First successful [4++ 2]-type polar cycloadditions of 2-benzothiopyrylium salt with dienes

2-Benzothiopyrylium salt 1 reacted via a [4(+) + 2]-type polar cycloaddition with dienes 2 in the presence of methanol to afford benzo-fused bicyclo[2.2.2] compounds 5, while in the absence of methanol cycloaddition of 1 with 2,3-dimethylbuta-1,3-diene 2a afforded a novel benzo-fused tricyclic compound 4a, whose structure has been confirmed by X-ray crystallography.