Photochemistry of cyclic enones. Part 8. 5-(But-3-enyl)-3-phenylcyclopent-2-enone and 5-(pent-4-enyl)-3-phenylcyclopent-2-enone

Abstract

Photolysis of 5-(but-3-enyl)-3-phenylcyclopent-2-enone in benzene solution using a Pyrex filter affords two products in the ratio 2:1. These are 1-phenyltricyclo[4.2.1.0]nonan-7-one and 8-phenyltricyclo[4.2.1.0]nonan-9-one. They are inseparable by column chromatography, but their structure follows from examination of the four separable lactones derived from the mixture by Baeyer-Villiger oxidation.

Photolysis of 5-(pent-4-enyl)-3-phenylcyclopent-2-enone under the same conditions affords only one product, 9-phenyltricyclo [5.2.1.0]decan-10-one, whose structure follows from examination of the two lactones derived by similar oxidation.

These results demonstrate that in these compounds the regioselectivity of ring closure depends on the length of the 5-side-chain.