- [2.2]Paracyclophane-derived N-acyloxazol-2(3H)-one as a suitable planar chiral auxiliary for the enantioselective synthesis of β-hydroxy acids
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The synthesis of optically active (+)-(R)-[2.2]paracyclophano[4,5-d]-oxazol-2(3H)-one is described. The aldol reaction of N-acyl derivatives of this planar chiral auxiliary with beazaldehyde occurs with good diastereo- and enantioselectivity.
- Fringuelli, Francesco,Piermatti, Oriana,Pizzo, Ferdinando,Ruzziconi, Renzo
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Read Online
- 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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Read Online
- 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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Read Online
- Siloxy Esters as Traceless Activators of Carboxylic Acids: Boron-Catalyzed Chemoselective Asymmetric Aldol Reaction**
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The catalytic asymmetric aldol reaction is among the most useful reactions in organic synthesis. Despite the existence of many prominent reports, however, the late-stage, chemoselective, catalytic, asymmetric aldol reaction of multifunctional substrates is still difficult to achieve. Herein, we identified that in situ pre-conversion of carboxylic acids to siloxy esters facilitated the boron-catalyzed direct aldol reaction, leading to the development of carboxylic acid-selective, catalytic, asymmetric aldol reaction applicable to multifunctional substrates. Combining experimental and computational studies rationalized the reaction mechanism and led to the proposal of Si/B enediolates as the active species. The silyl ester formation facilitated both enolization and catalyst turnover by acidifying the α-proton of substrates and attenuating poisonous Lewis bases to the boron catalyst.
- Fujita, Taiki,Kanai, Motomu,Mitsunuma, Harunobu,Sameera, W. M. C.,Yamane, Mina
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supporting information
p. 24598 - 24604
(2021/10/14)
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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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- 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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- Heterogeneous versus homogeneous copper(II) catalysis in enantioselective conjugate-addition reactions of boron in water
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We have developed CuII-catalyzed enantioselective conjugate-addition reactions of boron to α,β-unsaturated carbonyl compounds and α,β,γ,δ-unsaturated carbonyl compounds in water. In contrast to the previously reported CuI catalysis that required organic solvents, chiral CuII catalysis was found to proceed efficiently in water. Three catalyst systems have been exploited: cat. 1: Cu(OH)2 with chiral ligand L1; cat. 2: Cu(OH)2 and acetic acid with ligand L1; and cat. 3: Cu(OAc)2 with ligand L1. Whereas cat. 1 is a heterogeneous system, cat. 2 and cat. 3 are homogeneous systems. We tested 27 α,β-unsaturated carbonyl compounds and an α,β-unsaturated nitrile compound, including acyclic and cyclic α,β-unsaturated ketones, acyclic and cyclic β,β- disubstituted enones, acyclic and cyclic α,β-unsaturated esters (including their β,β-disubstituted forms), and acyclic α,β-unsaturated amides (including their β,β-disubstituted forms). We found that cat. 2 and cat. 3 showed high yields and enantioselectivities for almost all substrates. Notably, no catalysts that can tolerate all of these substrates with high yields and high enantioselectivities have been reported for the conjugate addition of boron. Heterogeneous cat. 1 also gave high yields and enantioselectivities with some substrates and also gave the highest TOF (43 200 h-1) for an asymmetric conjugate-addition reaction of boron. In addition, the catalyst systems were also applicable to the conjugate addition of boron to α,β,γ, δ-unsaturated carbonyl compounds, although such reactions have previously been very limited in the literature, even in organic solvents. 1,4-Addition products were obtained in high yields and enantioselectivities in the reactions of acyclic α,β,γ,δ-unsaturated carbonyl compounds with diboron 2 by using cat. 1, cat. 2, or cat. 3. On the other hand, in the reactions of cyclic α,β,γ,δ-unsaturated carbonyl compounds with compound 2, whereas 1,4-addition products were exclusively obtained by using cat. 2 or cat. 3, 1,6-addition products were exclusively produced by using cat. 1. Similar unique reactivities and selectivities were also shown in the reactions of cyclic trienones. Finally, the reaction mechanisms of these unique conjugate-addition reactions in water were investigated and we propose stereochemical models that are supported by X-ray crystallography and MS (ESI) analysis. Although the role of water has not been completely revealed, water is expected to be effective in the activation of a borylcopper(II) intermediate and a protonation event subsequent to the nucleophilic addition step, thereby leading to overwhelmingly high catalytic turnover. Copyright
- Kitanosono, Taku,Xu, Pengyu,Kobayashi, Shu
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supporting information
p. 179 - 188
(2014/01/06)
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- Stereoselective synthesis of (S)-dapoxetine: A chiral auxiliary mediated approach
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An imidazolidin-2-one chiral auxiliary mediated acetate aldol reaction was explored in the enantioselective synthesis of (S)-dapoxetine (SSRI). The diastereoselective aldol adduct was transformed to highly enantiopure (S)-dapoxetine with overall good yield.
