- Et2Zn-mediated rearrangement of bromohydrins
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(Chemical Equation Presented) A simple and highly efficient method for the rearrangement of bromohydrins mediated by Et2Zn to synthesize carbonyl compounds was described. Various β-bromo alcohols were treated with 0.6 equiv of Et2Zn to form a zinc complex in CH 2Cl2 at room temperature for 2 h, followed by 1,2-migration to give the corresponding carbonyl compounds. This remarkable and clean rearrangement is general for acyclic and cyclic bromohydrins, and a variety of ring-expansive and -contractive carbonyl compounds were obtained in good to excellent yields according to the feature of the starting bromohydrins. The functional group tolerance of organozinc reagents in this reaction will be useful in organic synthesis. The scope and limitations of this rearrangement process were also investigated.
- Li, Lezhen,Cai, Peijie,Guo, Qingxiang,Xue, Song
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p. 3516 - 3522
(2008/09/20)
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- A simple and efficient method for regioselective and stereoselective synthesis of vicinal bromohydrins and alkoxybromides from an olefin
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(Chemical Equation Presented) A very rapid and efficient method has been developed for the synthesis of vicinal bromohydrins and alkoxybromides directly from an olefin without any catalyst. The reaction was performed in CH 3CN-water (4:1) or alcohol using N,N-dibromo-p-toluenesulfonamide (TsNBr2) as the brominating agent. Excellent yields and regio- and stereoselectivities have been obtained. Bromohydrins are formed instantaneously, whereas formation of alkoxybromides takes 30-60 min.
- Phukan, Prodeep,Chakraborty, Pranita,Kataki, Dolly
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p. 7533 - 7537
(2007/10/03)
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- Stereoselective chemoenzymatic process for the preparation of optically enriched phenylglycidates as precursors of taxol side chain
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The present invention relates to a novel and efficient chemoenzymatic process of preparation of optically active trans alkyl phenylglycidates. The invention particularly discloses a novel process for the chemoenzymatic synthesis of two enantiomers of tran
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- Chemoenzymatic approach to optically active phenylglycidates: Resolution of bromo- and iodohydrins
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Enantiomerically enriched phenylglycidates, precursors of the taxol C-13 phenylisoserine side chain and diltiazem, were prepared by kinetic resolution of anti-2-bromo-3-hydroxy- and anti-3-hydroxy-2-iodophenylpropanoates to provide enantioriched (2R,3R)- and (2S,3S)-halohydrins. The bulkiness and inflexibility of bromo and iodo groups in halohydrins have made them inaccessible to the active site of most of the lipases utilized for the hydrolysis of their acyloxy derivatives. In a set of 22 hydrolases screened herein, including native as well as commercial enzymes, only Aspergillus niger (Lipase AS, AMANO) could catalyze the hydrolysis with high enantioselectivity (E = 176). The resolved halohydrins easily underwent transformation to the corresponding (2S,3R)- and (2R,3S)-phenylglycidates.
- Anand, Naveen,Kapoor, Munish,Koul, Surrinder,Taneja, Subhash C.,Sharma, Rattan L.,Qazi, Ghulam N.
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p. 3131 - 3138
(2007/10/03)
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- 1,2-asymmetric induction in radical reactions. Deuteration and allylation reactions of β-oxy-α-bromo esters
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Chiral radicals were generated by halogen abstraction reactions of β-oxy-α-bromo esters and their asymmetric deuteration and allylation reactions were studied.
- Curran,Ramamoorthy
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p. 4841 - 4858
(2007/10/02)
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- CHEMO-ENZYMATIC SYNTHESIS OF ALL ISOMERIC 3-PHENYLSERINES AND -ISOSERINES
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The synthesis of all isomers of 3-phenylserines and 3-phenylisoserines in enantiomerically pure form is presented.Diastereomerically pure educts (threo/erythro-2-azido-3-butanoyloxy-3-phenyl-propionic esters, threo/erythro-3-azido-2-butanoyloxy-3-phenylpropionic esters, threo-2-butanoylamino-3-butanoyloxy-3-phenylpropionic ester, erythro-3-butanoylamino-2-butanoyloxy-3-phenyl-propionamide) were prepared from cinnamic acid derivatives or via aldol condensations of benzaldehyde and suitable enolates in few steps.These racemates were resolved with lipases from Candida cylindracea (CC) and Pseudomonas fluorescens (P) and the obtained products were hydrogenated to 3-phenylserines and -isoserines.The influence of the acyl group in the enzymatic resolution of erythro-3-azido-2-acyloxy-3-phenylpropionic esters was investigated.
- Hoenig, H.,Seufer-Wasserthal, P.,Weber, H.
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p. 3841 - 3850
(2007/10/02)
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- α-Bromo-, α-chloro-, and α-trimethylsilyl-zinc ester enolates. New and universal Reformatsky-type Darzens and Peterson reactions
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Low temperature reactions of ethyl dihaloacetates with carbonyl compounds in the presence of zinc/silver-graphite give α-halo-β-hydroxyalkanoates, which are readily converted into glycidates by base.Unser the same conditions ethyl(bromo)(trimethylsilyl) acetate with aldehydes and ketones forms α,β-unsaturated esters by a Reformatsky-Peterson reaction combination.
- Fuerstner, Alois
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p. C33 - C36
(2007/10/02)
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