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

Verified step by step guidance

1. Step 1: Identify the target molecule as a bicyclic compound containing a cyclohexenone ring fused to a phenyl-substituted ring. This suggests the use of the Robinson annulation, which combines a Michael addition followed by an intramolecular aldol condensation.
2. Step 2: Determine the precursor molecules for the Robinson annulation. The cyclohexenone ring indicates that a β-keto ester or diketone will be used as the nucleophile, while the phenyl-substituted ring suggests a conjugated enone as the electrophile.
3. Step 3: Select the appropriate enone and nucleophile. For this synthesis, the enone could be phenyl vinyl ketone (Ph-CH=CH-COCH3), and the nucleophile could be a methyl ketone such as 2-ethyl-1,3-cyclohexanedione.
4. Step 4: Perform the Michael addition. The nucleophile (2-ethyl-1,3-cyclohexanedione) attacks the conjugated double bond of phenyl vinyl ketone, forming a new carbon-carbon bond and generating an intermediate with a β-diketone structure.
5. Step 5: Carry out the intramolecular aldol condensation. The intermediate undergoes cyclization, forming the six-membered ring and completing the Robinson annulation to yield the target bicyclic compound.

Key Concepts

Robinson Annulation

The Robinson annulation is a key synthetic method in organic chemistry that combines a Michael addition and an intramolecular aldol condensation. This reaction typically involves a cyclic ketone and an α,β-unsaturated carbonyl compound, leading to the formation of a six-membered ring. It is particularly useful for constructing complex cyclic structures, making it a valuable tool in the synthesis of natural products and pharmaceuticals.

Michael Addition

The Michael addition is a nucleophilic addition reaction where a nucleophile adds to an α,β-unsaturated carbonyl compound. This reaction is crucial in the Robinson annulation as it forms the initial bond between the nucleophile and the electrophile, setting the stage for subsequent reactions. Understanding the reactivity of the nucleophile and the electrophile is essential for predicting the outcome of the reaction.

Aldol Condensation

Aldol condensation is a reaction between aldehydes or ketones that contain α-hydrogens, leading to the formation of β-hydroxy carbonyl compounds. In the context of the Robinson annulation, this step occurs after the Michael addition, where the newly formed compound undergoes dehydration to yield an enone. This transformation is critical for creating the final cyclic structure and understanding the reaction mechanism.