University of Illinois Urbana-Champaign

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

Wolk, Martin Benson

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Description

Title
The condensation of alpha,beta-unsaturated orthoesters and activated ketones: A stereoselective crotonate Michael equivalent
Author(s)
Wolk, Martin Benson
Issue Date
1990
Doctoral Committee Chair(s)
Coates, Robert M.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Language
eng
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.
Type of Resource
text
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http://hdl.handle.net/2142/22024
Copyright and License Information
Copyright 1990 Wolk, Martin Benson

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