The chemoselectivity of carbonyl compounds have been stduied. Dimethyl sulfide and trimethylsilyl triflate(TMSOTf) selectively reacted with aldehydes to form $\alpha$-silyloxysulfonium salts in the presence of ketones at low temperature. Therefore ketone groups could chemoselectively react with other uncleophiles in the presence of aldehyde groups. The difference in the reactivity between cyclic acetals or ketals and acyclic acetals or ketals was studied for various nucleophiles.