Photoinduced molecular transformations. Part 152. Ring expansion based on a sensitized [2 + 2] photoaddition of enol ethers of cyclic ketones with olefins, followed by a β-scission of alkoxyl radicals generated from the resulting cyclobutanols. Two-carbon ring expansion of β-indanone, β-tetralone and β-suberone

Abstract

A new method for two-carbon ring expansions of the 5-, 6- and 7-membered rings of β-indanone, β-tetralone and β-suberone is described. The sequence involves a sensitized [2 + 2] photoaddition of the trimethylsilyl enol ethers of the cyclic ketones with acrylonitrile or methyl acrylate, followed by a β-scission of the alkoxyl radicals generated from the resulting cyclobutanols. The syntheses of functionalized benzocycloheptane, benzocyclooctane and benzocyclononane rings were achieved by this method. The ring expansion of cyclobutanols derived from the [2 + 2] photoaddition of trimethylsilyl enol ethers of β-tetralone and β-indanone with methyl acrylate afforded γ-lactones as byproducts, which arose from a transannular bonding of the carbonyl oxygen and the cationic centre of the carbocationic intermediates generated by β-scissions of the alkoxyl radicals.