- Sterically congested chiral N-acetyl 2-oxazolidinone as acetyl enolate equivalents in stereoselective aldol reactions
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Sterically constrained and highly congested N-acetyl-DMAOx (1a) serves well as a useful chiral acetyl enolate equivalent which permits a diastereoselective and direct approach to 'acetate' aldols (2) including statine derivative.
- Otsuka,Ishizuka,Kimura,Kunieda
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Read Online
- Solvent-induced chirality switching in the enantioseparation of regioisomeric hydroxyphenylpropionic acids via diastereomeric salt formation with (1R,2S)-2-amino-1,2-diphenylethanol
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The enantioseparation of three hydroxyphenylpropionic acid isomers via diastereomeric salt formation with (1R,2S)-2-amino-1,2-diphenylethanol has been demonstrated. The racemates of all three acid isomers were successfully separated with high efficiency (0.56–0.84) after single crystallization. For 2-hydroxy-3-phenylpropionic acid 4, the configuration of the less-soluble salt was controlled by the crystallization solvent: the (R)-4 salt was crystallized from water, while 2-propanol afforded the (S)-4 salt. The chiral recognition mechanism of the three acids was discussed based on the crystal structures of the diastereomeric salts.
- Kodama, Koichi,Nagata, Jun,Kurozumi, Nobuhiro,Shitara, Hiroaki,Hirose, Takuji
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p. 460 - 466
(2017/03/23)
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- Stereoselective acetate aldol reactions of α-silyloxy ketones
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TiCl4-mediated aldol reactions of chiral methyl α-silyloxy ketones with a variety of aldehydes provide the corresponding 1,4-syn aldol adducts with moderate to high stereocontrol. This transformation represents a new approach to substrate-controlled acetate aldol reactions and complements the 1,4-anti asymmetric induction produced by the related α-benzyloxy ketones. This new approach could be useful in the design of more efficient syntheses of natural products.
- Lorente, Adriana,Pellicena, Miquel,Romea, Pedro,Urpí, Fèlix
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p. 1023 - 1035
(2015/01/30)
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- Substrate evaluation of rhodococcus erythropolis SET1, a nitrile hydrolysing bacterium, demonstrating dual activity strongly dependent on nitrile sub-structure
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Assessment of Rhodococcus erythropolis SET1, a novel nitrile hydrolysing bacterial isolate, has been undertaken with 34 nitriles, 33 chiral and 1 prochiral. These substrates consist primarily of β-hydroxy nitriles with varying alkyl and aryl groups at the β position and containing in several compounds different substituents α to the nitrile. In the case of β-hydroxy nitriles without substitution at the α position, acids were the major products obtained, along with recovered nitrile after biotransformation, as a result of suspected nitrilase activity of the isolate. Unexpectedly, amides were found to be the major hydrolysis product when the β-hydroxy nitriles possessed a vinyl group at this position. To probe this behaviour further, additional related substrates were evaluated containing electron-withdrawing groups at the α position, and amide was also observed upon biotransformation in the presence of SET1. Therefore this novel isolate has also demonstrated NHase activity with nitriles that appears to be substrate-dependent.
- Coady, Tracey M.,Coffey, Lee V.,O'Reilly, Catherine,Lennon, Claire M.
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supporting information
p. 1108 - 1116
(2015/02/19)
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- Biotransformation of aromatic ketones and ketoesters with the non-conventional yeast Pichia glucozyma
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The non-conventional yeast Pichia glucozyma CBS 5766 has been used for the biotransformation of different aromatic ketones and ketoesters. The growth and biotransformation conditions were optimised for the reduction of acetophenone and under optimised conditions, propiophenone, butyrophenone and valerophenone were reduced to the corresponding (S)-alcohols with high yields and enantioselectivity. Ketoreductase(s) of Pichia glucozyma showed high catalytic activities also towards aromatic β- and γ-ketoesters, being often competitive with esterase(s). These concurrent activities allowed for the preparation of hydroxyesters, hydroxyacids and lactones often in a very selective manner.
