Show how you would use the Robinson annulation to synthesize the following compounds.
(c)

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Identify the target compound and determine the key structural features that suggest it can be synthesized using the Robinson annulation. The Robinson annulation typically forms a six-membered ring through a Michael addition followed by an intramolecular aldol condensation.

Break down the target compound into two precursor molecules: (1) a Michael acceptor (α,β-unsaturated ketone) and (2) a Michael donor (a ketone or β-diketone). Analyze the structure of the target compound to identify these components.

Propose the Michael addition step. Write the reaction where the enolate ion of the Michael donor attacks the β-carbon of the Michael acceptor, forming a new carbon-carbon bond. Represent this step using MathML for clarity.

Describe the intramolecular aldol condensation step. After the Michael addition, the intermediate undergoes enolate formation at the α-carbon of the ketone, followed by nucleophilic attack on the carbonyl group within the same molecule. This forms a six-membered ring. Use MathML to show the mechanism.

Conclude by showing the dehydration step, where the aldol product loses a molecule of water to form the α,β-unsaturated ketone in the final product. Highlight the formation of the conjugated system in the target compound.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Robinson Annulation

The Robinson annulation is a key synthetic method in organic chemistry that combines a Michael addition and an intramolecular aldol reaction. This process allows for the formation of six-membered rings, which are common in many natural products. The reaction typically involves a β-keto ester and an α,β-unsaturated carbonyl compound, leading to the formation of a cyclohexenone structure.

Michael Addition

The Michael addition is a nucleophilic addition reaction where a nucleophile adds to an α,β-unsaturated carbonyl compound. This step is crucial in the Robinson annulation, as it forms a new carbon-carbon bond, setting the stage for subsequent reactions. The nucleophile can be a variety of species, including enolates, which are often generated from β-keto esters.

Aldol Reaction

The aldol reaction involves the condensation of two aldehydes or ketones to form a β-hydroxy carbonyl compound. In the context of the Robinson annulation, the aldol reaction occurs after the Michael addition, where the intermediate formed undergoes dehydration to yield an α,β-unsaturated carbonyl compound. This step is essential for closing the ring and completing the synthesis of the desired cyclic structure.

Show how you would use the Robinson annulation to synthesize the following compounds.(c)