Photochemical reactions, 135th communication Photochemistry of Homoconjugated Cyclobutanones. II. Decisive Effect of gem ‐ Dimethyl Substitution on the Course of the Oxa‐di‐π‐methane Rearrangement

The synthesis and photolysis of the spirocyclobutanones 4–7 incorporating a cyclohexa‐, cyclohepta‐ and cyclooctadiene moiety, respectively, is described. On triplet excitation, these compounds undergo isomerization via a 1,2‐acyl shift involving one or both double bonds of the diene system. The presence of a gem ‐dimethyl group as in 1 , 4 and 7 dramatically changes the photoproduct distribution, since only these substrates leads to the products 3, 29 and 34 resulting from vinylogous ring closure ( Scheme 5 ). Those substrates without methyl substitution ( 5 and 6 ) give only products of a rearrangement involving one double bond.