- FeCl3·6H2O-catalyzed mukaiyama-aldol type reactions of enolizable aldehydes and acetals
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Mukaiyama-aldol type reactions of acetals derived from enolizable aldehydes with FeCl3·6H2O, an eco-friendly, low-cost, and stable catalyst, lead to β-methoxycarbonyl compounds with nearly quantitative yields. The methodology is extended to the parent aldehydes as starting materials, leading to the corresponding aldols with lower yields, but efficiently. Different alkyl and aryl substituted acetals and aldehydes have been tested in the reaction with linear and cyclic silyl enol ethers. Reactions are carried out in an open air atmosphere, and additives are not required. Acetals can be considered activating groups of the carbonyl moiety rather than a protecting group in this type of FeCl3·6H2O-catalyzed condensation.
- Rodríguez-Gimeno, Alejandra,Cuenca, Ana B.,Gil-Tomás, Jesús,Medio-Simón, Mercedes,Olmos, Andrea,Asensio, Gregorio
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p. 8263 - 8270
(2015/03/18)
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- On the Mechanism of Lewis Acid Mediated Nucleophilic Substitution Reactions of Acetals
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Lewis acid mediated nucleophilic substitution of acetals can occur by direct displacement (SN2) or oxocarbenium ion (SN1) mechanisms.With acyclic acetals, stereoselectivity increases with increasing steric bulk of the alkoxy group and with increasing polarity of the reaction medium.The enhanced stereoselectivity observed with acetals of secondary and tertiary alcohols is explained by perturbation of the approach trajectory of the nucleophilic alkene as it attacks the oxocarbenium ion.Highest stereoselectivity is seen in the reaction of 2-(1-phenylethyl)-4,4,5,5-tetramethyl-1,3-dioxolane (4) with enol silane 5; only one diastereomeric product (9s) is obtained, even in the relatively nonpolar solvent CH2Cl2.The TiCl4-mediated reactions of cyclic acetals 18c, 18t, 25, and 28 with silyl enol ether 5 show that in these systems the substitution does not occur by the SN2 mechanism.
- Mori, Ichiro,Ishihara, Kazuaki,Flippin, Lee A.,Nozaki, Kyoko,Yamamoto, Hisashi,et al.
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p. 6107 - 6115
(2007/10/02)
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