ch19 - Chapter 19 Synthesis and Reactions of -Dicarbonyl...

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Chapter 19 Synthesis and Reactions of β -Dicarbonyl Compounds: More Chemistry of Enolate Anions
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Chapter 19 2 Introduction β -Dicarbonyl compounds have two carbonyl groups separated by a carbon Protons on the α -carbon of β -dicarbonyl compounds are acidic (p K a = 9-10) The acidity can be explained by resonance stabilization of the corresponding enolate by two carbonyl groups
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Chapter 19 3 β -Dicarbonyl compounds can be synthesized by the Claisen condensation The acetoacetic ester and malonic acid syntheses use β - dicarbonyl compounds for carbon-carbon bond forming reactions The acetoacetic ester and malonic ester syntheses usually conclude with decarboxylation of a β -keto acid
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Chapter 19 4 The Claisen Condensation: Synthesis of β -Keto Esters Ethyl acetate undergoes a Claisen condensation when treated with sodium ethoxide The product is commonly called an acetoacetic ester Ethyl pentanoate undergoes an analogous reaction
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Chapter 19 5 The overall reaction involves loss of an α hydrogen from one ester and loss of ethoxide from another The mechanism is an example of the general process of nucleophilic addition-elimination at an ester carbonyl
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Chapter 19 6
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Chapter 19 7 The alkoxide base must have the same alkyl group as the alkoxyl group of the ester The use of a different alkoxide would result in formation of some transesterification products Esters with only one α hydrogen do not undergo Claisen condensation A second hydrogen on the α carbon is necessary so that it can be deprotonated in Step 3 This deprotonation drives the reaction to completion
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Chapter 19 8 The Dieckmann condensation is an intramolecular Claisen condensation Only 5- and 6-membered rings may be prepared in this way
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Chapter 19 9 Crossed Claisen Condensations Crossed Claisen condensations can lead to one major product when one of the two esters has no α hydrogen
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Chapter 19 10 Esters with one α hydrogen can react in Claisen condensations if they are deprotonated with a strong base and acylated with an acyl chloride
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This note was uploaded on 04/23/2009 for the course CHE 322 taught by Professor Staff during the Spring '08 term at SUNY Stony Brook.

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ch19 - Chapter 19 Synthesis and Reactions of -Dicarbonyl...

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