 14C-label attached to the chiral center of the molecule, was accomplished based on Evans' chiral oxazolidinone auxiliary method. Diastereoselective methylation of p -nitrophenylacetic acid derivative was used as a key step. The auxiliary was reductively removed, and the resulting primary alcohol was converted into the corresponding amine. Its sulfonylation, reduction of the aromatic nitro group, and acylation with 3,5-difluorobenzoyl chloride led to the final product. The synthesis of tritiated LY450108 is also detailed. Copyright © 2005 John Wiley & Sons, Ltd. [source] Densely grafted polyisocyanides synthesized by two types of polymerization techniquesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2003Yanqing Tian Abstract A series of novel polyisocyanide- graft -polystyrenes and polyisocyanide- graft -[polystyrene- block -poly(butyl acrylate)]s were synthesized through the grafting-through and grafting-from routes with two types of living polymerization techniques: polymerization with the Pd,Pt ,-ethynediyl dinuclear complex as the initiator and catalyst for the polyisocyanide backbone and atom transfer radical polymerization for the grafted side chain. Through the introduction of a chiral center at the side chain of the polyisocyanide backbone, helical grafted and graft block polyisocyanides were prepared through the grafting-from method. All of the obtained polymers exhibited polydispersities in the range of 1.07,1.41. This might have been the first time grafted polyisocyanides were prepared, especially helical grafted polyisocyanides, through the operation of two living polymerization techniques. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1871,1880, 2003 [source] Isolation of coumarins and ferulate from the roots of Angelica purpuraefolia and the antitumor activity of khellactonePHYTOTHERAPY RESEARCH, Issue 5 2007Hyeong-Kyu Lee Abstract A new coumarin, hydroxylomatin (1), was isolated from the CHCl3 -soluble fraction of the roots of Angelica purpuraefolia, along with one ferulate (2) and three other known coumarins (3,5) including khellactone (3). The structure of hydroxylomatin (1) was determined to be 3,,,5,-dihydroxy-3,,4,-dihydroseselin (1) by spectroscopic means including 2D-NMR. The modified Mosher's method was used to determine the chiral center at C-1 of compound 2. Khellactone (3) is a major compound of the roots of A. purpuraefolia. This study also examined the antitumor activity of khellactone (3) using a LLC mouse lung carcinoma in the BDF-1 mice and a NCI-H460 human lung carcinoma in a human tumor xenograft model in nude mice. This compound (3) inhibited LLC tumor growth with a T/C (mean value of treated group/mean value of control group) value of 12.9% at a dose of 5 mg/kg and 33.2% at a dose of 10 mg/kg, respectively, in a dose-dependent manner. In addition, it suppressed the growth of NCI-H460 tumor cells, accounting for 81.4% at a dose of 10 mg/kg in nude mice. Copyright © 2007 John Wiley & Sons, Ltd. [source] Chemistry of Ru(& 6 -1,3,5-cyclooctatriene)(,2 -dimethyl fumarate)2THE CHEMICAL RECORD, Issue 3 2006Take-Aki Mitsudo Abstract The chemistry of a novel zerovalent Ru complex, Ru(,6 -cot)(,2 -dmfm)2 (1) (cot=1,3,5-cyclooctatriene; dmfm=dimethyl fumarate), is reviewed with a focus on its reactivity toward phosphines, amines, and H2O, as well as arenes and p -quinones. A variety of novel zerovalent Ru complexes were synthesized from Ru(,6 -cot)(,2 -dmfm)2 (1), and it was shown that the complexes preferably bear both electron-donating and -accepting ligands simultaneously to exhibit thermodynamic stability. The first isolable zerovalent Ru aqua complexes were successfully prepared, and in these complexes, the generation of a chiral center on the O atom of the coordinated H2O was disclosed. In addition, the characteristic catalytic activity of 1 in organic synthesis was