First [4 + 2] Cycloaddition of 1-Phenyl-1-benzothiophenium Salts with Dienes

The reaction of 1-phenyl-1-benzothiophenium triflates 1 (R = H, Me, and Ph) with cyclopentadiene successfully proceeded to give the corresponding cycloadducts, respectively, in good yields. The single-crystal X-ray analysis of 1 (R = H) showed the endo isomer. Similarly, 1,3-diphenylisobenzofuran reacted with 1 (R = H) to give the corresponding cycloadduct. These novel [4 + 2] cycloadditions of 1-phenyl-1-benzothiophenium triflates as the first example clearly indicate the olefinic nature of the thiophene ring arising from the lack of aromaticity