Polar cycloaddition of 1-benzothiopyrylium salts with conjugated dienes and some transformations of the cycloadducts

Abstract

Polar cycloaddition of 1-benzothiopyrylium salts 2 with conjugated dienes proceeds regio- and stereo-specifically to afford the corresponding benzo-fused bicyclic sulfonium salts 4 in good yields. Reaction of the cycloadducts 4 with nucleophiles such as methanol or water causes ring opening to give 2-(but-2-enyl)- and 2-(but-3-enyl)-substituted 2H-1-benzothiopyrans 5 and 6, respectively. Treatment of the cycloadduct 4hA with a variety of bases affords the spirocyclopentene derivative 7 and the spiro-1,2-dioxolane derivative 8. A mechanism involving a biradical intermediate is discussed for the formation of the above products 7 and 8 on the basis of chemical evidence. Reduction of the cycloadduct 4hA with sodium borohydride or sodium cyanoborohydride is also described.