considered by reviewing recently developed novel reactions: (i) dimerization of 2,5-norbornadiene to pentacyclo[6.6.0.02,6.03,13.010,14]tetradeca-4,11-diene (PCTD), (ii) intramolecular hydroamination of aminoalkynes to cyclic imines, (iii) formal [4+2] cycloaddition of alkynes with dmfm to trans -4-cyclohexene-1,2-dicarboxylates, and (iv) co-dimerization of dihydrofurans with ,,,-unsaturated esters to 2-alkylidenetetrahydrofurans. The products obtained here are expected to be used as novel functional organic monomers. © 2006 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 6: 107,116; 2006: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20076 [source] The high-resolution structure of (+)- epi -biotin bound to streptavidinACTA CRYSTALLOGRAPHICA SECTION D, Issue 6 2006Isolde Le Trong (+)- Epi -biotin differs from (+)-biotin in the configuration of the chiral center at atom C2. This could lead to a difference in the mode of binding of (+)- epi -biotin to streptavidin, a natural protein receptor for (+)-biotin. Diffraction data were collected to a maximum of 0.85,Å resolution for structural analysis of the complex of streptavidin with a sample of (+)- epi -biotin and refinement was carried out at both 1.0 and 0.85,Å resolution. The structure determination shows a superposition of two ligands in the binding site, (+)-biotin and (+)- epi -biotin. The molecules overlap in the model for the complex except for the position of S1 in the tetrahydrothiophene ring. Differences in the conformation of the ring permits binding of each molecule to streptavidin with little observable difference in the protein structures at this high resolution. [source] Axially chiral N -(o -aryl)-4-hydroxy-2-oxazolidinone derivatives from diastereoselective reduction of N -(o -aryl)-2,4-oxazolidinediones: Thermally interconvertible atropisomers via ring-chain-ring tautomerizationCHIRALITY, Issue 7 2010Öznur Demir-Ordu Abstract The reduction of the axially chiral N -(o -aryl)-5,5-dimethyl-2,4-oxazolidinediones by NaBH4 yielded axially chiral N -(o -aryl)-4-hydroxy-5,5-dimethyl-2-oxazolidinone enantiomers having a chiral center at C-4, with 100% diastereoselectivity as has been shown by their 1H and 13C NMR spectra and by enantioselective HPLC analysis. The resolved enantiomeric isomers were found to interconvert thermally through an aldehyde intermediate formed upon ring cleavage via a latent ring-chain-ring tautomerization. It was found that the rate of enantiomerization depended on the size and the electronic effect of the ortho substituent present on the aryl ring bonded to the nitrogen of the heterocycle. Chirality 2010. © 2009 Wiley-Liss, Inc. [source] Diastereoisomeric assignment in a pacifenol derivative using vibrational circular dichroism,CHIRALITY, Issue 1E 2009Marcelo A. Muñoz Abstract The configuration of a chiral center in semisynthetic (,)-(2R,5R,5aR,8,,9aS)- 2,8-dibromo-2,5,9,9a-tetrahydro-5-hydroxy-5,8,10,10-tetramethyl-6H -2,5a-methano-1-benzoxepin-7(8H)-one (3 or 4), prepared in two steps from (,)-(2R,5R,5aR,7S,8S,9aS)-2, 7-dibromo-8-chloro-2,5,7,8,9,9a-hexahydro-5,8,10,10-tetramethyl-6H -2,5a-methano-1-benzoxepin-5-ol, known as pacifenol 1, has been determined using vibrational circular dichroism (VCD) measurements. The vibrational spectra (IR and VCD) of diastereoisomers 3 and 4 were calculated using density functional theory (DFT) at the B3LYP/DGDZVP level of theory for the two conformers that in each case account for the total energetic distribution found in the first 10 kcal/mol range. The DFT conformational optimization of the 8R diastereoisomer 3 indicates the cyclohexanone exists almost exclusively in a boat conformation with a ,-equatorial bromine atom and an ,-axial methyl group at the chiral center alpha to the carbonyl group, while for the 8S diastereoisomer 4 a 5:1 conformational distribution in favor of