Multi-step Synthesis
Multi-step synthesis involves transforming a starting material into a target molecule through a sequence of chemical reactions. It is the central art of preparative organic chemistry. Retrosynthesis, formalised by E. J. Corey (Nobel Prize 1990), is its fundamental logical tool.
Retrosynthetic Analysis
Retrosynthesis involves reasoning from the target back to precursors, applying disconnections to strategic bonds in the target molecule. The notation uses a retrosynthetic arrow (⇒):
Target ⇒ Synthon A + Synthon B
A synthon is an ideal fragment (ion or radical) derived from a disconnection. Each synthon corresponds to a real synthetic equivalent (commercial reagent or preparable precursor).
Criteria for choosing disconnections: - Break bonds formed in well-controlled reactions (C−C, C−O, C−N). - Minimise the number of steps and use accessible starting materials. - Identify stereocentres and plan how to control them.
Protecting Groups
When a molecule bears several reactive functional groups, it is often necessary to protect certain groups temporarily to prevent unwanted reactions. Three criteria for a good protecting group:
1. Installed easily and selectively. 2. Stable under the conditions of subsequent steps. 3. Removed easily under mild conditions without affecting the rest of the molecule.
| Group to protect | Common protecting group | Deprotection |
|---|---|---|
| Alcohol (−OH) | TMS ether (−OTMS), THP, benzyl | F⁻ (TBAF), HCl/MeOH, H₂/Pd |
| Amine (−NH₂) | Cbz (benzyloxycarbonyl), Boc (tert-butoxycarbonyl) | H₂/Pd, CF₃COOH |
| Aldehyde (−CHO) | Acetal (−CH(OR)₂) | dilute aqueous HCl |
| Carboxylic acid | Methyl or benzyl ester | LiOH, H₂/Pd |
Industrial Example: Aspirin Synthesis
Aspirin (acetylsalicylic acid) is manufactured industrially in one step from salicylic acid (derived from phenol) by acetylation with acetic anhydride (or acetyl chloride):
Salicylic acid + (CH₃CO)₂O → Aspirin + CH₃COOH
Catalysis by sulfuric acid or phosphoric acid accelerates the reaction. The critical quality point is purity control (residual salicylic acid → gastric irritation).
Industrial Example: Paracetamol Synthesis
Paracetamol (acetaminophen) is synthesised in three steps:
1. Nitration of phenol → p-nitrophenol (EAS directed to para by the −OH group). 2. Reduction of the nitro group to amine (H₂/Pd or Zn/HCl) → p-aminophenol. 3. Acetylation of the amine with acetic anhydride → paracetamol.
Key Strategies
- Convergence: build both halves of the molecule in parallel then assemble them (reduces step count and improves overall yield).
- Atom economy: prefer reactions that incorporate the maximum number of atoms from reagents into the product (additions > substitutions > eliminations).
- Stereocontrol: use chiral auxiliaries, asymmetric catalysts, or diastereoselective reactions to set stereocentres.
Mastery of multi-step synthesis is the ultimate skill of the organic chemist, providing access to high-value molecules (active pharmaceutical ingredients, materials).