- Contente, Martina Letizia,Molinari, Francesco,Zambelli, Paolo,De Vitis, Valerio,Gandolfi, Raffaella,Pinto, Andrea,Romano, Diego
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supporting information
p. 7051 - 7053
(2015/02/02)
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- Studies toward stereoselective bionanocatalysis on gold nanoparticles
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As yet, different enzymes were immobilized on gold nanoparticles both through adsorption and covalent binding. However, there is only one evaluation if such immobilization influenced enzyme enantioselectivity, which is an essential parameter in biocatalys
- Jedrzejewska, Hanna,Ostaszewski, Ryszard
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- Asymmetric β-boration of α,β-unsaturated N -acyloxazolidinones by [2.2]paracyclophane-based bifunctional catalyst
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An efficient copper-catalyzed asymmetric conjugate boration has been achieved by exploiting a new planar and central chiral bicyclic triazolium ligand. This protocol was highly efficient and gave a variety of chiral secondary alkylboronates in 97-99% ee. A preliminary mechanistic study supports the bifunctional nature of the catalyst.
- Zhao, Lei,Ma, Yudao,Duan, Wenzeng,He, Fuyan,Chen, Jianqiang,Song, Chun
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p. 5780 - 5783
(2013/01/15)
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- Reversal of selectivity in acetate aldol reactions of N-acetyl-(S)-4- isopropyl-1-[(R)-1-phenylethyl]imidazolidin-2-one
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Synergistic effects of the exo- and endocyclic chiral centers of an imidazolidinone-based auxiliary were investigated in the perspective of acetate aldol reactions. The reversal in diastereoselectivity was accomplished by lithium and titanium enolate reactions, which proceed through proposed open and closed transitions states, respectively. The aldol adducts were used in the stereoselective synthesis of fluoxetine.
- Khatik, Gopal L.,Kumar, Varun,Nair, Vipin A.
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scheme or table
p. 2442 - 2445
(2012/07/03)
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- Rhodium-catalyzed asymmetric addition of arylboroxines to β-alkoxyacrylate esters
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Asymmetric addition of arylboroxines to β-alkoxyacrylate esters proceeded in the presence of a rhodium complex coordinated with a chiral diene ligand to give high yields of β-alkoxy-β-arylcarboxylic acid esters with very high enantioselectivity.
- Nishimura, Takahiro,Kasai, Atsuyuki,Nagaosa, Makoto,Hayashi, Tamio
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supporting information; experimental part
p. 10488 - 10490
(2011/12/04)
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- 1,4-syn-Asymmetric induction in the titanium-mediated aldol reactions of chiral methyl α-silyloxy ketones
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Good levels of 1,4-syn asymmetric induction are obtained in the TiCl4-mediated aldol reaction of methyl α-silyloxy ketones with achiral aldehydes. Such methodology represents a new approach to the substrate-controlled acetate aldol reaction, which can be useful to design more efficient syntheses of natural products.
- Lorente, Adriana,Pellicena, Miquel,Romea, Pedro,Urpí, Fèlix
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scheme or table
p. 942 - 945
(2010/04/29)
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- 3-Pyrroline-1-carbonyl (Pyroc) group: A removable protecting group for the kinetic resolution of racemic carboxylic acids and alcohols through catalytic asymmetric acylation
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The O-3-pyrroline-1-carbonyl (O-Pyroc) group and 3-pyrrolinamide are useful removable protecting groups for the kinetic resolution of racemic α-hydroxycarboxylic acids, β-hydroxycarboxylic acids, 1,2-dicarboxylic acids, and 1,2-diols using the L-histidine
- Sakakura, Akira,Umemura, Shuhei,Ishihara, Kazuaki
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body text
p. 1647 - 1650
(2009/12/03)
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- Asymmetric synthesis of both antipodes of β-hydroxy nitriles and β-Hydroxy carboxylic acids via enzymatic reduction or sequential reduction/hydrolysis
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Use of isolated carbonyl reductases in the reduction of aromatic β-ketonitriles have completely eliminated the competing α-ethylation, which is often observed with whole cell biocatalysts. By choosing suitable recombinant carbonyl reductase, the reduction
- Ankati, Haribabu,Zhu, Dunming,Yang, Yan,Biehl, Edward R.,Hua, Ling
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experimental part
p. 1658 - 1662
(2009/08/08)
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- Chiral linker. Part 2: Synthesis and evaluation of a novel, reusable solid-supported open chain chiral auxiliary derived from m-hydrobenzoin for the diastereoselective reduction of α-keto esters
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Three novel m-hydrobenzoin derived chiral hydrobenzoin mono-alkyl ethers were synthesized and evaluated as open chain chiral auxiliaries in the L-selectrideR/ZnCl2 mediated stereoselective reduction of their corresponding phenyl glyoxylates, resulting in des of up to 91%. The optimized auxiliary structure was immobilized on commercially available Wang-resin by using the ether substituent as a sublinking unit and applied as a reusable solid-supported chiral auxiliary in the same type of reaction with only little loss of stereofacial selectivity.
