The tandem intermolecular Paterno-Buchi reaction: formation of tetrahydrooxepins

The Paterno-Buchi reaction is the [2 + 2] photocycloaddition between carbonyl compounds and electron rich alkenes to generate oxetane products. By the introduction of substituted cyclopropyl rings to the alkene components, the utility of this reaction has been extended to facilitate the synthesis of substituted tetrahydrooxepins. It is proposed that initial addition of oxygen radicals to cyclopropyl enol ethers generates cyclopropylmethyl radicals which, when the cyclopropane ring bears appropriate radical-stabilising groups (e.g. phenyl, CO,Et), undergo rapid fragmentation to form homoallylic 1,7-biradicals which then recombine to deliver the observed tetrahydrooxepin products, The importance of various radical-stabilising substituents on the efficiency of tetrahydrooxepin formation is examined.