Electrocyclic reactions. Part IX. Photolysis of trans,trans-2,4-dibromo-1,5-diphenylpenta-1,4-dien-3-one (αα′-dibromodibenzylideneacetone)

Abstract

Irradiation of trans,trans-αα′-dibromodibenzylideneacetone (I) in benzene–propane-2-ol or toluene at 30 °C under nitrogen at 300 nm yielded a little of the cis,trans-isomer(II) and, by loss of hydrogen bromide, one bicyclic product C₁₇H₁₁BrO and three polycyclic products (C₁₇H₁₁BrO)₂. The former is a 1-benzylidenenaphthalene-2-one derivative (VIIa); one of the latter is regarded as a pentacyclo [4.2.2.0^1,6.O^3,8.O^4,7]decane-2,5-dione derivative (X), and another is considered to be a tricyclo[6.2.0.0^3,6]decane-2,7-dione derivative (XVIIIa or b). The evidence for the structures of these photoproducts and the mechanisms of their formation are discussed.