- Khatik, Gopal L.,Sharma, Ratnesh,Kumar, Varun,Chouhan, Mangilal,Nair, Vipin A.
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p. 5991 - 5993
(2013/10/22)
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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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supporting information; experimental part
p. 2442 - 2445
(2012/07/03)
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- Asymmetric induction by (S)-4-isopropyl-1-phenylimidazolidin-2-thione in titanium-mediated aldol reactions and its application in enantioselective synthesis of (R)-baclofen
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The usefulness of (S)-4-isopropyl-1-phenylimidazolidin-2-thione as a chiral auxiliary in stereoselective propionate and acetate aldol reactions is discussed. Further, the enantioselective synthesis of (R)-baclofen by acetate aldol reaction using the chiral auxiliary was demonstrated. Georg Thieme Verlag Stuttgart New York.
- Khatik, Gopal L.,Khurana, Ravi,Kumar, Varun,Nair, Vipin A.
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experimental part
p. 3123 - 3132
(2011/10/31)
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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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- 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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- Asymmetric reduction of β-ketonitriles with a recombinant carbonyl reductase and enzymatic transformation to optically pure β-hydroxy carboxylic acids
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α-Ethylation is concomitant with the reduction of aromatic β-ketonitriles catalyzed by whole-cell biocatalysts. Use of isolated carbonyl reductase has completely eliminated this competing reaction. (R)-β-Hydroxy nitriles were obtained via a reduction cata
- Zhu, Dunming,Ankati, Haribabu,Mukherjee, Chandrani,Yang, Van,Biehl, Edward R.,Hua, Ling
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p. 2560 - 2563
(2008/02/05)
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- (1S,2S)-1-Amino-2-hydroxy-1,2,3,4-tetrahydronaphthalene: A new chiral auxiliary for asymmetric Reformatsky reactions
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(1S,2S)-1-Amino-2-hydroxy-1,2,3,4-tetrahydronaphthalene, synthesized via a chemoenzymatic approach from naphthalene, has been successfully used as a chiral auxiliary in Reformatsky-type reactions between the corresponding α-bromoacyloxazolidinone and carbonyl compounds.
- Orsini, Fulvia,Sello, Guido,Manzo, Angelo Maria,Lucci, Elvira Maria
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p. 1913 - 1918
(2007/10/03)
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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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- Stereoselective access to γ-nitro carboxylates, precursors for highly functionalized γ-lactams
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A straightforward synthesis of the enantiomerically pure nitro derivatives 31 and epi-32, which are particularly useful intermediates for the synthesis of highly functionalized γ-lactams, is presented. (+)-(R)-3-Hydroxy-3-phenylpropanoic acid (20) and its ethyl ester 25 were prepared from (+)-L-mandelic acid (21). Condensation of 20 with pivalaldehyde furnished the novel enantiomerically pure 1,3-dioxan-4-one 17, the absolute configuration of which was established by X-ray crystal-structure analysis. Treating the lithium enolate of 17 with the nitro alkene 18 led, in a Michael-type addition, to a 1:1 mixture of two diastereoisomeric products. The stereocontrol of the addition was limited to the novel stereogenic center next to the lactone function. When the lithium enolate of 25 was treated with 18, the same selectivity was observed but with a lower chemical yield. Very facile separation of the isomers was achieved later in the synthetic sequence, when one isomer cyclized selectively to the nitro lactone 31, while the other one was isolated as hydroxy ester epi-32. The relative configuration of racemic epi-32 could be established by X-ray crystal-structure analysis.
