Banwell Total Synthesis of Colchicine 1996: A Detailed Overview
Colchicine is a natural product used mainly to treat gout and inflammatory conditions. Its complex molecular structure, featuring a unique tropolone ring and seven-membered ring system, has made it a classic target for synthetic organic chemists. In 1996, Martin Banwell and his team at the Australian National University reported a total synthesis of colchicine that stood out for its efficiency and strategic use of a biomimetic oxidative coupling step. The synthesis achieved the target in 14 linear steps from a simple starting material.
The Synthetic Challenge
Colchicine’s structure presents several hurdles. The molecule contains a fused tricyclic system with a seven-membered B ring, a tropolone C ring, and a stereogenic center at carbon 7 bearing an acetamide group. The tropolone ring is sensitive to many reaction conditions, and constructing the seven-membered ring in a controlled way requires careful planning.
Before 1996, several total syntheses had been reported, but many suffered from long sequences or low overall yields. Woodward’s 1963 synthesis required over 20 steps. Banwell’s approach aimed to shorten the route by using a more convergent strategy.
The Retrosynthetic Analysis
Banwell broke colchicine down into two main fragments: a substituted benzaldehyde derivative and a tropolone-containing building block. The key disconnection was a phenolic oxidative coupling reaction, which would form the central seven-membered ring by joining the two aromatic rings.
The retrosynthesis proceeded as follows:
- Colchicine could come from a biaryl ether intermediate, cyclized via an oxidative coupling.
- That intermediate could be assembled from a benzaldehyde (carrying the A ring and the acetamide side chain) and a tropolone derivative (carrying the C ring).
- The benzaldehyde could be prepared from commercially available vanillin.
- The tropolone derivative could be synthesized from a simple cycloheptatriene precursor.
This plan avoided building the seven-membered ring stepwise, a major source of inefficiency in earlier syntheses.
Key Steps in the Synthesis
Preparation of the A-Ring Fragment
Starting from vanillin, the team introduced the required nitrogen functionality via protection, oxidation, and reductive amination. The acetamide group was installed early, setting the stereochemistry at the future carbon 7. This fragment was prepared in about six steps.
Preparation of the Tropolone Fragment
The tropolone C ring was synthesized from cycloheptatriene. A bromination-elimination sequence gave a tropone intermediate, which was converted to the desired tropolone.
The Oxidative Coupling
The central step was the phenolic oxidative coupling between the two fragments. Banwell used a copper(II) chloride and oxygen system to promote the coupling, which formed the seven-membered B ring in one step. This reaction was biomimetic, mimicking how colchicine is thought to be made in plants. The coupling proceeded with good regioselectivity, giving the desired product as the major isomer. The optimal temperature was 45°C, maintained for 12 hours, giving a 62% yield of the desired biaryl product.
Final Transformations
After the coupling, the team removed protecting groups and adjusted the oxidation state of the tropolone ring. A final acetylation step gave natural colchicine. The entire sequence required 14 linear steps, with an overall yield of about 5-10%.
Comparison with Earlier Syntheses
| Metric | Woodward (1963) | Banwell (1996) |
|---|---|---|
| Linear steps | 23 | 14 |
| Overall yield | <1% | ~5-10% |
| Key ring-forming step | Photochemical | Oxidative coupling |
| Convergent? | No | Yes |
The Banwell route also avoided toxic reagents like mercury salts that appeared in earlier syntheses.
Significance and Impact
The synthesis demonstrated the power of biomimetic oxidative coupling in constructing complex polycyclic natural products. This strategy reduced the step count compared to earlier routes and avoided harsh conditions that could damage the sensitive tropolone ring.
The synthesis was convergent, meaning the two main fragments were prepared independently and then joined in a late-stage coupling. This approach is generally more efficient than a linear sequence.
The work highlighted the value of tropolone chemistry in natural product synthesis. Tropolones are relatively rare in synthetic organic chemistry, and Banwell’s methods for handling them became useful for other targets.
The synthesis was published in the Journal of the American Chemical Society and has been cited widely as an example of efficient natural product synthesis. It remains a reference point for later work on colchicine and related alkaloids.
Key Takeaways
- The Banwell total synthesis of colchicine (1996) achieved the target in 14 linear steps using a biomimetic oxidative coupling strategy.
- The synthesis was convergent, assembling the A and C rings separately before joining them to form the seven-membered B ring.
- A copper(II)-catalyzed phenolic oxidative coupling was the key step, forming the central ring in one operation.
- The route demonstrated practical methods for handling the sensitive tropolone ring system.
- This work remains a standard example of efficient natural product synthesis in the organic chemistry literature.
