The Quick Answer
The two starting materials for a Robinson Annulation are:
Together, under basic conditions, they undergo a Michael addition followed by an intramolecular aldol condensation, building a new six-membered ring with a conjugated α,β-unsaturated ketone (cyclohex-2-enone).
The Two Starting Materials, Side by Side
Every Robinson Annulation begins with the same pairing — one nucleophilic carbon partner and one electrophilic carbon partner. Knowing which is which is the single most useful idea on this page.
α-Enolisable Ketone
- Must bear at least one α-hydrogen
- Base deprotonates the α-carbon → enolate
- The α-carbon becomes the new C–C bond site
Methyl Vinyl Ketone (MVK)
- α,β-unsaturated ketone, but-3-en-2-one
- β-carbon is electrophilic (1,4-addition site)
- Often released in situ from a Mannich base
How the Two Materials React — Mechanism in 4 Steps
The two starting materials react in sequence under basic conditions. Here are the four key steps:
Enolate Formation
Base removes an α-proton from the donor ketone, generating a resonance-stabilised enolate at the α-carbon.
Michael Addition (1,4-Addition)
The enolate (nucleophile) attacks the β-carbon of MVK in a conjugate 1,4-addition. A 1,5-diketone intermediate is formed.
Intramolecular Aldol Addition
A new enolate forms on the MVK-derived methyl group and attacks the other carbonyl in an intramolecular aldol reaction, closing the six-membered ring.
Dehydration (Aldol Condensation)
Loss of water from the β-hydroxy ketone gives the conjugated cyclohex-2-enone — the Robinson Annulation product.
Want the full electron-pushing version with curly arrows? Read the step-by-step mechanism →
Reactant 1 — The Michael Donor in Detail
The donor decides which α-carbon becomes the nucleophile and what substituents end up on the final cyclohexenone ring. It must have at least one α-hydrogen.
Common Donor Ketones
- Cyclohexanone — the textbook donor; gives a fused-ring decalone product.
- 2-Methylcyclohexanone — lets you study kinetic vs thermodynamic enolate control.
- 2-Methyl-1,3-cyclohexanedione — used in the Wieland–Miescher ketone synthesis (steroid precursor).
- Acetone, methyl ketones — work for simple, acyclic examples.
Reactant 2 — Methyl Vinyl Ketone (MVK)
Methyl vinyl ketone (but-3-en-2-one, CH₂=CH–CO–CH₃) is the canonical Michael acceptor for the Robinson Annulation. Its C=C double bond is conjugated to a carbonyl, so the β-carbon is strongly electrophilic and accepts enolates in a 1,4-addition.
MVK Properties
- IUPAC name: But-3-en-2-one
- Formula: C₄H₆O
- Role: Michael acceptor
- MW: 70.09 g/mol
Why Use a Mannich Base?
Free MVK polymerises easily. Chemists often use 1-(diethylamino)butan-3-one (a Mannich base) instead. Under basic conditions it eliminates the amine, slowly releasing MVK at low concentration — giving cleaner reactions with fewer side products.
Worked Example — Cyclohexanone + MVK
Combine cyclohexanone (donor) with methyl vinyl ketone (acceptor) in dilute KOH / ethanol:
- KOH deprotonates cyclohexanone at C-2 (α-carbon).
- The enolate adds to the β-carbon of MVK → 1,5-diketone.
- A second enolate on the MVK side attacks the cyclohexanone carbonyl → aldol addition closes a six-membered ring.
- Dehydration gives a bicyclic octalone (cyclohex-2-enone fused to cyclohexane).
Swap cyclohexanone for 2-methyl-1,3-cyclohexanedione and you get the famous Wieland–Miescher ketone — a key building block in steroid total synthesis.
Who Discovered It, and Why It Matters
The reaction is named after Sir Robert Robinson, who published the sequence in 1935. Robinson was awarded the 1947 Nobel Prize in Chemistry for his investigations of alkaloids and other natural products.
The Robinson Annulation became central to organic synthesis because it solves a recurring problem: building a six-membered ring with an α,β-unsaturated ketone — exactly the framework found in steroids, terpenes, and a huge family of polycyclic natural products. Virtually every steroid synthesis uses Robinson Annulation or a variant at a key step.
Common Mistakes Students Make
- Forgetting the α-hydrogen rule. A ketone with no α-H cannot act as the Michael donor.
- Confusing Michael Addition with Robinson Annulation. Michael addition is only step 1. The annulation requires the intramolecular aldol condensation that closes the ring. See the comparison →
- Too much MVK at once. High MVK concentrations promote polymerisation; use a Mannich base to release it slowly.
- Mislabelling the product. The product is a cyclohex-2-enone, not a saturated ketone or a diketone.
- Wrong regiochemistry. With unsymmetrical donors, the more acidic α-proton is removed first — always identify the correct enolate.