Intramolecular anionic ring opening of oxabicyclo[3.2.1] systems has been achieved and leads to an efficient route to bicyclo[5.3.0]decenes which are trans fused. Tethers containing heteroatoms (X = O, S, NMe) as well as all-carbon derivatives have been successfully cyclized under these conditions. It is not necessary to take any special precautions (high dilution, slow addition etc.) when carrying out the transmetalation-cyclization since intermolecular ring opening does not occur under these conditions.