The fragmentation of 5,6-epoxynorbornan-2-01 and 5,6-epoxynorbornan-2-one have been studied under both acidic and basic conditions. These processes show a clear preference for syn periplanar alignment of breaking bonds. We have recently reported a short and efficient synthesis of the ketol l_,l embodying all of the skeletal features of the taxane ring system. 2 The key step of this synthetic sequence involved the hydroxyl-directed epoxidation3 of 2, presumably giving intermediate epoxy alcohol 3 which underwent fragmentation in situ to provide a quantitative yield of 4. The -ease of this fragmentation is surprising since the Cl-C2 bond and the C3-0 bond (the breaking bonds) in 2 have a syn periplanar relationship. We have attempted, unsuccessfully, to prepare the isomeric B epoxide by epoxidation of the TMS ether of 2. Presumably both faces of this olefin are sterically shielded; the B-face is shielded by the geminal methyl group, and the o-face is shielded by the silyl ether. Fragmentation reactions have been the a clear preference for the anti periplanar subject of numerous studies. In several of these alignment of breaking bonds has been observed. 4
There exists no study, to our knowledge, in which a preference has been demonstrated for syn periplanar alignment of breaking bonds in a fragmentation process.5