Photochemical studies. 26. Irradiation-induced transformations of homoconjugated dienones. Highly selective photorearrangements in the spiro[5.5]undeca-1,3-dien-7-one system

Homoconjugated ketones with extended unsaturation: Wavelength‐selective, regioselective, diastereoselective, and enantiospecific photochemical transformations of methyl 7‐oxospiro[5.5]undeca‐1,3‐and ‐2,4,‐diene‐2‐carboxylate

“…4 rearrangement to occur under these conditions. This may indicate that the same excited triplet is attained in this system, by both direct and sensitized excitation, contrary to previously encountered behaviour [2]. The cis-isomer c-11 could not be obtained, neither by photochemical means nor by chemical transformations (ode infra).…”

“…2 + .2 mode of the ODPM rearrangement, represented by either A or B in Scheme 9, in both the direct and sensitized phototransformations, pointing to a reasonable picture of excited states (Scheme 10). At this stage of mechanistic comprehension of these and previously [2] Scheme 9 R = C0,Me. R' = H R = H, R' = C0,Me Scheme 10 R' R = CO,Me, R' = H R = H, R' = C0,Me described findings, however, it is difficult to pinpoint the exact reason for the differences in the behaviour of the COOMe-substituted oxospiro [5.5]undecadienes from the unsubstituted or Me-substituted ones [2].…”

“…2 + ,2 trans-cycloaddition product t-11. We had expected [2] a less regioselective behaviour, i.e., some . 2 + .…”

“…-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…”

“…(25%) (formylbuty1)benzoate 10 (lo%), and the [1,2]-acyl shift (ODPM) product t-11 (60 YO ; see below). iii) At and above 340 nm (cut-off filter) and at ca.…”