Silicon‐Directed Nazarov Cyclizations. Part VI. The anomalous cyclization of vinyl dienyl ketones

Reactions, of various vinyl dienyl ketones (see la–e, li) with FeCl, give rise to β,γ‐unsaturated α‐vinyl‐cyclo‐pentenones (see 2a–e, 2i, Table 2). The reaction succeeds for vinyl dienyl ketones with substituents on either double bond. Aryl dienyl and alkyl dienyl ketones (see 1f–h) do not cyclize cleanly. The effects of substituents on the rate of reaction is discussed in terms of the mechanism of the rearrangement. A ¹³C‐labeling study establishes the pathway as an unusual 1‐hydroxypentadienyl‐cation electrocyclization to a cyclopentenyl cation which collapses via a pinacol rearrangement to the α‐vinyl ketone.