The conjugated triene supplies six π electrons, a 4n+2 electrocyclic system. Its thermal closure is disrotatory, whereas photochemical closure is conrotatory. Tracking the terminal ethyl groups to the shown trans product requires conrotation for this starting geometry. Therefore the allowed condition is photochemical conrotatory, D.
More Pericyclic Reactions and Photochemistry: Electrocyclic, cycloaddition and sigmatropic reactions. Diels-Alder reaction, Claisen and Cope rearrangements and their applications in organic synthesis. FMO method of analysis. Woodward-Hoffmann rules. Photochemistry of alkenes, arenes and carbonyl compounds. Photo-oxidation and photo-reduction. Di-π-methane rearrangements, Norrish type-I and II reactions. Paternό-Büchi reaction, Photo- Curtius and Wolff rearrangements. Barton and Hofmann-Lӧffler-Freytag reactions. questions
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