Is an appreciable difference in reactivity between

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Unformatted text preview: eful – crossed Claisen condensations are useful, if there is an appreciable difference in reactivity between the two esters; when one of them has no α-hydrogens 28 Organic Organic Lecture Series Crossed Claisen Condensations – the ester with no α-hydrogens is generally used in excess Only this enolate can be formed 29 Organic Lecture Series Claisen condensations as routes to ketones This segment was added 30 Organic Organic Lecture Series Claisen condensations as routes to ketones The result of Claisen condensation, saponification, acidification, and decarboxylation is a ketone: 31 Organic Lecture Series Enamines Enamines are formed by the reaction of a 2° amine with the carbonyl group of an aldehyde or ketone the 2°amines most commonly used to prepare enamines are pyrrolidine and morpholine: O N H Pyrrolidine N H M orpholine 32 Organic Organic Lecture Series Preparation of Enamines Acid catalyst is usually TsOH; azeotropic removal of H2O. 33 See Chapter 16 for details Organic Lecture Series Enamines-Alkylation The value of enamines is that the β-carbon is nucleophilic (same C that was α to carbonyl) – enamines undergo SN2 reactions with methyl and 1°haloalkanes, α-haloketones, and αhaloesters 34 Enamines-Alkylation Organic Organic Lecture Series treatment of the enamine with 1 eq of an alkylating agent gives an iminium halide: hydrolysis of the iminium halide (salt) gives the alkylated aldehyde or ketone: O N Br- O O + + HCl/ H 2 O 2-Allylcyclohexanone + N ClHH Morpholinium chloride 35 Organic Lecture Series hydrolysis of the iminium halide (salt) gives the alkylated aldehyde or ketone: 36 Organic Organic Lecture Series Enamines-Acylation enamines undergo acylation when treated with acid chlorides and acid anhydrides 37 Organic Lecture Series Synthetic Advantages of Enamines vs Enolates 1) Avoids proton transfer. 2) Regiochemistry of alkylation can be controlled. (For un-symmetric ketones) 3) Avoids polyalkylation. 4) Avoids O-alkylation. 38 Organic Organic Lecture Series Acetoacetic Ester Synthesis The acetoacetic ester (AAE) synthesis is useful for the preparation of mono- and disubstituted acetones of the following types 39 Organic Lecture Series Acetoacetic Ester Synthesis – consider the AAE synthesis of this target molecule, which is a monosubstituted acetone 40 Organic Organic Lecture Series Acetoacetic Ester Synthesis – Step 1: formation of the enolate anion of AAE – Step 2: alkylatio...
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