Novel [4+2]-cycloaddition of 1-phenyl-1-benzothiophenium salts with dienes. Experimental evidence for a lack of aromaticity in the thiophene ring

The [4 + 2]-cycloaddition reaction of 1-phenyl-1-benzothiophenium triflates has been conducted for the first time. [4 + 2]-Cycloaddition with dienes such as cyclopentadiene and 1,3-diphenyl-isobenzofuran occurs successfully to give cycloadducts. This result indicates that the C=C bond of the thiophene ring acts as a 2pi electron component in the cycloaddition reaction. Cycloadducts were formed in high yields with high stereoselectivity. However, the cycloaddition with other less reactive dienes such as 2,3-dimethyl-1,3-butadiene did not take place. The structure and stereochemistry of cycloadduct 2a were analyzed by NMR techniques. Furthermore, reaction of the cycloadducts with sodium methoxide in methanol gave the ring-opened products in high yields.