a chair conformation with an ,-axial bromine atom and a ,-equatorial methyl group is calculated, suggesting due to well-known chair versus boat relative stabilities that the plausible diastereoisomer would be the 8S molecule. A comparison of the IR spectrum of the reaction product with the calculated spectra of 3 and 4 provided no means for the diastereoisomeric assignment, while from comparison of the VCD spectra it became immediately evident that the rearranged molecule possesses the 8R absolute configuration as shown in 3, in concordance with a single crystal X-ray diffraction study that could be refined to an R -factor of 2.9%. Chirality 21:E208,E214, 2009. © 2009 Wiley-Liss, Inc. [source] Conformational spaces of the gastrointestinal antisecretory chiral drug omeprazole: Stereochemistry and tautomerismCHIRALITY, Issue 1 2006Hava Caner Abstract A study of the conformational spaces of the chiral proton pump inhibitor (PPI) drug omeprazole by semiempirical, ab-initio, and DFT methods is described. In addition to the chiral center at the sulfinyl sulfur atom, the chiral axis at the pyridine ring (due to the hindered rotation of the 4-methoxy substituents) was considered. The results were analyzed in terms of the 5-methoxy and 6-methoxy tautomers and the two pairs of enantiomers (R,P)/(S,M) and (R,M)/(S,P). Five torsion angles were systematically explored: the backbone rotations defined by D1 (N3,C2,S10,O11), D2 (C2,S10,C12,C13), and D3 (S10,C12,C13,N14) and two methoxy rotations defined by D4 (C6,C5,O8,C9) and D5 (C16,C17,O19,C20). Significant energy differences were revealed between the 5- and 6-methoxy tautomers, the extended and folded conformations, and the (S,M) and (S,P) diastereomers. The "extended M" conformation of the 6-methoxy tautomer of (S)-omeprazole was found to be the most stable conformer. © 2005 Wiley-Liss, Inc. Chirality [source] Helical- and ahelical-dependent chiral recognition mechanisms in capillary electrophoresis using amylose as the selectorELECTROPHORESIS, Issue 8 2009Weili Wei Abstract The present study discovered that helical structures of amylose were not always responsible for its chiral recognition abilities in CE. Several enantiomers with different structures were selected as models. Based on ultraviolet,visible spectroscopy and 13C NMR measurements, it was found that helical structures were gradually destroyed by temperature elevation and almost entirely transformed to extended ahelical structures above 60°C. Then, CE and 1H NMR chiral recognitions were investigated at different temperatures; chiral selectivity of the enantiomers varied in two different ways. Summarily, helical structures were necessary only for chiral separations of the enantiomers with small (<0.78,nm) and flexible molecular structures. However, for the gauche enantiomers (>0.78,nm) with high steric hindrances over their chiral centers, ahelical structures alone can realize chiral recognitions. By using iodine as a helix including competitor, it was further proved that helical structures functioned through the inclusive complexations only in the chiral separations of small enantiomers and had no effect for the others. The underlying mechanisms of the functions of helical and ahelical structures in molecular level were discussed as well. [source] Tellurated Schiff Bases Formed from {2-[(4-Methoxyphenyl)telluro]ethyl}amine and Bis(2-aminoethyl) Telluride with o -Hydroxyacetophenone: Synthesis and Complexation Reactions with HgII, PdII and RuII , Crystal Structures of the Ligands, [Ru(p -cymene)Cl{H2NCH2CH2TeC6H4 -4-OCH3}]Cl·H2O and [RuCl{4-MeOC6H4TeCH2CH2NHCH(CH3)C6H4 -2-O,}]EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 5 2004Raghavendra Kumar P. Abstract {2-[(4-Methoxyphenyl)telluro]ethyl}amine and bis(2-aminoethyl) telluride on treatment with o -hydroxyacetophenone gave the Schiff bases 