- Schuster, Christian,Knollmueller, Max,Gaertner, Peter
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p. 3211 - 3223
(2007/10/03)
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- Asymmetric synthesis of β-hydroxy acid via stereoselective dirhodium(II)-catalyzed C-H insertion of α-alkoxydiazoketone
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A new methodology for the asymmetric synthesis of β-hydroxy acid was developed. Dirhodium(II)-catalyzed C-H insertion of α-alkoxydiazoketone (3), which was prepared from primary alkyl halide (1) and readily available chiral α-hydroxy acid (2), gave stereo
- Yakura, Takayuki,Tanaka, Takeshi,Ikeda, Masazumi,Uenishi, Jun'ichi
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p. 471 - 473
(2007/10/03)
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- Selective reductions. 59. Effective intramolecular asymmetric reductions of α-, β-, and γ-keto acids with diisopinocampheylborane and intermolecular asymmetric reductions of the corresponding esters with B-chlorodiisopinocampheylborane
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A comparison of the stereochemistry of the products obtained from the intramolecular asymmetric reduction of a series of keto acids with (-)-diisopinocampheylborane and intermolecular asymmetric reduction of the corresponding series of keto esters with (-)-B-chlorodiisopinocampheylborane ((-)-DIP-Chloride) has been made. The stereochemistry of the hydroxy acids from the reduction of keto acids is dependent only on the enantiomer of the reagent used. The stereochemistry of the products from the reduction of keto esters is also consistent, except those of aliphatic α-keto esters. α-, β-, and γ-keto acids provide the corresponding hydroxy acids in 77-98% ee, and the α- and γ-keto esters afford the hydroxy esters in 82-≥99% ee. β-Keto esters do not undergo reduction. Although the reduction of δ-keto acids does not proceed under the same reaction conditions, the reduction of δ-keto esters is facile. All of the products from the reduction of γ-keto acids and esters and δ-keto esters were converted to the corresponding lactones. This study revealed that DIP-Chloride is an efficient reagent for the reduction of α-keto esters at low temperatures.
- Ramachandran, P. Veeraraghavan,Pitre, Sangeeta,Brown, Herbert C.
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p. 5315 - 5319
(2007/10/03)
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- Chemoenzymatic synthesis2 of both enantiomers of fluoxetine, tomoxetine and nisoxetine: Lipase-catalyzed resolution of 3-aryl-3-hydroxypropanenitriles
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A facile preparation of (±)-3-hydroxy-3-phenylpropanenitrile has been carried out by ring-opening of styrene oxide with NaCN in aqueous ethanol. Subsequent kinetic resolution of this material via lipase-mediated transesterification gave the S-alcohol and R-acetate in excellent yields and high enantioselectivities, particularly with lipase PS-C 'Amano' II. The effect of solvents and immobilization of the lipase has also been investigated. It is interesting to note that the use of immobilized lipase for this transesterification process in hydrophobic solvents (diisopropyl ether, toluene and hexane) enhanced the reaction rate drastically and gave optimal yields with high enantioselectivity (>99%). Moreover, enantiopure 3-hydroxy-3-phenylpropanenitrile products have been converted via enantioconvergent routes into the (R)- and (S)-enantiomers of the important anti-depressants fluoxetine, tomoxetine, nisoxetine and norfluoxetine.
