UBC Theses and Dissertations
Intramolecular photochemical cycloaddition of nonconjugated dienes Boire, Brian Anthony
The photolysis of isogermacrone (68) has been investigated. Exclusive "straight" cycloaddition occurs to form syn (70) and anti (71) 1,7- dimethyl-4-isopropylidene-tricyclo[188.8.131.52[sup 2,6]decane-3-one. The structures of the photoproducts have been confirmed by an independent synthesis. A mechanism, for the photolysis of 68, involving an intermediate 1,4-diradical is postulated in order to account for the stereochemistry of the products. The direct and triplet-sensitized photochemistry of the three geometric isomers of diethyl deca-2,8-diene-l,10-dioate (79 - 81) has been studied. The triplet reaction is one of rapid cis,trans isomerization accompanied by slower 2 + 2 internal cyclization in a "straight" manner to give four of the six possible stereoisomeric diethyl bicyclo[4.2.0]-octane-7,8-dicarboxylates (82 - 85). The stereochemistry of these products as well as the triplet nature of the reaction are indicative of a two step mechanism involving 1,4-diradical intermediates. Possible explanations for the direction of initial bond formation in these reactions are also discussed. The direct (singlet) reaction of the deca-2,8-diene-l,10-dioates is one of trans to cis isomerization followed by α,β to β,γ double bond migration from the Cis isomer; the sole deconjugated product is diethyl trans,trans-deca-3,7-diene-l,10-dioate. A possible explanation for this stereoselectivity is advanced. Reasons for an investigation of the photolysis of cyclonona-2,6-dienone (128) and a scheme for its synthesis are presented.
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