Cationic cyclization
Prins cyclization
How the Prins cyclization works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Homoallylic alcohol, aldehyde (CH₂O), Brønsted or Lewis acid (H₂SO₄, TFA, BF₃·OEt₂, SnCl₄). Conditions: CH₂Cl₂ or aq. acid, 0–25 °C. Bonds formed: C–C (ring closure); C–O (ring ether); C–Nu or C=C; bonds broken: C=C π; C=O π. Stereochemistry: Chair TS; 2,6-cis-substituted tetrahydropyrans usually favoured. Rate and selectivity: Oxocarbenium formation is fast; alkene attack is rate-limiting.
At a glance
Step 1: Hemiacetal, then oxocarbenium ion
Acid activates a carbonyl (typically formaldehyde); the homoallylic alcohol's own oxygen attacks it first, forming a hemiacetal that ionizes to a resonance-stabilized oxocarbenium ion.
Step 2: Alkene attacks: tetrahydropyran ring closes
The pendant alkene's π electrons then attack that oxocarbenium ion intramolecularly — the defining C–C bond-forming step — closing a tetrahydropyran ring and generating a new carbocation within the ring.
Step 3: Proton loss or nucleophilic trapping
Loss of a proton (or trapping by an external nucleophile) gives a substituted tetrahydropyran — a reliable way to build the oxygen-containing six-membered rings found throughout polyketide natural products, in a single cationic cascade.
Abbreviations and terms
- Nu
- Nucleophile. An electron-rich species that donates an electron pair to form a new bond.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- TFA
- Trifluoroacetic acid. A strong organic acid (pKa ≈ 0.5), used to remove Boc groups and protonate carbonyls.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- BF3
- Boron trifluoride. Lewis acid (usually as the Et₂O complex).
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| Prins cyclization | Prins-pinacol tandem reaction | Terpene cyclization (squalene → lanosterol-type) | |
|---|---|---|---|
| Reagents | Homoallylic alcohol, aldehyde (CH₂O), Brønsted or Lewis acid (H₂SO₄, TFA, BF₃·OEt₂, SnCl₄) | Homoallylic diol bearing a tertiary carbinol, CH₂O (or acetal), Lewis acid (SnCl₄, BF₃·OEt₂) or Brønsted acid | Squalene 2,3-oxide (model polyene shown), oxidosqualene cyclase (acid residue, Asp) |
| Conditions | CH₂Cl₂ or aq. acid, 0–25 °C | CH₂Cl₂, –78 to 25 °C | Aqueous, 37 °C, enzyme active site holding a pre-folded chair–chair conformation |
| Stereochemistry | Chair TS; 2,6-cis-substituted tetrahydropyrans usually favoured | Chair-like cyclisation; migration with retention at the migrating carbon | Pre-organised folding sets all ring-fusion stereocentres |
| Rate / selectivity | Oxocarbenium formation is fast; alkene attack is rate-limiting | Cyclisation is rate-limiting; the shift is fast | Enzyme-controlled; epoxide protonation triggers the whole cascade |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Prins cyclization?
Homoallylic alcohol, aldehyde (CH₂O), Brønsted or Lewis acid (H₂SO₄, TFA, BF₃·OEt₂, SnCl₄). Conditions: CH₂Cl₂ or aq. acid, 0–25 °C.
Which bonds are formed and which are broken?
Formed: C–C (ring closure); C–O (ring ether); C–Nu or C=C. Broken: C=C π; C=O π.
What is the stereochemical outcome?
Chair TS; 2,6-cis-substituted tetrahydropyrans usually favoured
What controls the rate or the selectivity?
Oxocarbenium formation is fast; alkene attack is rate-limiting
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go