What Is Robinson Annulation?
The Robinson annulation is a classic carbon–carbon bond-forming reaction that combines a Michael addition with an intramolecular aldol condensation to construct cyclohexenone rings from a methyl vinyl ketone (or related α,β-unsaturated ketone) and a cyclic or acyclic ketone. Since its discovery in 1935 by Sir Robert Robinson, this transformation has become indispensable in total synthesis, particularly for the construction of polycyclic terpenes and steroids. One of the most celebrated examples is the preparation of the Wieland-Miescher ketone (WMK) from 2-methyl-1,3-cyclohexanedione and methyl vinyl ketone (MVK) using pyrrolidine in benzene. The product is a key intermediate for the synthesis of terpenes (e.g., taxol, vitamin D analogs) and steroids.
The Robinson annulation (also referred to as the Robinson-Mannich base route) is a two-step cascade process that furnishes substituted 2-cyclohexenones. The classical protocol employs a cyclic ketone or an acyclic ketone and MVK under basic conditions. The name "annulation" derives from the Latin annulus (ring), reflecting the formation of a new six-membered ring fused to an existing one. Over time, variants using other Michael acceptors (e.g., acrolein, ethyl vinyl ketone) and different bases (e.g., pyrrolidine, LDA, KOH/MeOH) have been developed, but the core principle remains unchanged.
- Reagents: Base (e.g., pyrrolidine, KOH, NaOMe, LDA); often a solvent (benzene, toluene, MeOH, THF).
- Reactants: A cyclic or acyclic ketone (enolate/enamine precursor, e.g., cyclohexanone) + an α,β-unsaturated ketone (typically methyl vinyl ketone, MVK).
- Products: A substituted 2-cyclohexenone (a six‑membered α,β‑unsaturated ketone).
- Reaction Type: Tandem reaction – Michael addition followed by intramolecular aldol condensation (annulation).
- Related Reactions: Hajos-Parrish-Eder-Sauer-Wiechert reaction, Stork-Danheiser annulation, Michael–Dieckmann annulation.
Fig 1. Robinson annulation reaction and its mechanism. [1]
Mechanism of Robinson Annulation
The Robinson annulation proceeds via two distinct, sequentially linked steps: (1) Michael addition, followed by (2) intramolecular aldol condensation.
- Enolate Formation: Under basic conditions, the starting carbonyl compound is deprotonated at the α-position to form an enolate.
- Michael Addition: The enolate performs a 1,4-conjugate addition to the α,β-unsaturated carbonyl compound, generating a 1,5-dicarbonyl intermediate. This step is crucial because it sets up the chain length required for ring closure.
- Intramolecular Aldol Condensation: The new carbonyl-containing intermediate forms another enolate, which then attacks one of its own carbonyl groups intramolecularly. This cyclization produces a β-hydroxy ketone.
- Dehydration: The β-hydroxy ketone then undergoes elimination of water to yield the final α,β-unsaturated cyclohexenone.
Experimental Tips for Robinson Annulation
- Control the Base Strength: A base that is too strong can cause over-alkylation, self-condensation, decomposition, or undesired Michael side reactions. Common choices include sodium ethoxide, potassium tert-butoxide, sodium hydroxide, potassium carbonate, or catalytic amines in some systems.
- Workup and Purification: Products are often conjugated enones and may be sensitive to prolonged strong base exposure. Neutralization and prompt purification are recommended.
- Handling MVK: MVK is a lachrymator and a potent alkylating agent – handle in a fume hood with proper PPE (gloves, goggles). It is prone to spontaneous polymerization; store at –20°C under inert atmosphere and use freshly distilled or stabilized (with hydroquinone) material. Slow addition of MVK to the reaction mixture via syringe pump (over 1–3 hours) minimizes polymer formation.
Application Examples of Robinson Annulation
- Example 1: Qing Zhang et al. developed a spiro-pyrrolidine catalyst to achieve a highly enantioselective Robinson annulation, enabling the rapid construction of fused cis-dibenzofuran frameworks featuring adjacent stereocenters and all-carbon quaternary centers. This asymmetric synthetic approach provides an effective strategy for the synthesis of (–)-morphine and its analogs. [2]
- Example 2: Vojtech Kapras et al. reported a unique method for the asymmetric total synthesis of a non-natural enantiomer of pregnanolone, wherein the asymmetry was introduced via a highly atom-economical organocatalytic Robinson annulation. [3]
Fig 2. Synthetic examples via Robinson annulation.
Related Products
| CAS No. | Structure | Product | Inquiry |
| 1122-24-3 |  | Cyclohexanone,2-ethenyl- | Inquiry |
| 1193-47-1 |  | Cyclohexanone,2,2-dimethyl- | Inquiry |
| 13368-65-5 |  | Cyclohexanone,3-methyl-,(3R)- | Inquiry |
| 13619-73-3 |  | Cyclohexanone,3-ethoxy-(8ci,9ci) | Inquiry |
| 14073-97-3 |  | Cyclohexanone,5-methyl-2-(1-methylethyl)-,(2S,5R)- | Inquiry |
| 14376-79-5 |  | Cyclohexanone,3,3,5,5-tetramethyl- | Inquiry |
| 1444-65-1 |  | Cyclohexanone,2-phenyl- | Inquiry |
| 14765-30-1 |  | Cyclohexanone,2-(1-methylpropyl)- | Inquiry |
| 946-33-8 |  | Cyclohexanone,2-(phenylmethyl)- | Inquiry |
| 141632-21-5 |  | 3-(2-Fluorophenyl)cyclohexanone | Inquiry |
| 6289-46-9 |  | Dimethyl 1,4-cyclohexanedione-2,5-dicarboxylate | Inquiry |
| 6297-22-9 |  | Methyl 4-ketocyclohexanecarboxylate | Inquiry |
| 139025-93-7 |  | 4-Morpholinocyclohexanone | Inquiry |
| 13482-22-9 |  | 4-Hydroxycyclohexanone | Inquiry |
| 105640-07-1 |  | 4-(4-Hydroxyphenyl)Cyclohexanone | Inquiry |
References
- Jie Jack Li. Name Reactions-A Collection of Detailed Mechanisms and Synthetic Applications, Sixth Edition, 2021, 474-476.
- Zhang, Qing, et al. Nature Communications 10.1 (2019): 2507.
- Kapras, Vojtech, et al. Organic letters 20.4 (2018): 946-949.
Please kindly note that our products are for research use only.
