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The condensation of alpha,beta-unsaturated orthoesters and activated ketones: A stereoselective crotonate Michael equivalent

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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, R. M.
Department / Program: Chemistry
Discipline: Chemistry
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): Chemistry, Organic
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.
Issue Date: 1990
Type: Text
Language: English
Rights Information: Copyright 1990 Wolk, Martin Benson
Date Available in IDEALS: 2011-05-07
Identifier in Online Catalog: AAI9026355
OCLC Identifier: (UMI)AAI9026355

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