Photochemical Reactions. 135th Communication. Photochemistry of Homoconjugated Cyclobutanones. II. Decisive Effect of gem-Dimethyl Substitution on the Course of the Oxa-di-?-methane Rearrangement

Article abstract of DOI:10.1002/hlca.19840670319

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 isomerisation 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 lead 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.