Efficient Syntheses of Thiochromans via Cationic Cycloadditions

α-(Benzotriazolyl)methyl thioethers 1a−e reacted with styrenes under Lewis acid catalysis to give novel polysubstituted thiochromans (3,4-dihydro-2H-1-benzothiopyrans) 3−14 and 16−20 in generally high yields. Most thiochromans were isolated as one diastereomer following recrystallization. The configuration and conformation of the products are predicted on the basis of their NMR data. A stepwise reaction, proceeding via a [4+ + 2] cationic polar cycloaddition mechanism, is proposed.