- Kamal, Ahmed,Khanna,Ramu
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p. 2039 - 2051
(2007/10/03)
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- Efficient Lipase-Catalyzed Kinetic Resolution and Dynamic Kinetic Resolution of β-Hydroxy Nitriles. Correction of Absolute Configuration and Transformation to Chiral β-Hydroxy Acids and γ-Amino Alcohols
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Chemoenzymatic dynamic kinetic resolution of β-hydroxy nitriles 1 has been carried out using Candida antarctica lipase B and a ruthenium catalyst. The use of a hydrogen source to depress ketone formation in the dynamic kinetic resolution yields the corresponding acetates 2 in good yield and high enantioselectivity. It is shown that the ruthenium catalyst and the enzyme can be recycled when used in separate reactions. We also report on the preparation of various enantiomerically pure β-hydroxy acid derivatives and γ-amino alcohols from 1 and 2. The latter compounds were also used to establish the correct absolute configuration of 1 and 2.
- Pamies, Oscar,Baeckvall, Jan-E.
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p. 947 - 952
(2007/10/03)
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- Asymmetric Diels-Alder, Michael, and aldol reactions using a planar chiral 1,3-oxazol-2(3H)-one derived from (R)-(+)-4-hydroxy-[2.2]paracyclophane
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(+)-(R)-[2.2]Paracyclophane[4,5-d]-1,3-oxazol-2(3H)-one exhibiting planar chirality has been used as a chiral auxiliary in asymmetric Diels-Alder, Michael, and aldol reactions of α,β-unsaturated carboxy and enolate imides, respectively. The endo-exo- and face-diastereoselectivity is good and is controlled by the spatial relationship between the prochiral center and the C9-C10 ethylene bridge of the [2.2]paracyclophane moiety. The chiral auxiliary is easily removed and quantitatively recovered.
- Cipiciani, Antonio,Fringuelli, Francesco,Piermatti, Oriana,Pizzo, Ferdinando,Ruzziconi, Renzo
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p. 2665 - 2670
(2007/10/03)
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- Ultrasound in enzymatic resolution of ethyl 3-hydroxy-3-phenylpropanoate
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The enzymatic hydrolysis of ethyl-3-hydroxy-3-phenylpropanoate using ultrasound bath and PCL, PLE and CRL enzymes was studied. The application of ultrasound bath led to an appreciative decrease in the reaction time of enzymatic hydrolysis without a signif
- Ribeiro, Carlos Magno R.,Passaroto, Elisa N.,Brenelli, Eugênia C.S.
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p. 6477 - 6479
(2007/10/03)
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- Efficient intramolecular asymmetric reductions of α-, β-, and γ-keto acids with diisopinocampheylborane1
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(equation presented) α-, β-, and γ-Keto acids are reduced with diisopinocampheylborane at room temperature to the corresponding hydroxy acids with predictable stereochemistry in very high ee. The γ-hydroxy acids produced were conveniently cyclized to the corresponding lactones. This provides a simple synthesis of 4-hexanolide, a component of the pheromone secreted by the female dermestid beetle Trogoderma glabrum.
- Veeraraghavan Ramachandran,Brown, Herbert C.,Pitre, Sangeeta
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- Asymmetric Samarium-Reformatsky Reaction of Chiral α-Bromoacetyl-2-oxazolidinones with Aldehydes
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The samarium(II) iodide mediated asymmetric Reformatsky-type reaction of chiral 3-bromoacetyl-2-oxazolidinones with various aldehydes was studied. A series of chiral 4-substituted 2-oxazolidinones 1-3 and 5,5-disubstituted "SuperQuat" oxazolidinones 4-5 were employed as chiral auxiliaries of the α-bromoacetic acid. The reaction of 1 with various aldehydes gave the α-unbranched β-hydroxy carboximides in good yields with high diastereomeric excess values (up to >99% de). The majority of the reaction product derived from 5,5-diphenyl SuperQuat 5 were highly crystallinity; a single recrystallization yielding a diastereomerically pure product with the other diastereomer not detectable by spectroscopic methods. The absolute configurations of the β-hydroxy carboximides were determined by signs of optical rotations of the corresponding known ethyl esters referring to the literature values. Hydrolytic cleavage of the appended of β-hydroxy moieties from the auxiliary SuperQuats was readily achieved under mild conditions using lithium hydroxide; the corresponding carboxylic acids and the returned SuperQuats were obtained in good yields without any evidence of racemization. The first step of the reaction is the reduction of the α-bromo group to produce the samarium enolate, which adds to an aldehyde. The absolute configuration of the adduct (7i) derive from benzaldehyde was found to be R, with the samarium enolate favoring the transition state predicted from chelation control of the reagent; this is in analogy to the discussion that has been used for the corresponding titanium enolate. The stereochemistry of the reaction may be explained by incorporating the Nerz-Stormes-Thornton chair transition structure model.
