Photochemistry of Linearly Conjugated Cyclohexadienones in Solution'

Objects of study: The cyclohexa-2,4-dienones 1, II and 111. Special methods of examination Low temperature spectroscopy and Mauser diagrams. Main results of investigation: Cyclo/seco isomeri7ation of a it*,nexcited reactant to a transient dienylketene, able to recyclize and/or to add a protic nucleophile, furnishes derivatives of a substituted hexadienoic acid in high yield. Alternatively, a cyclohexa-2,4-dienone/hicyclo-I 3.1.0 hex-2-enone rearrangement of a *,jt..excited reactant may take place. Whether the latter reaction occurs (as with III under special conditions) or not (as with I and II under any conditions investigated) is a question of the spatial symmetry of the lowest excited state. The light-induced ring opening of the linearly conjugated cyclohexadienone XX represents the key reaction of a three-stage synthesis of dimethylcrocetin starting from 2,6-dimetbylphenol and 1,4-dibromobut2-ene.