Robinson annulation: push the electrons yourself
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Organic Chemistry 2 · Free interactive demo
Cyclopentanone and methyl vinyl ketone react with sodium ethoxide in ethanol, then heat, to build a fused bicyclic enone. The ketone enolate first adds to the end of the enone, making a 1,5-diketone; a second enolate closes a six-membered ring, and dehydration puts a C=C bond next to the remaining ketone.
Draw all 18 arrows across eight steps above. Each arrow is checked as you draw it, and Hint reveals the next move a little at a time.
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The Robinson annulation mechanism, step by step
A Robinson annulation joins a Michael addition to an intramolecular aldol condensation, building a six-membered enone ring. Ethoxide makes only a small equilibrium population of ketone enolate, which is replenished as it reacts. Of the possible enolates of the 1,5-diketone, the side-chain methyl enolate closes the six-membered ring; heating then favors E1cB dehydration to the conjugated product.
The 9 steps of the demo, in the exercise's own words. Try drawing each one above before you read it.
Form the ketone enolate
A ketone α C–H (pKa approximately 19 to 20) is less acidic than ethanol (pKa approximately 16), so only a small fraction of enolate is present at any moment. It is replenished as it reacts.
Michael addition to the enone
Conjugation makes the terminal carbon of methyl vinyl ketone electrophilic. Attack there (1,4-addition) keeps the product anion stabilized as an enolate.
Protonate the new enolate on carbon
Ethanol (pKa approximately 16) is a stronger acid than a ketone α C–H (pKa approximately 19 to 20), so protonating this enolate is favorable. The product is a 1,5-diketone.
Form the enolate that can close a ring
Several α carbons can be deprotonated reversibly. Only the enolate at the side-chain methyl carbon can reach the ring carbonyl carbon while making a six-membered ring, which is the ring size that forms most easily.
Close the six-membered ring
An intramolecular aldol addition. The new six-membered ring contains the four side-chain carbons and the two adjacent carbons of the original ring.
Protonate the alkoxide
An alkoxide and ethoxide are bases of similar strength (their alcohols have pKa values of approximately 16 to 18), so proton exchange with ethanol is fast. The product is a β-hydroxy ketone.
Deprotonate between the ketone and the alcohol
With heat, the aldol product loses water by the E1cB route, which starts from an enolate.
Expel hydroxide
Hydroxide is normally a poor leaving group, but here it leaves from an anion next to a carbonyl, and the new C=C bond is conjugated with the ketone. Heat drives this step.
Fused cyclohexenone
A new six-membered ring is fused to the original five-membered ring. Its new C=C double bond is conjugated with the ketone: 1,2,3,6,7,7a-hexahydro-5H-inden-5-one. All four carbons of methyl vinyl ketone ended up in the new ring. The cyclopentanone oxygen left as hydroxide, and ethoxide ended up as ethanol.
How the mechanism is drawn
Cyclopentanone and but-3-en-2-one (methyl vinyl ketone) with sodium ethoxide in ethanol, then heat. One ethanol/ethoxide molecule transfers protons throughout the reaction; Na⁺ is a spectator. Follow the path to the fused bicyclic enone 1,2,3,6,7,7a-hexahydro-5H-inden-5-one. Competing self-aldol reactions and polymerization of the enone are not modeled; the new stereocenter at the ring carbon that forms the first C–C bond is not assessed.