Ring transformation of the adducts of the polar cycloadditions of 2-benzothiopyrylium salts

Abstract

Ring transformation of the adducts 1 of the polar cycloadditions of 2-benzothiopyrylium salts induced by a variety of bases and reducing agents has been investigated. Treatment of the 9-unsubstituted cycloadducts 1a, 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, treatment of compound 1a with weak bases such as triethylamine, diethylamine and potassium acetate gave no compounds 2a or 3a, but only ring-opened compounds 4a in high yields via S_N2-type processes. In these reactions, weak bases acted as nucleophiles. In contrast, the 9-benzoyl cycloadduct 1d reacted with both weak and strong bases to afford, mainly, the similar ring-transformed product 2d. Whilst the strong base LDA gave compound 3d(as a minor product) in addition to compound 2d, weak bases such as diethylamine and butylamine also acted as nucleophiles to give the corresponding ring-opened compounds 4d as minor products. A mechanistic interpretation of the above reactions is presented. When 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 diiodide are also described.