Organic Chemistry · Carbonyl Condensation Reactions
The Robinson Annulation Reaction
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The Robinson Annulation A reaction sequence that builds a new ring Full entry → (Robert Robinson, 1935) fuses two reactions of this chapter into a ring-forming powerhouse: a Michael addition Conjugate (1,4) addition of an enolate to an enone's β-carbon Full entry → followed by an Intramolecular aldol condensation Enolate attack on a carbonyl in the same molecule, then dehydration Full entry → (with dehydration). The net result is a cyclohexenone — a six-membered ring containing an α,β-unsaturated ketone — built from a ketone (Michael donor) and an enone (Michael acceptor, classically methyl vinyl ketone). Two new C–C bonds form: the Michael bond (donor α-C → acceptor β-C) and the aldol bond (the ring-closing bond). Because it reliably constructs the cyclohexenone ring that pervades steroids, terpenes, and alkaloids, the Robinson annulation has been a cornerstone of synthesis for ninety years — the Wieland–Miescher ketone C₁₁H₁₄O₂ bicyclic enone-dione from 2-methyl-1,3-cyclohexanedione + MVK Full entry →, made in one step from 2-methyl-1,3-cyclohexanedione and methyl vinyl ketone, is still a standard entry point for steroid total syntheses.
Why this matters
- Ring construction: builds a six-membered enone ring — a motif found throughout steroids, terpenes, and natural products — from simple precursors.
- Two C–C bonds in one operation: Michael addition plus intramolecular aldol creates the ring and installs functionality in a single practical sequence.
- Steroid synthesis: the Wieland–Miescher ketone (2-methyl-1,3-cyclohexanedione + MVK) is a classic annulation product and standard steroid-synthesis starting material.
- Exams: product prediction, mechanism, and retrosynthesis are standard items.
The college version
Core Concepts
Anatomy of the annulation
Two partners are needed: a Michael donor (ketone or 1,3-dicarbonyl with an α-hydrogen, e.g., cyclohexanone, 2-methylcyclohexanone, 2-methyl-1,3-cyclohexanedione) and a Michael acceptor enone, classically methyl vinyl ketone (MVK), CC(=O)C=C. The enone is chosen so that, after the Michael step, the two carbonyls are separated by exactly the right tether for the intramolecular aldol to close a six-membered ring — the preferred size for intramolecular aldols (kinetically and thermodynamically favorable).
Step 1: Michael addition → 1,5-dicarbonyl
Base removes an α-proton from the donor; the enolate attacks the β-carbon of MVK (Topic 10). Protonation gives a 1,5-dicarbonyl. Example: cyclohexanone + MVK → 2-(3-oxobutyl)cyclohexanone (CC(=O)CCC1CCCCC1=O, C₁₀H₁₆O₂).
Step 2: Intramolecular aldol condensation
Base removes the α-proton from the side-chain methylene (adjacent to the side-chain ketone). The enolate attacks the ring carbonyl intramolecularly (arrow: enolate electrons form the new C–C bond as the carbonyl π electrons move onto oxygen), giving a cyclic β-hydroxy ketone (aldol). Under the reaction conditions the aldol dehydrates (E1cb: base removes an α-proton; hydroxide leaves, forming the C=C), producing the conjugated cyclohexenone. Conjugation makes dehydration essentially irreversible — the sequence's thermodynamic driving force.
The products
For cyclohexanone + MVK: a fused bicyclic octalone — two six-membered rings, one a cyclohexenone. For 2-methylcyclohexanone + MVK: the octalone CC12CCCCC1=CC(=O)CC2 (C₁₁H₁₆O), methyl at the ring junction (a quaternary center). For 2-methyl-1,3-cyclohexanedione + MVK: the Wieland–Miescher ketone (CC12CCC(=O)C=C1CCCC2=O, C₁₁H₁₄O₂), an enone-dione with a quaternary stereocenter.
