The title molecules were shown to photorearrange with remarkable selectivity. Wavelength variation steers the rearrangement modes, of which the [1,2]-acyl shift was found to be largely regioselective, diastereoselective, and enantiospecific. Chemical intercorrelation of products and mechanistic studies were carried out all along. The potential significance of these photochemical processes in selective synthetic schemes is discussed.Introduction. -We recently described the interesting photochemical behaviour of a certain class of homoconjugated ketones with extended unsaturation, namely 5-acylcyclohexa-1,3-dienes. The spiro[5.5]undeca-l,3-dien-7-one system was found to be of particular interest and versatility [2], and an outline of its irradiation-induced transformations is given in Scheme 1. The wavelength selectivity is evident and remarkable: i) high-energy (254 nm) irradiation brings about an electrocyclic opening of the cyclohexadiene ring to the corresponding conjugated trienone, which is highly photoreactive and leads to a variety of secondary products; ii) at 300 nm, a -cleavage takes place, giving the