- Organometallic reactions in aqueous media - Bismuth-mediated crossed aldol type reactions
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Bismuth metal, upon activation by zinc fluoride, can effect the crossed aldol reaction between α-bromocarbonyl compounds and aldehydes in aqueous media. The reaction was found to be regiospecific and syndiastereoselective.
- Lee, Yoon Joo,Chan, Tak Hang
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p. 1406 - 1412
(2007/10/03)
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- Kinetics and Mechanism of the Ivanov Reaction: Reaction of Aldehydes and Ketones with Phenylacetic Acit Magnesium Enediolate
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Stopped-flow kinetics of the reaction between aldehydes or ketones and the magnesium enediolate of phenylacetic acid in THF are second order at enediolate concentrations -3 M.At concentrations of 10-3 - 3*10-2 M, the formation of a bis(enediolate) requires a more complex kinetic equation.Second-order rate constans are reported for the reaction of the enediolate with a number of aldehydes and ketones at 25 deg C.Entropies of activation for cyclohexanone, benzaldehyde, 2-methylpropanal, and 2,2-dimethylpropanal are positive, and enthalpy-entropy compensation is observed.Ef fects of cycloalkanone ring size and benzaldehyde substituents are small and are ascribable to a transition state with a very small C-C bond order.A two-step mechanism is proposed, with preequilibrium formation of a coordination intermediate which, in some cases, is accompanied by a change in the magnesium coordination number.Effects of alkyl groups on aldehyde and ketone reactivity stem mainly from steric desolvation.
- Toullec, Jean,Mladenova, Margarita,Gaudemar-Bardone, Francoise,Blagoev, Blagoi
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p. 2563 - 2569
(2007/10/02)
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- STEREOSELECTIVE ADDITIONS TO CARBOXYLIC ACID DIANIONS AND β-LACTONE SUBSTITUTED ESTER ENOLATES. APPLICATION TO THE SYNTHESIS OF RACEMIC EPI-BLASTMYCINONE, δ-MULTISTRIATINE, PARACONIC ESTERS AND LIGNANTYPE DILACTONES
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New stereoselective syntheses are reported for racemic 4-epi-blastmycinone (6) and δ-multistriatine (13) utilizing the anti-configurated γ,δ-unsaturated β-hydroxy-carboxylic acids 2a/b.A diastereo- and enantioselective aldoltype addition of phenylacetic acid dianion to benzaldehyde has been achaived by employing optically active alkoxide amide bases.Finally, highly stereocontrolled additions to the novel β-lactone substituted ester enolates 22 are described.
- Mulzer, Johann,Lasalle, Peter de,Chucholowski, Alexander,Blaschek, Ursula,Bruentrup, Gisela,et al.
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p. 2211 - 2218
(2007/10/02)
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- KETENE BIS(TRIMETHYLSILYL) ACETALS. CROSS-ALDOL TYPE CONDENSATION REACTIONS WITH ALDEHYDES AND SCHIFF BASES
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Condensation of the title acetals with aldehydes and Schiff bases in the presence of titanium tetrachloride is reported for the first time.It leads to β-hydroxyacids and to β-lactams via a cross-aldol type reaction, with good yields.
- Dubois, Jacqes-Emile,Axiotis, Georges
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p. 2143 - 2146
(2007/10/02)
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- Stereochemistry of the Addition of Carboxylic Acid Dianions to Aldehydes under Kinetic and Thermodynamic Control - Synthesis and Configurational Assignment of 2,3-Disubstituted threo- and erythro-3-Hydroxycarboxylic Acids
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Under kinetically controlled conditions (-50 deg C, 10 min) the carboxylic dianions 2 add to aldehydes 3 to give the threo/erythro-adducts 4/5 (Scheme 1); the threo-selectivity markedly increases with the bulkiness of the substituents of 2 or 3 and decreases with the charge/radius ratio of the counter-ions of 2.From these results a syn-transition state with a HOMO-LUMO ineraction between 2 and 3 is derived (Scheme 3).For appropriate substituents a far higher threo-selectivity is observed under thermodynamically (22-50 deg C, 1-3 days) than under kinetically controlled conditions.We describe the isolation of the hydroxy acids 6 and 7, which are formed from 4 and 5 on acidic hydrolysis, and show how their configurations can be unambiguously assigned on the basis of 1H-NMR data.
- Mulzer, Johann,Zippel, Matthias,Bruentrup, Gisela,Segner, Johannes,Finke, Juergen
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p. 1108 - 1134
(2007/10/02)
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