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The solution photochemistry of some Diels-Alder adducts Ngan, Yuen Mui


In order to extend the study of the photochemistry of substituted butadiene-α, β-unsaturated ketone Diels-Alder adducts, 3,4-dimethyl-3- cyclohexenyl phenyl ketone (47) and cis and trans-2,5-dimethyl-3-cyclo- hexenyl methyl ketone (48 and 56) were synthesized and photolyzed. The photoproducts from 3,4-dimethyl-3-cyclohexenyl phenyl ketone were obtained in very low yields with difficuclty, and appeared to have arisen via a hydrogen abstraction process. The photoreaction of cis-2,5-dimethyl-3- cyclohexenyl methyl ketone resulted in the formation of 8-oxa-2,5,7- trimethyltricyclo[]³[sup ,]⁷]octane (80A), 7-oxa-2,5,8-trimethyltricyclo-3 8 []³[sup ,]⁸]octane (81B) and 7-hydroxy-4,7-dimethylbicyclo[4.2.0]oct-2-ene (82C) , all in low yield. The first two products were produced by an intramolecular Paterno-Buchi reaction, while the third was formed from a Norrish Type II reaction. Photolysis of trans-2,5-dimethyl-3-cyclo- hexenyl methyl ketone afforded the major product 7-oxa-3,6,8-trimethyl- bicyclo[3.2.1]oct-2-ene (83B) in low yield, for which a mechanism involving a diradical intermediate and an unusual intramolecular 1,4-hydrogen transfer was postulated. In addition the photochemistry of the trans-1,3,5-hexatriene- p-benzoquinone Diels-Alder adduct (t-59) was investigated. Selective n-π* excitation of this material led to a single product, tetracyclo[6.4.0. 2 11 4 9 0.²[sup ,]¹¹ 0⁴[sup ,]⁹ ]dodec-6-ene-3,10-dione (127) in high yield, the result of a novel regiospecific intramoleculat 2+2 photoaddition. A two step mechanism involving a diradical intermediate was proposed, and the observed photoreaction was discussed in terms of conformational and radical stability effects.

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