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3.8.3 Synthesis of Five-Membered Rings through Electrocyclization of Pentadienyl Cations: The Nazarov Cyclization

DOI: 10.1055/sos-SD-203-00232

Gaspar, B.; Trauner, D.Science of Synthesis: Stereoselective Synthesis, (20113386.

The formation of cyclopentenones via the 4π electrocyclic ring closure of pentadienyl cations, which are generated by either Lewis or Brønsted acid activation of divinyl ketones, is known as the Nazarov cyclization.[‌15‌,‌16‌] The reaction is generally accepted to proceed via pentadienyl cation 15, which cyclizes in a conrotatory fashion to oxyallyl cation 16 (Scheme 5). Elimination followed by protonation of the enolate and ketoenol tautomerization affords the cyclopentenone 17. Thanks to the well-defined mechanism, the challenges of regio- and stereoselection have in many cases been successfully addressed, rendering the reaction synthetically very useful.

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