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Ch. 22 - Conjugated Systems 2: Pericyclic Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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Tutti i libri di testoMullins 1st EditionCh. 22 - Conjugated Systems 2: Pericyclic ReactionsProblema 54a
Capitolo 21, Problema 54a

Only after working Assessment 22.53, predict the product of the following reactions.
(a) Chemical reaction diagram showing reactants and conditions: LDA, Me3SiCl, heat, and H3O+ for product prediction.

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1
Identify the starting material as an α,β-unsaturated ester with a ketone group. The structure shows a carbonyl group adjacent to a double bond, which is a common site for enolate formation.
The first reagent, LDA (Lithium diisopropylamide), is a strong, non-nucleophilic base used to deprotonate the most acidic hydrogen. In this case, it will deprotonate the α-hydrogen adjacent to the carbonyl group, forming an enolate ion.
The second step involves Me₃SiCl (Trimethylsilyl chloride), which is used to silylate the enolate ion. This step protects the enolate by forming a silyl enol ether, which is more stable and less reactive than the enolate itself.
Upon heating, the silyl enol ether can undergo a [1,3]-sigmatropic rearrangement or other thermal rearrangements, depending on the specific structure and conditions. This step is crucial for rearranging the molecular structure.
Finally, the addition of H₃O⁺ (acidic workup) will hydrolyze the silyl enol ether back to a ketone or aldehyde, completing the reaction sequence and yielding the final product.

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LDA (Lithium Diisopropylamide) as a Base

LDA is a strong, non-nucleophilic base commonly used in organic synthesis to deprotonate weakly acidic protons, such as those adjacent to carbonyl groups. In this reaction, LDA will likely deprotonate the alpha hydrogen of the ester, generating an enolate ion, which is a key intermediate for further reactions.
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Acid-Base Catalysis Concept 3

Silyl Enol Ether Formation

The reaction with Me3SiCl (trimethylsilyl chloride) following LDA treatment suggests the formation of a silyl enol ether. This occurs when the enolate ion reacts with Me3SiCl, resulting in the protection of the enolate as a silyl enol ether, which is stable and can be used in subsequent reactions without undergoing unwanted side reactions.
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Formation of Enolates

Hydrolysis and Product Formation

The final step involving H3O+ indicates an acidic workup, which typically serves to hydrolyze silyl enol ethers back to their corresponding carbonyl compounds. This step is crucial for regenerating the carbonyl group and completing the transformation, leading to the final product of the reaction sequence.
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Hydrolysis of Thioesters Concept 2
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The [2 + 2] cyclization is especially useful when done intramolecularly. 

(a) What makes intramolecular reactions more favorable? 

(b) Predict the product of the following cyclization (slight modification of a reaction from Angew. Chem. 2011, 50, 5149)

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The Claisen rearrangement can be used to attach allyl groups to benzene via initial alkylation of a phenol.

(a) Predict the identity of A and B

(b) provide an arrow-pushing mechanism for each step. [The phenol alkylation is simply a Williamson ether synthesis reaction.]


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The Eschenmoser–Claisen reaction is a variant of the Claisen reaction studied in this chapter. Based on your answers to Assessments 22.53–22.55, predict a product of this reaction.

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Only after working Assessment 22.53, predict the product of the following reactions.

(b)

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The Ireland–Claisen reaction is a variant of the Claisen reaction studied in this chapter.

(a) Suggest a mechanism for the first step.

(b) Predict the product (B) that would result from heating intermediate A.

(c) Hydrolysis of B gives a carboxylic acid.

Suggest a mechanism for this reaction.

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The Johnson–Claisen reaction is a variant of the Claisen reaction studied in this chapter. (a) Suggest a mechanism for the first step. (b) Predict the product (B) that would result from heating intermediate A.

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