Irradiation-Induced Transformations of Homoconjugated Dienones.¹ Highly Selective Photorearrangements in the Spiro[5.5]undeca-1,3-dien-7-one System

We present a study of the photorearrangements in the homoconjugated (β,γ,δ,∊-unsaturated) ketone la system, viz., derivatives of spiro[5.5]undeca-1,3-dien-7-one (2, 3, 4) which are of mechanistic interest and considerable synthetic potential because of their high wavelength selectivity, regioselectivity, and diastereoselectivity. Thus, in direct irradiation of 2–4 at 254 nm, electrocyclic opening of the cyclohexadiene ring to trienone products (5, 9) occurs, along with some α-cleavage and β-H abstraction to give aromatic aldehydes (10, 11). These are the main products at λ ≥ 300 nm while at λ ≥ 340 nm an oxa-di-π-methane (ODPM) rearrangement takes place to give exclusively the trans-tricyclo[5.4.0.0^7.11]undeca-9-en-2-ones (12t, 13t, and 14t). Sensitized irradiation of 2, 3, and 4 yielded the latter along with 1,2-acyl shift products of a second kind: cis- and trans-tricyclo[5.4.0.0⁷’⁹]undeca-10-en-2-ones (15–17). Quantum yields, triplet sensitizer energy, quenching, and solvent effects were studied. The involvement of multiple excited states is inferred, and mechanistic models are discussed.