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|Title:||The condensation of alpha,beta-unsaturated orthoesters and activated ketones: A stereoselective crotonate Michael equivalent|
|Author(s):||Wolk, Martin Benson|
|Doctoral Committee Chair(s):||Coates, Robert M.|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||The acid-catalyzed condensation of $(E)$-triethyl orthocrotonate, (Z)-triethyl orthocrotonate, and triethyl orthoacrylate with activated ketones at 25-100$\sp\circ$C followed by hydrolysis gave a series of $\delta$-keto ester adducts. The process is acid-catalyzed and occurs readily with cyclic and acyclic ketones containing an electron-withdrawing group $\alpha$ to the ketone carbonyl. The general trend in reactivity with respect to the substituents is: RCHO $>$ RCN $>$ RNO$\sb2$ $\gg$ RCOMe $>$ RCO$\sb2$R with cyclopentanones reacting faster than the corresponding cyclohexanones.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)
The syn stereochemistry of three condensation products has been proven by independent synthesis, X-ray crystallographic analysis, and NMR analysis of trans-fused lactones obtained by sodium borohydride reduction. The net reaction of the rearrangement-hydrolysis is the diastereoselective (typically $>$80:20) synthesis of a 1,5-dicarbonyl compound from a ketone (donor) and an unsaturated orthoester (acceptor) equivalent to a Michael addition via a Claisen rearrangement.
The mechanism for the orthoester condensation most likely consists of a series of equilibria involving the reversible, acid-catalyzed formation of an oxonium ion from the $\alpha$,$\beta$-unsaturated orthoester, reversible complexation of the oxonium ion with ketone, proton loss, (3,3) sigmatropic rearrangement, and reversible acid-catalyzed orthoester formation. The reaction is not a standard catalytic Michael addition since no reaction was observed under orthoester condensation conditions when ethyl crotonate was substituted for the orthoester.
The stereochemistry of the condensation is proposed to arise from a rapid, diastereoselective Claisen rearrangement of an allyl vinyl orthoester intermediate. The proven stereochemistry of three orthoester condensation products is consistent with a chair transition state.
|Rights Information:||Copyright 1990 Wolk, Martin Benson|
|Date Available in IDEALS:||2011-05-07|
|Identifier in Online Catalog:||AAI9026355|
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