4-MeOC6H4TeCH2CH2N=C(CH3)C6H4 -2-OH (L1) and 2-HOC6H4(CH3)C=NCH2CH2TeCH2CH2N=C(CH3)C6H4 -2-OH (L3), respectively. The reduction of L1 and L3 with NaBH4 resulted in 4-MeOC6H4TeCH2CH2NHCH(CH3)C6H4 -2-OH (L2) and 2-HOC6H4(CH3)CHNHCH2CH2TeCH2CH2NHCH(CH3)C6H4 -2-OH (L4), respectively, which have 1 or 2 chiral centers. The 1H and 13C NMR spectra of L1 to L4 were found to be characteristic. Treatment of L1 with [Ru(p -cymene)Cl2]2 resulted in [Ru(p -cymene)(4-MeOC6H4TeCH2CH2NH2)Cl]Cl·H2O (1) whereas in the reaction of L2 with [Ru(p -cymene)Cl2]2, the p -cymene ligand is lost resulting in [RuCl(L2 -H)] (4). The reactions of L1, L3 and L4 with HgBr2 resulted in complexes of the type [HgBr2·(L)2] while Na2PdCl4 reacted with L1 to give [PdCl(L1 -H)]. The solid-state structures of L1, L3, 1 and 4 were determined by single-crystal X-ray diffraction studies. The very swift formation of the tellurated amine from a tellurated Schiff base (L1) by hydrolysis has been observed for the first time and has resulted in 1. The Ru,N and Ru,Te bond lengths in 1 are 2.142(3) and 2.6371 (4) Å, respectively. The replacement of the p -cymene ligand with a hybrid organotellurium ligand (L2 -H), resulting in 4, is also a first example of its kind. The Ru center in 4 has a square-planar geometry, with the Ru,N, Ru,Te, Ru,O and Ru,Cl bond lengths being 2.041(6), 2.4983(8), 2.058(5) and 2.308(2) Å, respectively. In the crystals of 4 there are secondary intermolecular Te···Cl interactions and intermolecular N,H···O hydrogen bonds. This is the first example in which coordinated Te in a complex is engaged in two intermolecular secondary interactions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] N -(4-Nitrophenylsulfonyl)- and N -(Fluorenylmethoxycarbonyl)- N -ethyl Amino Acid Methyl Esters , A Practical ApproachEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2010Emilia Lucia Belsito Abstract An efficient one-pot preparation of N -ethyl- N -4-nitrophenylsulfonyl (nosyl) amino acid methyl esters was accomplished by a simple N -ethylation reaction by using triethyloxonium tetrafluoroborate in the presence of N,N -diisopropylethylamine. The N -ethylated amino acid methyl esters are obtained with total retention of stereochemistry at the original chiral centers. To further broaden the scope of this methodology, the N -ethylated nosyl-protected compounds are easily converted in the more practical fluorenylmethyloxycarbonyl (Fmoc)-protected derivatives. The cleavage of methyl ester by using a mild and neutral method enables the preparation of N -ethyl amino acids that are building blocks suitable for introduction into a peptide chain. The methodology works well with both nosyl- and Fmoc-based solution-phase peptide synthesis. [source] First Total Synthesis of the Potent Anticancer Natural Product Dideoxypetrosynol A: Preparation of the "Skipped" (Z)-Enediyne Moiety by Oxidative Coupling of Homopropargylphosphonium YlideEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 28 2008Benjamin W. Gung Abstract Dideoxypetrosynol A is a C30 polyacetylenic alcohol with C2 symmetry. The first total synthesis of both enantiomers of the potent anti-cancer natural product (+)- and (,)-dideoxypetrosynol A is reported. The key step is an oxidative coupling of a homopropargylphosphonium ylide to prepare the "skipped" (Z)-enediyne moiety. The natural dideoxypetrosynol A was isolated as a racemic mixture as shown in structure 1. The absolute configurations of the chiral centers are established for the (+)- and (,)-enantiomers using Burgess' enzymatic resolution procedure with Pseudomonas AK lipase. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Manifestation of a Chiral Smectic C Phase in Diphenylbutadiene-Cored Bolaamphiphilic Sugars,ADVANCED FUNCTIONAL MATERIALS, Issue 11 2008Suresh Das Abstract A series of symmetrical bolaamphiphiles possessing a diphenylbutadiene core and glucopyranoside head groups linked together by oligomethylene spacers, were synthesized and their thermotropic liquid crystalline properties investigated by polarized light optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optic switching. In spite of the presence of chiral centers, amphiphilic sugars in general do not exhibit macroscopic chirality and this phenomenon is attributed to strong hydrogen bonding between sugar head groups resulting in microphase-segregated layer like arrangements. In the present study all the molecules investigated exhibited the smectic C* phase, i.e., tilted lamellar phase with macroscopic chiral ordering of the molecules. The stability of this phase increased with increase in the length of the oligomethylene spacers. Whereas for derivatives with spacers containing ,4 methylene groups, the smectic C* phase was observed only in the cooling phase, for those containing spacers with ,5 methylene groups this phase was observed both in the heating and cooling cycles. The absorption and fluorescence spectra of these materials suggest that the unusual observation of macroscopic chirality in these bolaamphiphiles containing free hydroxyl groups could be attributed to self-aggregating behavior of the diphenylbutadiene core. [source] Asymmetric Michael Addition Reaction of 3-Substituted Oxindoles to Nitroolefins Catalyzed by a Chiral Alkyl- Substituted Thiourea CatalystADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 2-3 2010Xin Li Abstract A simple alkylthiourea was found to be an effective catalyst for the Michael addition reaction of 3-substituted oxindole to nitroolefins. A number of 3,3,-substituted oxindole derivatives, which have two vicinal quaternary-tertiary chiral centers were synthesized with up to 99% yield, 19:1 dr and 98% ee. [source] Discrimination between diastereoisomeric dipeptides by IR,UV double resonance spectroscopy and ab initio calculationsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2005Ali G. Abo-Riziq Abstract We studied diastereoisomeric dipeptides, containing two chiral centers, by comparing ab initio calculations with laser desorption jet-cooling experiments. We studied the hetero-dipeptides LL,VF (L-Val-L-Phe) and DL,VF and the homo-dipeptides LL,FF (L-Phe-L-Phe) and LD,FF. Changing one of the chiral centers in each molecule leads to changes in the spectra that can be used to distinguish between diastereoisomeric pairs. We observed three different conformers for LL,VF, four for DL,VF, two for LL,FF, and one for LD,FF. By comparing the results from IR,UV double resonant spectroscopy with ab initio calculations, we can draw conclusions about the conformational structures. At the same time, the experimental data serve as a test for the computational results. We discuss the possibilities and limitations of the interplay between theory and experiment. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Mixed Aromatic Acyloin Condensations with Recombinant Benzaldehyde Lyase: Synthesis of ,-Hydroxydihydrochalcones and Related ,-Hydroxy KetonesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6-7 2003Monica Sanchez-Gonzalez Abstract Recombinant benzaldehyde lyase (BAL), expressed and purified from E.,coli strain JM-109, was used to catalyze the condensation of a series of methoxybenzaldehydes and phenylacetaldehyde in the synthesis of ,-(R)-hydroxydihydrochalcones. Enantiomerically pure 1-hydroxy-1,3-diphenylpropan-2-ones and o -anisoin were also obtained as products of the BAL reaction. The R absolute configurations of chiral centers were determined by CD spectroscopy. ,-(R)-Hydroxydihydrochalcones and 1-hydroxy-1,3-diphenylpropan-2-ones are valuable synthons for chemoenzymatic syntheses of flavonoids. This is the first synthesis of ,-(R)-hydroxydihydrochalcones by a microbial enzyme. [source] Polymer-bound alkyltriazenes for