- Fukuzawa, Shin-Ichi,Matsuzawa, Hiroshi,Yoshimitsu, Shin-Ichi
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p. 1702 - 1706
(2007/10/03)
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- Enantioselective synthesis of β-hydroxy carboxylic acids: Direct conversion of β-oxocarboxylic acids to enantiomerically enriched β-hydroxy carboxylic acids via neighboring group control
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β-Oxocarboxylic acids can be reduced to the corresponding β-hydroxy carboxylic acids employing DIP-Cl(TM) as a reducing agent. The β-carboxylic substituent exerts a remarkable neighboring group effect on the reduction. The reaction presumably proceeds in an intramolecular fashion through a 'rigid' bicyclic transition state assembly, which produces enantioselectivities approaching 99%.
- Wang, Zhe,Zhao, Chunlin,Pierce, Michael E.,Fortunak, Joseph M.
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p. 225 - 228
(2007/10/03)
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- The ene reaction of 3-methylene-2,3-dihydrofuran with aldehydes
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The Lewis acid-promoted ene reaction of 3-methylene-2,3-dihydrofuran, 1, with aldehydes gives the corresponding alcohols in good to excellent yield.
- Miles, William H.,Berreth, Christina L.,Anderton, Christian A.
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p. 7893 - 7896
(2007/10/03)
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- New chiral acetate imide enolate for stereoselective aldol reactions
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The chiral imide acetate 4 reacts upon lithium and titanium enolate formation conditions with aldehydes in a stereoselective manner. Remarkably, aldols obtained from aromatic and α,β-unsaturated aldehydes exhibited opposite stereochemistry than those obtained from aliphatic aldehydes.
- Palomo, Claudio,Oiarbide, Mikel,Gonzalez, Alberto,Garcia, Jesus M.,Berree, Fabienne,Linden, Anthony
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p. 6931 - 6934
(2007/10/03)
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- THE ALDOL-TYPE CONDENSATION OF LITHIUM, BORON AND TITANIUM ENOLATES OF ISOCARANYL ACETATES WITH BENZALDEHYDE. THE METAL AND CHIRAL AUXILIARY EFFECTS
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A new approach to chiral enolates by using a chiral ester (isocaranyl acetates) and a chiral organometallic reagent (boranes and titanates) is reported.The effect of this double enantiodifferentiation and the effects of the isomerism of chiral auxiliary (2- and 4-isocaranyl) and that of the metal (Li, B, Ti) on the enantioselectivity of aldol condensation of 2- and 4-isocaranyl acetates with benzaldehyde are discussed.Lithium and boron enolates of isocaranyl acetates add preferentially to the si-face of benzaldehyde while the addition of analogous titanium enolates occurs preferably on the re-face of the aldehyde.The enolborinate which gives the highest ee value is that obtained by transmetallation reaction of the lithium enolate of (+)-2-isocaranyl acetate with (-)-di-4-isocaranylchloroborane.Obversely the enoltitanate prepared from (-)-4-isocaranyl acetate and the LDA-(+)-di-2-isocaranoxy(chloro)cyclopentadienyl titanate system is that which gives the highest enantiofacial discrimination.The chiral auxiliary effect has been used to plan the synthesis of (+)-4α-acetoxy-3β-trimethylsilyloxy-trans-carane, the titanium enolate of which gives (R)-(+)-β-hydroxy-β-phenylpropionic acid in high enantiomeric excess.
