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L_11_09 - 318N Krische Lecture 11 T Claisen and Dieckmann...

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318N – Krische, Lecture 11: T - 02/24/09, Claisen and Dieckmann Reactions, Conjugate Addition to Enones Esters also participate in base-mediated condensation reactions. The products formed are beta-keto esters. If this ester condensation reaction is conducted intramolecularly, it is termed the “Claisen Condensation”. If it is conducted intramolecularly, it is termed the Dieckmann condensation. The Claisen and Dieckmann condensations are just addition-elimination reactions. The starting ester serves as the electrophile and the enolate of the starting ester serves as the nucleophile. In the case of ethyl esters, ethoxide is the leaving group. SAME STORY: (1) ADDITION – ester enolate adds to the carbonyl carbon of the ester to form a TETRAHEDRAL INTERMEDIATE, which (2) undergoes ELIMINATION by expelling ethoxide. The choice of base in Claisen condensations is important. If you are using ethyl esters, then you should use sodium ethoxide as base. If you are using methyl esters, then you should use sodium methoxide as
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