Exploration of the Nazarov Cyclization Reaction
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Allenyl aryl ketones were synthesized and underwent the Nazarov cyclization using Lewis acid catalysts. The aromaticity of the AAKs is destroyed during the electrocyclization and interception of the carbocation intermediates was unsuccessful. The oxyallyl cation likely re-aromatizes more quickly than it can be intercepted by a nucleophile. The oxyallyl cation derived by the Nazarov reaction of an allenyl vinyl ketone (AVK) is trapped at position a. Sterically hindered nucleophiles trap at position c, but nothing has trapped at position b. Synthesis of a tethered AVK for which intramolecular trapping of the carbocation would be geometrically favorable only at position b was not successful. Divinyl ketones containing internal β-substituents isomerize to undergo the Nazarov reaction. The internal substituent increases the steric interactions in the reactive conformer. An AVK that is disubstituted was synthesized but the efficiency of the cyclization was considerably lowered. However, trapping was successful using allyltrimethylsilane.