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Photochemistry of Diels-Alder adducts Gayler, Rudolf Erich

Abstract

The photochemistry of a variety of Diels-Alder adducts of general structure I has been investigated. The adducts chosen for study were those formed between the dienophiles p-benzoquinone, 1,4-naphthoquinone, and duroquinone and the dienes 2,3-dimethylbutadiene, trans,trans-2,4-hexadiene, 2-methylbutadiene, and 1,3-pentadiene. Three substituent-dependent photochemical reaction types were observed: (1) For adducts with C₅α and/or C₈α methyl groups, γ-hydrogen abstraction was preferred and led to the novel ring system II. (2) For adducts lacking these γ-hydrogen atoms, an unusual new reaction course prevailed. This consisted of β-hydrogen atom abstraction from C₅ (or equivalently C₈) to give the diradical III. Closure of this diallylic diradical in three of the possible four ways (1,6-bonding, 3,8-bonding and 3,6-bonding) and subsequent ketonization gave the observed products of general structure IV, V and VI respectively, although not all three products were observed in each case. The relative amounts of these products were strongly solvent dependent. A methyl group at C₆ directed the β -hydrogen abstraction to the C₈ position in accord with expectations based on radical stability. This same overall reactivity obtained for the corresponding naphthoquinone Diels-Alder adducts. (3) Finally, for the duroquinone-2,3-dimethylbutadiene adduct an unusual cyclobutanone-con- taining photoproduct VII was observed in addition to those resulting from β -hydrogen abstraction. Based on deuterium labeling studies, this product was suggested to arise via direct C₈ hydrogen abstraction by the C₃ enone carbon atom followed by 2,8 bonding. Photoproducts of the type IV,VI and VII undergo novel thermal rearrangements for which mechanisms are proposed. In addition ene-dione VII undergoes a particularly intriguing base-catalyzed rearrangement which is formally the result of a 1,2-carbanionic shift. In summary, the photochemical reactions described demonstrate interesting variations on the familiar γ-hydrogen abstraction process, show the feasibility and preliminary scope of excited state β-hydrogen abstraction processes (heretofore virtually unknown), and appear to provide a general method for the synthesis of tricyclic ring systems of the type II and V which are closely related to certain naturally occurring sesquiterpenes. [See Thesis for Diagrams]

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