Electrocyclic reactions. Part III. Some reactions of 2,4-dimethyl-1,5-diphenylpenta-1,4-dien-3-one (αα′-dimethyldibenzylideneacetone)

Abstract

αα′-Dimethyldibenzylideneacetone (I) by treatment with hot hydrochloric acid givestrans-2,5-dimethyl-3,4-diphenylcyclopent-2-enone (Va). With hot hydriodic acid the minor product is the cyclopentenone (Va) and the major product is trans-2,5-dimethyl-trans-3,4-diphenylcyclopentanone (VIa). The cyclopentenone (Va) undergoes only partial reduction with hot hydriodic acid to the cyclopentanone (VIa). The results suggest a conrotatory thermal ground state electrocyclisation of a pentadienyl cation to a cyclopentenyl cation, followed by loss of a proton and ketonisation, or by reduction. The high yields of cyclic products, relative to those observed for trans,trans-dibenzylideneacetone, are notable.