RING TRANSFORMATION OF THE ADDUCTS OF THE POLAR CYCLOADDITIONS OF 2-BENZOTHIOPYRYLIUM SALTS

Ring transformation of the adducts 1 of the polar cycloadditions of 2- benzothiopyrylium salts induced by a variety of bases and reducing age nts has been investigated. Treatment of the 9-unsubstituted cycloadduc ts la, b, c with strong bases such as lithium diisopropylamide (LDA); sodium hydride and potassium carbonate afforded the vinylcyclopropane derivatives 2 and the 1,5-methano-2-benzothionines 3. In contrast, tre atment of compound la with weak bases such as triethylamine, diethylam ine and potassium acetate gave no compounds 2a or 3a, but only ring-op ened compounds 4a in high yields via S(N)2-type processes. In these re actions, weak bases acted as nucleophiles. In contrast; the 9-benzoyl cycloadduct Id reacted with both weak and strong bases to afford, main ly, the similar ring-transformed product 2d. Whilst the Strong base LD A gave compound 3d (as a minor product) in addition to compound 2d, we ak bases such as diethylamine and butylamine also acted as nucleophile s to give the corresponding ring-opened compounds 4d as minor products . A,mechanistic interpretation of the above reactions is presented. Wh en heated at high temperature, the vinylcyclopropane 2d was converted into the cyclopentene derivative 5. Ring transformations of compounds 1 with reducing agents such as sodium borohydride and samarium diiodid e are also described.