mild racemization-free esterification of amino acid and peptide derivativesJOURNAL OF PEPTIDE SCIENCE, Issue 10 2004Joachim Smerdka Abstract A novel tool for polymer-assisted solution phase (PASP) esterification of amino acid and peptide derivatives has been developed. When treated with carboxylic acids, polymer-bound alkyltriazenes react with a loss of nitrogen and transfer of the alkyl moiety to the carboxylate anion to form the corresponding alkyl esters. There are no limitations with regard to either the protecting groups or the nature of the amino acid. Furthermore no racemization occurs at the chiral centers of the amino acids as demonstrated by chiral GC-MS analyses. Alkyltriazene-resins were also applied successfully to the esterification of peptide acids and other peptidic structures, such as tripalmitoyl- S -glyceryl-cysteine (Pam3Cys). The triazene-mediated esterification reaction is exceptionally mild, and there is no need for prior activation of the carboxy groups. This method is therefore particularly suitable for the alkylation of complex peptidomimetic structures prone to racemization and for acid-sensitive structures. Copyright © 2004 European Peptide Society and John Wiley & Sons, Ltd. [source] Stereocontrolled anionic alternating copolymerization of ethylphenylketene with benzaldehyde by a bisoxazoline ligandJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2004Daisuke Nagai Abstract Anionic copolymerization of ethylphenylketene with benzaldehyde with butyllithium or diethylzinc as the initiator proceeded in a perfect 1:1 alternating manner to produce the corresponding polyester, whose repeating unit had two adjacent chiral centers. The relative stereochemistry between these two chiral centers was successfully controlled by the addition of (S,S)-(-)-2,2,-isopropylidenebis(4- tert -butyl-2-oxazoline), producing the corresponding polyester that had excellent diastereoselectivity (erythro -configuration : threo -configuration = 4:96). The diastereomeric ratio was determined by high-performance liquid chromatography analysis of the diol, which was obtained by reductive degradation of the polyester while maintaining the configuration of the repeating unit. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5384,5388, 2004 [source] Synthesis and NMR spectroscopic studies of allylsulfanyl- N1 -alkyl- N4 -phenyl-1,4-phenylenediamines and their cyclization products, 2,3-dihydro-1-benzothiophenes and thiochromansMAGNETIC RESONANCE IN CHEMISTRY, Issue 12 2004Alan R. Katritzky Abstract Regioselective addition of allylthiol at the C-3 position adjacent to the nitrogen carrying the phenyl group of the 1,4-phenylenediamine moiety of compounds 1,4 was rigorously confirmed by 1D NOE difference in combination with gHMBC experiments. The structures of 1,4-phenylenediamines 1,4, allylsulfanyl- N1 -alkyl- N4 -phenyl-1,4-phenylenediamines 5,8 and cyclization products 9,14 were completely analyzed in both CDCl3 and DMSO- d6 solutions. The 1H and 13C NMR spectra of 10 and 11, which contain two chiral centers, exhibit duplication for several signals, indicating the existence of two diastereomeric forms. The full structures of 5 and 9 were unambiguously confirmed by x-ray crystallography. The 1H and 13C NMR spectra of all compounds were assigned using one- and two-dimensional NMR techniques (APT, DEPT, 1D NOE difference, COSY, NOESY, HETCOR, gHMQC and gHMBC). Copyright © 2004 John Wiley & Sons, Ltd. [source] Tetrameric aggregate of 1,c -3-diphenyltetran- r -1-ol via an R44(8) homodromic ringACTA CRYSTALLOGRAPHICA SECTION C, Issue 2 2009Daniel R. Vega The compound 1,c -3-diphenyltetran- r -1-ol (systematic name: 1,c -3-diphenyl-1,2,3,4-tetrahydro- r -1-naphthol), C22H20O, which possesses the tetrahydronaphthalene core that is found in a large number of natural products, crystallizes with Z, = 4 and with the four