- Fringuelli, Francesco,Piermatti, Oriana,Pizzo, Ferdinando,Scappini, Anna Maria
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p. 195 - 200
(2007/10/02)
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- Enantioselective Aldol Type Reactions of Acetate Titanium Enolate with α,β-Unsaturated Aldehyde-TiCl4 Complex
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The camphor-derived N-acetyloxazolidinethione has been used to effect enantioselective aldol type reaction of the derived thioimide titanium enolate with unsaturated aldehyde-TiCl4 complex.The observed facial selectivity is best explained by an open trans
- Yan, Tu-Hsin,Hung, An-Wei,Lee, Hui-Chun,Liu, Wen-Hung,Chang, Chii-Shin
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p. 691 - 700
(2007/10/03)
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- Asymmetric Aldol Type Reactions of Acetate Imide Enolates
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Titanium-mediated chelation-controlled aldol type reactions of acetate thioimide with representative aldehydes proceed with high ?-facial differentiation.An investigation of the influence of the nature of the imide enolate ligands and the Lewis acids in t
- Yan, Tu-Hsin,Hung, An-Wei,Lee, Hui-Chun,Chang, Chii-Shin,Liu, Wen-Hung
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p. 3301 - 3306
(2007/10/02)
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- Hetero Diels-Alder reaction involving (E)-3-(tert-butyldimethylsiloxy)-1-(2',3',4',6'-tetra-O-acetyl -β-D-gl ucopyranosyloxy)buta-1,3-diene and p-nitrobenzaldehyde
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The title reaction, when carried out in carbon tetrachloride in the presence of a catalytic amount of Eu(fod)3, leads mainly to the cis-cycloadduct 6a; the product undergoes isomerisation to its epimer 7a in the presence of the catalyst in dich
- Lowe,Stoodley
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p. 6351 - 6354
(2007/10/02)
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- The catalytic asymmetric aldol reaction of silyl ketene acetals with aldehydes in the presence of a chiral borane complex. Nitroethane as a highly effective solvent for catalytic conditions
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Catalytic asymmetric aldol reactions of silyl ketene acetals with aldehydes in the presence of 20 mol % of the chiral borane reagent, prepared in situ from the p-nitrobenzenesulfonamide of (S)-valine and BH3-THF complex, gave β-hydroxy carboxylic acid esters in good chemical yields along with excellent enantioselectivity under conditions that employ nitroethane as an effective solvent.
- Kiyooka, Syun-Ichi,Kaneko, Yuichi,Kume, Ken-Ichi
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p. 4927 - 4930
(2007/10/02)
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- Stereoselective Aldol Reactions with (R)- and (S)-2-Hydroxy-1,2,2-triphenylethyl Acetate and Related Glycol Monoacetates
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The enolate 7a, formed by double deprotonation of the ester 5a, is added to aldehydes.The influences of the enolate gegenion, of the solvent, and of the reaction temperature on the ratio of the isomeric products 9:10 are studied.The highest degrees of dia
- Devant, Ralf,Mahler, Ulrike,Braun, Manfred
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p. 397 - 406
(2007/10/02)
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- Stereoselective Aldol Reaction with Chiral Secondary Acetamides
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The deprotonated acetamides 4a - c and 5a - c are added to prochiral carbonyl compounds.The influence of the solvent, of the reaction temperature, and of the enolate gegenion on the ratio of the isomeric products 8/9, 18/19, and 26/27, respectively, are studied.The highest degree of diastereoselectivity are observed, when the titanium enolate of the acetamide 4a or the threefold deprotonated N-acetyl-α-phenylglycinol (5a) is used.The diastereomers 18a - d, formed in excess in the addition of 5a to aldehydes, are isolated in pure from by a single recrystallization, and afford the enantiomerically pure β-hydroxy carboxylic acids 3a - d.Thereby, the chiral auxiliary, α-phenylglycinol (14), is recovered.
