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Conformational effects in the photochemistry of tetrahydro-1,4-naphthoquinones Jennings, Barry Michael

Abstract

The photochemistry of a variety of tetrahydro-1,4-naphthoquinones (structure 1) has been investigated. These were synthesized by Diels-Alder reaction of corresponding p-quinones and acyclic-1,3-dienes [diagram omitted]. Three substituent-dependent types of reactions were observed: (1) for adducts possessing hydrogen substituents at C₄a and C₈a (bridgehead position), the prevalent process was one of abstraction of a β-hydrogen atom from C₅(or equivalently, C₈) by excited carbonyl oxygen. In general, three product types were then observed, derived from carbon-carbon bond formation (proceeded in two instances by conformational rotation about the C₄a - C₈a bond) in the bisallylic radical so produced. Placing a methyl or phenyl substituent at C₂ renders the β-hydrogens non-equivalent, and abstraction occurs in accord with expectations based on the formation of the more stable biradical intermediate. In adducts possessing bridgehead substituents, rotation about the C₄a - C₈a bond in the biradical is suppressed and only a single product type (enone alcohol) is formed, which possesses the same relative conformation as the biradical and starting adduct. In the case where the bridgehead substituents are carboxymethyl, however, secondary photolysis in benzene solution occurs, giving rise to a product where the bridgehead substituents are nearly eclipsed. (2) For the adduct possessing exo-methyl substituents at positions 5 and 8 as well as methyls at the bridgehead positions and at and C₂ and C₃, hydrogen abstraction was partially suppressed in favor of a process tentatively concluded to involve Ύ~hydrogen abstraction by excited enone carbon, giving rise to a product, the formation of which again requires little conformational change in the biradical. (3) For adducts possessing endo-methyl substituents at C₅ and C₈ as well as bridgehead substituents, a novel oxetane product was observed, formally the result of a cycloaddition reaction involving the remote double bond and one of the carbonyl groups. The oxetane derived from the duroquinone adduct was found to be photolabile, giving back starting material and, eventually, a quantitative conversion to a novel cage diketone. The reactivity differences for these systems, as well as for those previously studied in our laboratory, are interpreted as being due to the effects of substituents on the energy barrier to conformational isomerization in the biradical intermediates. Finally, the potential utility of these photochemical reactions for the synthesis of unusual ring systems is noted.

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