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Intramolecular photochemical cycloaddition of nonconjugated dienes

University of British Columbia

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[5.3.0.0[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.

Text

2011-03-14

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http://hdl.handle.net/2429/32426

Chemistry

University of British Columbia

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