- Devant, Ralf,Braun, Manfred
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p. 2191 - 2207
(2007/10/02)
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- ASYMMETRIC ALDOL REACTION OF AMIDE ENOLATES BEARING trans-2,5-DISUBSTITED PYRROLIDINES AS CHIRAL AUXILIARIES
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Condensation of aldehydes with Zr enolate of chiral N-propionyl-trans-2,5-disubstituted pyrrolidine proceeded with high diastereo- and diastereofaceselection.Stereoselectivity in the reaction of the corresponding N-acetyl compounds was also examined.
- Katsuki, Tsutomu,Yamaguchi, Masaru
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p. 5807 - 5810
(2007/10/02)
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- CHIRAL ACETATE ENOLATE EQUIVALENT FOR THE SYNTHESIS OF β-HYDROXY ACIDS AND ESTERS: X-RAY CRYSTAL STRUCTURE OF RR,SS-5-C5H5)Fe(CO)(PPh3)(COCH2CH(OH)CH2CH3)>
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The aluminium anolate derived from the iron acetyl complex 5-C5H5)Fe(CO)(PPh3)COCH3>, in contrast to the lithium enolate, undergoes highly strereoselective aldol reactions with aldehydes to generate RR,SS-β-hydroxyacyl complexes which on decomlexation liberate β-hydroxy acids or esters.Determination of the molecular structure of RR,SS-5-C5H5)Fe(CO)(PPh3)(COCH2CH(OH)CH2CH3> allowed assignment of the relative configuration of the new chiral centre.
- Davies, Stephen G.,Dordor-Hedgecock, Isabelle M.,Warner, Peter
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p. 213 - 224
(2007/10/02)
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- (R)- AND (S)-2-ACETOXY-1,1,2-TRIPHENYLETHANOL - EFFECTIVE SYNTHETIC EQUIVALENTS OF A CHIRAL ACETATE ENOLATE
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The enolate 3, easily available by double deprotonation of (R)-2-acetoxy-1,1,2-triphenylethanol (5), adds in a highly stereoselective manner to aldehides.Hydrolysis of the adducts 6/7 affods the acids 2.
- Braun, Manfred,Devant, Ralf
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p. 5031 - 5034
(2007/10/02)
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- Resolution of Conglomerates with the Assistance of Tailor-made Impurities. Generality and Mechanistic Aspects of the "Rule of Reversal". A New Method for Assignment of Absolute Cofiguration
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A new general and efficient method for kinetic resolution of racemic conglomerates by crystallization in the presence of "tailor-made" additives is described.The process is explained in terms of stereoselective adsorption of the resolved additive at the surface of the growing crystals of the enantiomer of the same absolute configuration, resulting in a drastic decrease in their rate of growth and thus allowing preferential crystallization of the opposite enantiomer ("rule of reversal").Some empirical resolutions reported in the literature are rationalized through this mechanism, and appropriate additives for the resolution of new systems are designed and successfully applied.The crystallization of the conglomerates (R,S)-glutamic acid hydrochloride (Glu*HCl), (R,S)-threonine (Thr), (R,S)-(p-hydroxyphenyl)glycine p-toluenesulfonate (pHpgpTs), and (R,S)-asparagine hydrate (Asn*H2O) in the presence of other amino acids, used as additives, has been studied in particular.It is demonstrated that the additives are occluded in the bulk of the homochiral crystal in typical amounts of 0.5-1.5percent, while they are not found in the bulk of the crystals of the antipode.The possible role of the additives in nucleation and dissolution of the affected crystals is considered.A new method for the assignment of absolute configuration of chiral molecules is proposed.
- Addadi, L.,Weinstein, S.,Gati, E.,Weissbuch, I.,Lahav, M.
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p. 4610 - 4617
(2007/10/02)
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- SYNTHESES ASYMETRIQUES DE β-HYDROXYACIDES PAR CONDENSATION D'ANIONS ENOLATES D'ESTERS α-SULFINYLE CHIRAUX SUR DES COMPOSES CARBONYLES
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A stereospecific synthesis of (R)-(+)-t-butyl p-tolylsulfinyl acetate 2 is described.The aldol-type condensation of this reagent leads to β-hydroxyacids in good chemical and optical yields.The determination of the absolute configuration of the condensatio
- Mioskowski, Charles,Solladie, Guy
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p. 227 - 236
(2007/10/02)
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