Studies of chemoselectivity in Diels–Alder reactions of 2-(p-tolylsulfinyl)-1,4-benzoquinone and styrenes: formation of (p-tolylsulfinyl)-1,4-phenanthrenequinones

Diels–Alder reactions of 2-(p-tolylsulfinyl)-1,4-benzoquinone 1 with 4-methoxystyrenes 2a–b in polar solvents afforded 2- and 3-(p-tolylsulfinyl)-1,4-phenanthrenequinones 4a–b and 5a–b as major compounds. A second cycloaddition of 5a and 4a with 3,4-dimethoxystyrene 2a gave access to C- and S-shaped pentacyclic molecules 10 and 11 in a highly regioselective way.