Novel synthesis of an eight-membered compound by the ring expansion reaction of a two-carbon unit was developed using the stannyl anion generated from Me3SiSnBu3 and CsF in DMF. cis-and trans-cyclooctenone derivatives were synthesized from cyclohexanone derivatives having vinyl iodide in a tether by treatment with Me3SiSnBu3 and CsF in DMF in a stereospecific manner: The trans-cyclooctenone derivative was isomerized to the cis-isomer in the presence of Me3SiSnBu3 and CsF. It is known that the trans-eight-membered ring is an asymmetric compound. Using this procedure, (+)- and (-)-trans-cyclooctenone derivatives could be synthesized from the corresponding optically active cyclohexanone derivatives.