A novel tandem [4++2] cycloaddition–elimination reaction: 2-alkenyl-4,4-dimethyl-1,3-oxathianes as synthetic equivalents for α,β-unsaturated thioaldehydes

The first tandem cationic [4++2] polar cycloaddition–elimination reaction of 1-thia-1,3-butadienyl cations with olefins to afford directly 3,4-dihydro-2H-thiopyrans is described. The cations were easily accessible by treatment of monothioacetals , particularly the 2-alkenyl-4,4-dimethyl-1,3-oxathianes , with a hard Lewis acid. In this novel reaction, 2-alkenyl-4,4-dimethyl-1,3-oxathianes were utilized as synthetic equivalents for highly reactive α,β-unsaturated thioaldehydes. The effect of geminal dimethyl substituents on the oxathianes and the mechanistic aspect of the reaction are considered. The reactionʹs asymmetric version was also investigated using a chiral oxathiane derived from (−)-(1R,2R,5R)-2-(1-mercapto-1-methylethyl)-5-methylcyclohexanol. Although the enantioselectivities were moderate, the whole process can be done under odorless conditions.