molecules forming a hydrogen-bonded cyclic aggregate. The aliphatic six-membered rings are present with two different conformations in the molecules of the asymmetric unit. A comparison with similar fragments reveals their conformational flexibility. In addition, the structure demonstrates the relative stereochemistries of the chiral centers, which are important since the title compound is used in the stereoselective synthesis of compounds with therapeutic activity. [source] 1-(,-Aminobenzyl)-2-naphthol: A New Chiral Auxiliary for the Synthesis of Enantiopure ,-Aminophosphonic AcidsCHEMISTRY - A EUROPEAN JOURNAL, Issue 27 2009Kirill Abstract Ooh Betti! A new diastereoselective synthesis of ,-aminophosphonates has been developed based on the reaction of trialkyl phosphites with chiral imines derived from (R)- or (S)-1-(,-aminobenzyl)-2-naphthol (Betti base; see scheme, X=H, CH3, or Br). The reaction proceeds with high diastereoselectivity. Treatment with HCl results in the formation of the desired ,-aminophosphonic acids. A new diastereoselective synthesis of ,-aminophosphonates has been developed, based on the reaction, in the presence of trifluoroacetic acid, of trialkyl phosphites with chiral imines derived from (R)- or (S)-1-(,-aminobenzyl)-2-naphthol. The reaction proceeds at room temperature in toluene with high diastereoselectivity. The major diastereomer can be separated by crystallization from an appropriate solvent. The relative configuration of both chiral centers of the major diastereomer was determined by single-crystal X-ray structure analysis. The desired ,-aminophosphonic acids can be obtained in enantiopure form by treatment of the corresponding diastereomers with HCl. [source] Spontaneous Resolution of Chiral Polyoxometalate-Based Compounds Consisting of 3D Chiral Inorganic Skeletons Assembled from Different Helical UnitsCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2008Ya-Qian Lan Dr. Abstract Four enantiomerically pure 3D chiral POM-based compounds, [Ni2(bbi)2(H2O)4V4O12],2,H2O (1,a and 1,b) and [Co(bbi)(H2O)V2O6] (2,a and 2,b) (bbi=1,1,-(1,4-butanediyl)bisimidazole) based on the achiral ligand, different vanadate chains, and different metal centers have been synthesized by hydrothermal methods. Single-crystal X-ray diffraction analyses revealed that 1,a and 1,b, and 2,a and 2,b, respectively, are enantiomers. In 1,a and 1,b two kinds of vanadate chains with different screw axes link Ni cations to generate 3D chiral inorganic skeletons, which are connected by the achiral bbi ligands to form complicated 3D 3,4-connected chiral self-penetrating frameworks with (72,8)(72,82,92)(73,82,10) topology. They represent the first examples of chiral self-penetrating frameworks known for polyoxometalate (POM) systems. Contrary to 1,a and 1,b, in 2,a and 2,b the vanadate chains link CoII cations to generate 3D chiral inorganic skeletons, which are assembled from two kinds of heterometallic helical units of opposite chirality along the c axes. The chiral inorganic skeletons are connected by bbi to form 3D 3,4-connected chiral POM-based frameworks with (62,8)2(62,82,102) topology. It is believed that the asymmetrical coordination modes of the metal cations in 1,a,2,b generate the initial chiral centers, and that the formation of the various helical units and the hydrogen bond interactions are responsible for preservation of the chirality and spontaneous resolution when the chirality is extended into the homochiral 3D-networks. This is the first known report of chiral POM-based compounds consisting of 3D chiral inorganic skeletons being obtained by spontaneous resolution upon crystallization in the absence of any chiral source, which may provide a rational strategy for synthesis of chiral POM-based compounds by using achiral ligands and POM helical units. [source] | ||||||||||||||||||||||||||||||||||