What are the two starting materials for Robinson annulation? Multiple Choice a) An α,β-Unsaturated carbonyl compound and an enolate b) Ketoester and an enolate c) 1,5-Dicarbonyl compound and an enolate d) 1,3-Dicarbonyl compound and an enolate
The Correct Answer and Explanation is:
Correct Answer: a) An α,β-Unsaturated carbonyl compound and an enolate
Explanation
The Robinson annulation is a classic organic reaction that forms six-membered rings and is particularly useful in the synthesis of complex cyclic molecules such as steroids and natural products. The reaction is a combination of two fundamental reactions: the Michael addition and the intramolecular aldol condensation.
The two starting materials required for a Robinson annulation are:
- An α,β-unsaturated carbonyl compound (commonly a ketone or aldehyde with a conjugated double bond),
- An enolate (usually derived from a 1,3-dicarbonyl compound such as a β-ketoester or β-diketone).
Here’s how the reaction proceeds:
- Michael Addition: The enolate (nucleophile) attacks the β-carbon of the α,β-unsaturated carbonyl compound (electrophile). This forms a 1,5-dicarbonyl intermediate via conjugate (1,4-) addition.
- Aldol Condensation: The resulting 1,5-dicarbonyl compound undergoes an intramolecular aldol condensation. The enolate formed from one carbonyl attacks the other carbonyl group, forming a new carbon-carbon bond and resulting in ring formation.
- Dehydration: The aldol product typically loses water, forming a double bond and resulting in a conjugated cyclic enone product.
Why the other choices are incorrect:
- b) Ketoester and an enolate: This is too vague. While a ketoester may form an enolate, it must react with an α,β-unsaturated carbonyl to qualify for Robinson annulation.
- c) 1,5-Dicarbonyl compound and an enolate: The 1,5-dicarbonyl compound is typically an intermediate, not a starting material.
- d) 1,3-Dicarbonyl compound and an enolate: A 1,3-dicarbonyl forms the enolate, but another reactant (α,β-unsaturated carbonyl) is required.
Thus, option (a) accurately describes the necessary starting materials for the Robinson annulation.
