Cyclopentenone from electrocyclic ring closure

The acid-catalysed ring-closure of divinyl ketones to cyclopentenones (equation 6), the Nazarov reaction6-8, represents a conrotatory electrocyclization of 4jr-cyclopentadienyl cations. The conrotatory course of the reaction was confirmed for the case of the dicyclo-hexenyl ketone 7, which yielded solely the tricyclic ketone 8 on treatment with phosphoric acid (equation 7)3b. Cycloalkanocyclopentenones 10 with c/s-fused rings are obtained from the trimethylsilyl-substituted ketones 9 (n = 1, 2 or 3) and iron(III) chloride and... 

The Nazarov cyclization is an example of a 47r-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear predictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constructed to aUow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with Ae pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatoiy) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

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