Polar cycloaddition of 2-benzothiopyrylium salts with conjugated dienes

Abstract

2-Benzothiopyrylium salts 4 and 6 underwent polar cycloaddition with conjugated dienes to afford benzo-fused bicyclic sulfonium salts 7 and 8, respectively, having sulfur at a bridgehead position, in good yields. The salt 4 bearing an electron-withdrawing group at the 3-position was much more reactive than was the salt 6. The structures of the cycloadducts have been established by X-ray crystallography of compound 7a, indicating a cis-fused configuration. In contrast, attempted reaction of the salt 4 with furan as a heterocyclic diene resulted in the electrophilic substitution of the furan by the salt, giving compound 9. The cycloadducts 7 underwent retro-addition to generate 2-benzothiopyrylium ion 4, which was easily trapped with other dienes or active methyl compounds to give the corresponding adduct 7 or 1-alkylated 1H-2-benzothiopyran 5, respectively. Reactions of the cycloadducts 7 with a variety of nucleophiles caused ring opening to afford 1-allyl-12 and 1-homoallyl-substituted 1H-2-benzothiopyrans 13 in good yields. On the other hand, the cycloadduct 8, when treated with nucleophiles, underwent a novel ring transformation along with nucleophilic ring-opening.