- Magnesium bis(diisopropylamide), a useful reagent for regio- and stereoselective synthesis of kinetic silyl enol ethers
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Less highly substituted silyl enol ethers are regiospecifically prepared in high yield, around room temperature under kinetic conditions, from unsymmetrical cyclic ketones and magnesium bis(diisopropylamide) [(DA)2Mg] in THF/heptane. This high regioselectivity is markedly higher than these reported for bromomagnesium diisopropylamide (DAMgBr); it is also similar to this of LDA/DME at -78°C, but (DA)2Mg can be used at room temperature. In addition, a high E-enolization stereoselectivity is observed for benzylic ketones, reverse of this obtained with LDA.
- Lessene, Guillaume,Tripoli, Regis,Cazeau, Philippe,Biran, Claude,Bordeau, Michel
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Read Online
- Halogenative difluorohomologation of ketones
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A method for the difluorohomologation of ketones accompanied by halogenation of a C-H bond is described. The reaction involves silylation, difluorocarbene addition using Me3SiCF2Br activated by a bromide ion, and halogenation of intermediate cyclopropanes with N-bromo- or N-iodosuccinimide. The whole process is performed without isolation of intermediates. The resulting α,α-difluoro-β-halo-substituted ketones can be readily converted into fluorine containing pyrazole derivatives and oxetanes.
- Fedorov, Oleg V.,Kosobokov, Mikhail D.,Levin, Vitalij V.,Struchkova, Marina I.,Dilman, Alexander D.
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p. 5870 - 5876
(2015/06/16)
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- The reduction of α-silyloxy ketones using phenyldimethylsilyllithium
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Phenyldimethylsilyllithium reacts with acyloin silyl ethers RCH(OSiMe3)COR 8 to give regiodefined silyl enol ethers RCH=C(OSiMe2Ph)R 9, and hence by hydrolysis ketones RCH2COR 10. The yields can be high but are usually moderate. The mechanism of this reduction is established to involve a Brook rearrangement (Scheme 6) rather than a Peterson elimination (Scheme 1). Although the mechanism appears to be the same in each case, the stereochemistries of the silyl enol ethers 9 are opposite in sense in the aromatic series (R = Ph, Scheme 7) and the aliphatic series (R = cyclohexyl, Scheme 8), with the major aromatic silyl enol ether being the thermodynamically less stable isomer E-PhCH=C(OSiMe2Ph)Ph E-9aa, and the major aliphatic silyl enol ether being the thermodynamically more stable isomer Z-c-C6H11CH= C(OSiMe2Ph)-c-C6H11 Z-9ba. This is a consequence of anomalous anti-Felkin attack in the aromatic series. The reaction with the silyl ether ButCH(OSiMe3)COPh 13b is normal in giving Z-ButCH= C(OSiMe2Ph)Ph Z-38 (Scheme 11), but reduction of the silyl ether 8a with lithium aluminium hydride is also anti-Felkin giving with high selectivity the meso diol PhCH(OH)CH(OH)Ph 39. The reaction between Phenyldimethylsilyllithium and the acyloin silyl ether 8d (R = But) does not give the ketone ButCH2COBut, but gives instead the anti-Felkin meso diol ButCHOHCHOHBut 40 also with high selectivity (Scheme 12). Silyllithium and some related reagents react with trifluoromethyl ketones 46 and 48 to give α,α-difluoro silyl enol ethers 47 and 49 (Scheme 14).
- Fleming, Ian,Roberts, Richard S.,Smith, Stephen C.
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p. 1215 - 1228
(2007/10/03)
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- Chemistry of oxaziridines. 14. Asymmetric oxidation of ketone enolates using enantiomerically pure (camphorylsulfonyl)oxaziridine
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The reagent-controlled asymmetric oxidation of tri- and tetrasubstituted ketone enolate anions 4 and 8 by enantiomerically pure (camphorylsulfonyl)oxaziridine 2 has been investigated. The stereoselectivities for oxidation of trisubstituted enolates 4a-d are good to excellent, 60-95% ee, while those for tetrasubstituted enolates 4e and 8 are lower; i.e., 21-30% ee. Isolated chemical yields for both types of enolate anions are good to excellent. The sodium enolate anions of 4a-d, which could be oxidized at -78°C, gave both higher yields and stereoselectivities than the corresponding lithium or zinc enolates, which required warming to higher temperatures for complete oxidation. The presence of HMPA generally had a deleterious effect on the stereoinduction. However, for oxidation of (E)- and (Z)-4d the highest ee's were observed in the presence of this additive. Investigation of the stereoselective trends reveals that the enolate substitution pattern and the enolate solution structure are the most important stereocontrol elements. The role that the enolate geometry has in the stereoinduction is less clear although Z enolates seem to exhibit higher stereoselectivities than the E enolates. The results obtained in this study have been formulated into a mechanistic rational involving an SN2-type substitution of the enolate anion on oxaziridine 2 via an "open" transition state.
- Davis, Franklin A.,Sheppard, Aurelia C.,Chen, Bang-Chi,Haque, M. Serajul
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p. 6679 - 6690
(2007/10/02)
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