- Meisterhans, Christian,Linden, Anthony,Hesse, Manfred
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p. 644 - 656
(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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- 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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- 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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- Diastereoselective allylation of aldehydes with an enantiopure 2-sulfinylallyl halide under environmentally friendly barbier-type conditions
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(Formula presented) A simple, efficient, and diastereoselective zinc-promoted allylation of aldehydes with enantiopure (SS)-3-chloro-2-(p-tolylsulfinyl)-1-propene [(SS)-1] under aqueous Barbier conditions is described. The observed d
- Marquez, Francesc,Llebaria, Amadeu,Delgado, Antonio
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p. 547 - 549
(2007/10/03)
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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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- Highly diastereoselective aldol reactions with camphor-based acetate enolate equivalents
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New lithium enolates of α-hydroxy ketones, derived from camphor, are evaluated for asymmetric aldol reactions in the presence of lithium chloride. The diastereoselectivity of the reactions between the lithium enolate of 3 and a variety of achiral aldehydes is strongly influenced by the lithium chloride salt. In these instances, the achieved levels of asymmetric induction, typically 95:5 dr, are in the range of those attained in aldol reactions involving the lithium enolate of the methyl ketone 4, which is sterically more demanding. The resulting aldol adducts are easily transformed into β-hydroxy carboxylic acids, ketones, and aldehydes with concomitant recovery of the camphor, the chiral controller of the process, which can be reused.
- Palomo, Claudio,Oiarbide, Mikel,Aizpurua, Jesus M.,Gonzalez, Alberto,Garcia, Jesus M.,Landa, Cristina,Odriozola, Ibon,Linden, Anthony
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p. 8193 - 8200
(2007/10/03)
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- D-xylose derived oxazolidin-2-ones as chiral auxiliaries in stereoselective aldol reactions
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Chiral N-acylated oxazolidin-2-one derivatives from D-xylose have been shown to undergo diastereoselective aldol reactions via their lithium imide enolates to afford β-hydroxylated products. Aliphatic substrates and aldehydes were shown to yield 'non-Evan
- Luetzen, Arne,Koell, Peter
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p. 1193 - 1206
(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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- Diastereoselectivity in the Preparation of β-Silyl Esters from αβ-Unsaturated Esters and Amides Attachhed to Chiral Auxiliaries
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The conjugate addition of the phenyldimethylsilyl-cuprate reagent to cinnamate and crotonate esters and amides 1 of various known chiral auxiliaries, a-e, is diastereoselective.The sense of diastereoselectivity of silyl-cuprate addition to the esters 1b-d, 8 and 9 is different from established precedent based on carbon-cuprates, but is normal for silyl-cuprate addition to the amide 1a, the imines 1e and 21, and the oxazolidine 6.The chiral auxiliary e gives the best results of those tested, and the silicon-containing group can be removed from the chiral auxiliary using alkoxide ion in aprotic media, making available β-silyl esters 27-29 of high enantiomeric excess, with recovery of the chiral auxiliary 30.
- Fleming, Ian,Kindon, Nicolas D.
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p. 303 - 316
(2007/10/02)
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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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- 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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- 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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- Use of 1,3-dioxin-4-ones and related compounds in synthesis. 46. 2-(l-menthyloxycarbonyl)-2,6-dimethyl-1,3-dioxin-4-one and related compounds. Relationship between facial selectivity and pyramidalization
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Preparation and ground-state reactions (conjugate addition and catalytic hydrogenation) of a 1,3-dioxin-4-one having an l-menthyloxycarbonyl group at the 2-position as the only substituent (7a) and its 6-methyl derivative (8a), as well as their 2-methyl derivatives (5a and 6a), are reported. The X-ray structure of the 2-(methoxycarbonyl)-2-phenyl derivative (4b) was also determined. The result, when combined with previously reported X-ray crystallographic analyses on related dioxinones, suggests that all of the dioxinones take a sofa conformation of the six-membered ring, whose top face is always on the same side as the pyramidalization of the enone function in the ring. Conjugate addition reaction of Sa leads to exclusive addition on the bottom face, while addition to 7a gives the product corresponding to the top face addition. Catalytic hydrogenation of 6a and 8a is also reported. Though the selectivity is much lowered as compared with the conjugate addition, the same facial selectivities are again observed. A comparison with the exclusive top face attack on 2-tert-butyl-1,3-dioxin-4-one (1) and its 2-methyl derivative 2 and bottom face attack on 2-(l-menthyloxycarbonyl)-2-phenyl-1,3-dioxin-4-one (3a) makes it clear that the prediction of facial selectivity in these ground-state reactions (conjugate addition and catalytic hydrogenation) based on pyramidalization is not always correct. A novel hypothesis which accounts for all of the above results (sofa conformation of the hetero ring, pyramidalization at the enone portion, and facial selectivity) is presented.
