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6.5: Intramolecular Claisen Condensations - The Dieckmann Cyclization

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    469403
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    The Dieckmann Cyclization

    Diesters can undergo an intramolecular reaction, called the Dieckmann condensation, to produce cyclic beta-keto esters. This reaction works best with 1,6-diesters, which produce five-membered rings, and 1,7-diesters which produce six membered rings.

    Examples

    Mechanism

    The mechanism of the Dieckmann condensation is the same as a Claisen condensation. An alkoxide base removes an alpha-hydrogen from one of the esters to form an ester enolate. The enolate then adds to the carbonyl carbon of the other ester to form a tetrahedral alkoxide intermediate. The alkoxide reforms the carbonyl bond which causes the elimination of the –OR leaving group and forms a cyclic beta-keto ester. The high acidity of the beta-keto ester allows it to be deprotonated by the reactions base to form a second enolate. Like the Claisen condensation, this deprotonation step drives the equilibrium towards the products and is required for the reaction to occur. A full equivalent of base is necessary during this reaction.

    1) Enolate formation

    Dieckmann Mechanism Step 1.svg

    2) Nucleophilic attack

    Dieckmann Mechanism Step 2.svg

    3) Removal of leaving group

    Dieckmann Mechansim Step 3.svg

    4) Deprotonation

    Dieckmann Mechanism Step 4.svg

    5) Protonation

    Dieckmann Mechanism Step 5.svg

    Further Reactions of the Dieckmann Cyclization Product

    Further Reactions.svg

    The cyclic beta-keto ester product of a Dieckmann cyclization can be modified by reaction similar to those used in the acetoacetic ester synthesis (Section 22-7). The acidic alpha-hydrogens of the beta-keto ester allow it to easily be deprotonated and alkylated in an alpha-substitution reaction. Having a carbonyl group in the beta position allows the ester to be removed through decarboxylation. The combination of these three reactions (1) Dieckmann cyclization, (2) alpha alkylation, and (3) decarboxylation provides an efficient method for preparing 2-substituted cyclopentanones and cyclohexanones.


    6.5: Intramolecular Claisen Condensations - The Dieckmann Cyclization is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Steven Farmer, Dietmar Kennepohl, Layne Morsch, Gamini Gunawardena, & Gamini Gunawardena.