Retrosynthetic thinking
Work backward from the target cyclohexenone: (1) "delete" the enone C=C (the dehydration step); (2) break the aldol C–C bond to reveal the two carbonyls of the 1,5-dicarbonyl; (3) break the Michael C–C bond to reveal the donor and the acceptor. This identifies exactly which ketone and enone to use.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Michael addition alone | Robinson annulation | Michael alone gives an open 1,5-dicarbonyl; the annulation adds an intramolecular aldol + dehydration to close the ring |
| Intramolecular aldol | Intramolecular Claisen | Aldol: enolate + carbonyl → β-hydroxy ketone; Claisen: enolate + ester → β-keto ester (alkoxide leaves) |
| Which carbonyl is attacked in the aldol | Which enolate forms | Enolate forms at the side-chain methylene and attacks the ring carbonyl, closing a six-membered ring |
| Conjugated enone product | Isolated ketone | The conjugated enone is more stable, making dehydration irreversible and driving the sequence |
| Donor attack at acceptor β-carbon | Donor attack at acceptor carbonyl | Only β-attack (Michael) sets up the 1,5-dicarbonyl needed for the aldol step |
| Wieland–Miescher ketone (C₁₁H₁₄O₂) | Octalone from 2-methylcyclohexanone (C₁₁H₁₆O) | 1,3-dione donor → enone-dione; simple ketone donor → mono-ketone octalone |
Eli explains
The same idea, in plain words
Explain it like I’m 10
The Robinson annulation is like building a bracelet: first you snap two beads together (Michael addition makes a chain), then you bend the chain around and snap the two ends together to close the circle (intramolecular aldol makes the ring). The circle is a special six-bead ring with a picture on it (the enone), and you made the whole bracelet in one go.
Worked example
Example 1: Cyclohexanone + MVK — the complete mechanism
Reaction: cyclohexanone (O=C1CCCCC1) + methyl vinyl ketone (CC(=O)C=C) with catalytic NaOH, heat.
Step 1 — Michael addition. Hydroxide forms the cyclohexanone enolate; its α-carbon attacks MVK's β-carbon; MVK's π electrons move to its carbonyl oxygen; protonation gives 2-(3-oxobutyl)cyclohexanone (CC(=O)CCC1CCCCC1=O, C₁₀H₁₆O₂).
Step 2 — Enolate for the aldol. Base removes a proton from the side-chain methylene (adjacent to the side-chain ketone); the enolate is delocalized onto the side-chain carbonyl oxygen.
Step 3 — Intramolecular aldol. The enolate carbon attacks the ring carbonyl (arrow: enolate electrons form the new C–C bond; ring C=O π electrons move to oxygen), closing a six-membered ring: ring C1, ring C2, the two side-chain CH₂ groups, the side-chain carbonyl carbon, and the enolate carbon.
Step 4 — Dehydration. Base removes the α-proton; hydroxide leaves, forming the C=C conjugated with the ring carbonyl. Product: the fused bicyclic octalone — two six-membered rings, one a cyclohexenone.
Step 5 — Formula check. Overall C₆H₁₀O + C₄H₆O − H₂O = C₁₀H₁₄O:
DBE = 2(10) + 2 - 142 = 4
Two rings + one C=O + one C=C = 4 ✓ — a bicyclic enone.
Example 2: The Wieland–Miescher ketone
Reaction: 2-methyl-1,3-cyclohexanedione (CC1C(=O)CCCC1=O) + MVK → Wieland–Miescher ketone (CC12CCC(=O)C=C1CCCC2=O).
Step 1 — Why this donor? The 1,3-dione is acidic (pKa ≈ 11–13), forming its enolate under mild base; the C2 methyl becomes the product's quaternary stereocenter.
Step 2 — Michael addition. The dione enolate attacks MVK's β-carbon, giving the 1,5-dicarbonyl intermediate.
Step 3 — Intramolecular aldol. Enolate formation at the side-chain methylene and attack on a ring carbonyl close the ring.
Step 4 — Dehydration. Loss of water gives the conjugated enone, the Wieland–Miescher ketone, C₁₁H₁₄O₂:
DBE = 2(11) + 2 - 142 = 5
Two rings + two C=O + one C=C = 5 ✓.
