Nazarov cyclisation template

(2E,5E)-Hepta-2,5-dien-4-one is protonated to a pentadienyl cation, which closes conrotatory to a trans-dimethyl oxyallyl cation; loss of a proton and tautomerisation give 3,4-dimethylcyclopent-2-en-1-one.

Mechanism step by step

A protonated divinyl ketone closes by 4π conrotatory electrocyclisation to a cyclopentenone

1. Activation – Acid (or a Lewis acid) on the C=O oxygen gives a pentadienyl cation
2. 4π Electrocyclisation – Four π electrons, thermal: conrotatory, so the two methyls end up trans (accepted mechanism)
3. Elimination – Water removes a proton next to the oxyallyl cation (drawn with + on the other allyl terminus): a cyclopentadienol
4. Tautomerisation – The enol turns into the ketone (acid catalysed): the conjugated cyclopentenone

Key points

• Woodward–Hoffmann: a pentadienyl cation has 4 π electrons, so the thermal ring closure is conrotatory (the photochemical one disrotatory).
• (E,E)-Substituents at the termini end up trans in the oxyallyl cation; the elimination step can erase one of the two new stereocentres.
• Strong Brønsted or Lewis acids (H₂SO₄, H₃PO₄, BF₃·OEt₂, FeCl₃, SnCl₄) promote it; the C=C usually ends up in the more substituted position.
• Silicon-directed (Denmark) and catalytic asymmetric variants control where the double bond and stereocentres end up.

Drawn with 86 atoms, 12 arrows, 6 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 35 Textbook
• Fleming, Molecular Orbitals and Organic Chemical Reactions, Wiley 2010 Textbook
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook