THE TANDEM INTERMOLECULAR PATERNO-BUCHI REACTION - FORMATION OF TETRAHYDROOXEPINS

The Paterno-Buchi reaction is the [2 + 2] photocycloaddition between c arbonyl compounds and electron rich alkenes to generate oxetane produc ts. By the introduction of substituted cyclopropyl rings to the alkene components, the utility of this reaction has been extended to facilit ate the synthesis of substituted tetrahydrooxepins. It is proposed tha t initial addition of oxygen radicals to cyclopropyl enol ethers gener ates cyclopropylmethyl radicals which, when the cyclopropane ring bear s appropriate radical-stabilising groups (e.g. phenyl, CO,Et), undergo rapid fragmentation to form homoallylic 1,7-biradicals which then rec ombine to deliver the observed tetrahydrooxepin products, The importan ce of various radical-stabilising substituents on the efficiency of te trahydrooxepin formation is examined.