Introduction
The Robinson Annulation is versatile: any ketone bearing an α-hydrogen can be used as the Michael donor, and any α,β-unsaturated ketone (usually MVK) can be the Michael acceptor. The examples below show how the choice of donor ketone determines the final ring structure.
All examples follow the same four-step sequence: enolate formation → Michael addition → intramolecular aldol → dehydration.
Example 1: Cyclohexanone + MVK → Octalone
This is the textbook Robinson Annulation. Cyclohexanone (donor) reacts with methyl vinyl ketone (acceptor) under dilute KOH in ethanol.
Cyclohexanone
- Cyclic ketone
- α-H at C-2 and C-6
- Gives a fused bicyclic product
Methyl Vinyl Ketone
- MVK, but-3-en-2-one
- β-carbon electrophilic
- 1,4-addition site
Steps
Enolate at C-2
KOH removes H from C-2 of cyclohexanone giving a resonance-stabilised enolate.
Michael Addition
C-2 enolate adds to the β-carbon of MVK, giving a 1,5-diketone (cyclohexanone + CH₂–CH₂–CO–CH₃ chain).
Aldol Ring Closure
The MVK methyl group enolate attacks the cyclohexanone carbonyl → six-membered β-hydroxy ketone ring fused to cyclohexane.
Dehydration
Loss of water gives the octalone (bicyclo[4.4.0]dec-1(6)-en-2-one), a fused bicyclic enone.
Example 2: Wieland–Miescher Ketone
The Wieland–Miescher ketone is one of the most important Robinson Annulation products in organic synthesis. It is the key intermediate in many steroid and terpenoid total syntheses.
2-Methyl-1,3-cyclohexanedione
- 1,3-diketone donor
- Highly acidic α-H between the two carbonyls
- Reacts under mild conditions
Methyl Vinyl Ketone
- MVK, standard acceptor
- Often used as Mannich base precursor
- Gives bicyclic diketone product
Product: The Wieland–Miescher ketone is a bicyclic compound with two ketone functionalities and a conjugated enone system. It is used as a chiral building block in the synthesis of steroids, sesquiterpenes, and other polycyclic natural products.
Example 3: Acyclic Variant — Acetone + MVK
Robinson Annulation also works with acyclic ketones. Acetone (propan-2-one) reacts with MVK under base to give an acyclic 1,5-diketone which then cyclises.
Acetone
- Simplest methyl ketone
- 6 equivalent α-H atoms
- Gives monocyclic product
Cyclohex-2-enone
- Simple cyclohex-2-enone
- No fused ring (acyclic donor)
- Parent enone framework
Summary Table of Examples
| Donor Ketone | Acceptor | Product | Application |
|---|---|---|---|
| Cyclohexanone | MVK | Octalone (bicyclic enone) | Textbook example |
| 2-Methyl-1,3-cyclohexanedione | MVK | Wieland–Miescher ketone | Steroid synthesis |
| Acetone | MVK | Cyclohex-2-enone | Simple acyclic example |
| 2-Methylcyclohexanone | MVK | 4a-Methyloctalone | Enolate regioselectivity study |
| Cyclopentanone | MVK | Bicyclo[3.4.0] enone | Five-six fused ring system |