Step 5 — Significance. A quaternary ring-junction carbon plus enone and ketone functionality is precisely what steroid skeletons need — from two cheap components.
Example 3: Retrosynthetic analysis of a target cyclohexenone
Target: 4a-methyl-4,4a,5,6,7,8-hexahydronaphthalen-2(3H)-one (CC12CCCCC1=CC(=O)CC2, C₁₁H₁₆O).
Step 1 — Remove the C=C (dehydration). Replace the enone double bond with the β-hydroxy ketone of the aldol.
Step 2 — Break the aldol bond. The ring-closing bond opens to a 1,5-dicarbonyl: 2-(3-oxobutyl)-2-methylcyclohexanone.
Step 3 — Break the Michael bond. Disconnect donor α-C from acceptor β-C: the pieces are 2-methylcyclohexanone (donor) and methyl vinyl ketone (acceptor).
Step 4 — Conclusion. The target comes from the Robinson annulation of 2-methylcyclohexanone + MVK — the methyl lands at the ring junction, exactly where needed.
Key takeaways
- Robinson annulation = Michael addition + intramolecular aldol condensation; product = cyclohexenone (six-membered ring with an enone).
- Partners: ketone/1,3-dicarbonyl donor + enone acceptor (typically MVK).
- Two new C–C bonds: the Michael bond (donor α-C → acceptor β-C) and the aldol bond (ring closure).
- Michael step gives a 1,5-dicarbonyl; the aldol closes a six-membered ring; E1cb dehydration gives the conjugated enone and drives the sequence.
- Wieland–Miescher ketone: 2-methyl-1,3-cyclohexanedione + MVK → C₁₁H₁₄O₂; standard steroid-synthesis intermediate with a quaternary center.
- Intramolecular aldols work best for 5–6-membered rings; the Michael tether must position the enolate to reach the carbonyl.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Name the two reactions that make up the Robinson annulation, and state what functional group the product contains.
Show answer
Michael addition (conjugate addition of an enolate to an enone) + intramolecular aldol condensation (with dehydration). The product is a cyclohexenone — a six-membered ring with an α,β-unsaturated ketone.
Cyclohexanone + MVK: what is the Michael product, and what happens to it next?
Show answer
The Michael product is 2-(3-oxobutyl)cyclohexanone (a 1,5-dicarbonyl). It then undergoes intramolecular aldol and dehydration to give the fused bicyclic octalone.
Why is the dehydration step essentially irreversible, and why does that matter?
Show answer
The product C=C is conjugated with the carbonyl (an enone), which is much more stable than an isolated alkene; dehydration therefore runs to completion and pulls the sequence forward. Without it the aldol would be reversible.
What is the Wieland–Miescher ketone, and why is it historically important?
Show answer
C₁₁H₁₄O₂, made in one step from 2-methyl-1,3-cyclohexanedione and MVK; its quaternary center and enone make it a classic steroid-synthesis intermediate.
A target is a cyclohexenone with a methyl at the ring junction. What donor and acceptor would you choose, and how would you prove it by retrosynthesis?
Show answer
Donor: 2-methylcyclohexanone; acceptor: MVK. Retrosynthesis: remove the enone C=C, break the aldol bond, then the Michael bond.
Cyclohexanone + MVK overall gives C₁₀H₁₄O. Verify with a DBE calculation and interpret the number.
Show answer
DBE = (2(10) + 2 − 14)/2 = 4: two rings + one carbonyl + one C=C = four degrees of unsaturation, consistent with the bicyclic enone product.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Annulation
- A reaction sequence that builds a new ring
- Michael addition
- Conjugate (1,4) addition of an enolate to an enone's β-carbon
- Intramolecular aldol condensation
- Enolate attack on a carbonyl in the same molecule, then dehydration
- Wieland–Miescher ketone
- C₁₁H₁₄O₂ bicyclic enone-dione from 2-methyl-1,3-cyclohexanedione + MVK
Sources & references
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