- Sato, Masayuki,Murakami, Masayuki,Sunami, Satoshi,Kaneko, Chikara,Furuya, Toshio,Kurihara, Hiroyuki
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p. 4279 - 4287
(2007/10/02)
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- An Unusual Enantioselective Aldol Type Reaction of Acetate Boryl Enolate Derived from Chiral Thioimide
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The camphor-derived N-acetyloxazolidinethione has been used to effect enantioselective aldol type reactions of the derived 9-BBN enolate with a variety of aldehydes.Mechanistically, the observed facial selectivity is best explained by a boatlike transitio
- Yan, Tu-Hsin,Hung, An-Wei,Lee, Hui-Chun,Chang, Chii-Shin
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p. 8187 - 8191
(2007/10/02)
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- Catalytic Asymmetric Aldol-Type Reactions Using a Chiral (Acyloxy)borane Complex
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In the presence of 20 molpercent of a chiral (acyloxy)borane (CAB) complex prepared from BH3*THF and a chiral mono-O-acylated tartaric acid, achiral silyl enol ethers or ketene silyl acetals react with achiral aldehydes to afford the corresponding aldol-t
- Ishihara, Kazuaki,Maruyama, Tohru,Mouri, Makoto,Gao, Qingzhi,Furuta,Kyoji,Yamamoto, Hisashi
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p. 3483 - 3491
(2007/10/02)
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- Stereoselective Titanium-Mediated Aldol Reactions of Menthyl Acetate Enolates, and Allylations with Benzaldehyde
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The enolates of (+)- and (-)-menthyl acetate, formed by transmetallation from lithium with chloro(cyclopentadienyl)bis(1,2:5,6-di-O-isopropylidene-α-D-glucofuranosato-O3)titanium (1), react with benzaldehyde from the Re-face wit
- Cambie, Richard C.,Coddington, Jan M.,Milbank, Jared B. J.,Pausler, Maria G.,Rustenhoven, Job J.,et al.
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p. 583 - 591
(2007/10/02)
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- Optically pure, crystalline 'acetate'-aldols from Nacetylbornane-10,2-sultam
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O-Silylation of N-acetylsultam 2 gives O-silyl-N,O-ketene acetal 3 which undergoes TiCl4 promoted addition to aromatic and aliphatic aldehydes to afford, after flash chromatography, diastereomerically pure, crystalline aldols 5 which are readil
- Oppolzer, Wolfgang,Starkemann, Christian
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p. 2439 - 2442
(2007/10/02)
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- TOWARDS CHIRAL REFORMATSKY REAGENTS
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Chiral β-hydroxyacids are obtained via the Reformatsky reaction of lactic acid derivatives with carbonyl compounds in 11-100percent optical purities.
- Basavaiah, D.,Bharathi, T. K.
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p. 2035 - 2040
(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 and 'anti'-Selective Aldolisations of Acetates and Propionates
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Starting from acetates 1 and propionates 6, TiCl4-mediated addition of their silylketene acetals 2 and 7 to aldehydes gave aldols 4 and 9, respectively, with high ?-face and 'anti' differentiation (Schemes, and Tables 1 and 2).Alternation of the (E/Z)-eno
- Oppolzer, Wolfgang,Marco-Contelles, Jose
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p. 1699 - 1703
(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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- (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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- ENANTIOSELECTIVE ALDOL CONDENSATIONS VIA BORON ENOLATES. A STERIC MODEL FOR ASYMMETRIC INDUCTION.
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Chiral boron enolates have been shown to be effective in stereoregulated aldol condensations.A transition state model is proposed for chirality transfer.
- Evans, D. A.,Taber, T. R.
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p. 4675 - 4678
(2007/10/02)
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