Chapter 23 Textbook slides

Chapter 23 Textbook slides - CHAPTER 23 CHAPTER 23 Enolates...

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CHAPTER 23 Enolates and the Claisen Condensation: Synthesis of -Dicarbonyl Compounds: Acyl Anion quivalents Equivalents
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-Dicarbonyl Compounds: Claisen Condensations 23-1 Claisen condensations form -dicarbonyl compounds. Ethyl acetate reacts with a stoichiometric amount of sodium ethoxide to give ethyl 3-oxobutanoate.
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The first three steps in the mechanism are reversible.
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Because ethoxide ion is consumed in Step 4, a stoichiometric amount of sodium ethoxide is required, rather than a catalytic amount to insure that some ethoxide is always available to catalyze Step 1. To prevent transesterification, both the alkoxide and the ester are sually derived from the same alcohol usually derived from the same alcohol.
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Protons flanked by two carbonyl groups are acidic. The acidity of hydrogens flanked by two carbonyl groups is enhanced by resonance stabilization of the resulting anion.
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When only a single -hydrogen is present, the Claisen condensation fails because the addition product lacks the additional acidic proton necessary to drive the equilibrium. The existence of the unfavorable equilibrium can be demonstrated by the reversal of a Claisen condensation ( retro-Claisen ondensation condensation ):
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Claisen condensations can have two different esters s reactants. as reactants. Mixed Claisen condensations start with two different esters, are typically unselective and furnish product mixtures unless one of e esters has no ydrogens the esters has no -hydrogens.
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Intramolecular Claisen condensations result in yclic compounds. cyclic compounds. The Dieckmann condensation is an intramolecular version of the Claisen condensation which produces cyclic 3-keto esters. The method works best for the formation of five- and six-membered rings.
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Ketones undergo mixed Claisen reactions. Ketones are more acidic than esters and as a result are deprotonated and react before the ester has a chance to undergo self-condensation. The products may be -diketones, -ketoaldehydes or other -dicarbonyl compounds.
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Retrosynthetic analysis clarifies the synthetic utility f the Claisen condensation. of the Claisen condensation. Three facts govern the use of Claisen condensations in synthetic
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Chapter 23 Textbook slides - CHAPTER 23 CHAPTER 23 Enolates...

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