The reaction of 2, 3-bis (phenylsulfonyl)-1, 3-butadiene with the anion of various 1- substituted dimethyl 1-pentenedioates has been investigated with the purpose of devising a tandem conjugate addition-[3+ 2]-anionic cyclization route for the synthesis of bicyclo [3.3. 0] octenes. The reaction proceeds with complete stereospecificity as was evidenced by treating (E)-and (Z)-dimethyl (3-cyano-2-propenyl) propanedioate with NaH in the presence of the ...