system of antimonypentachloride, chlorotrimethylsilane and tin(II) iodide, α-substituted cyclic ethers are stereoselectively prepared from lactones by successive treatment with 1-(t-butyldimethylsiloxy)-1-ethoxyethene and silyl nucleophiles such as triethylsilane, allyltrimethylsilane and trimethylsilyl cyanide. These catalysts also promote the reaction of γ-, δ-, and e-trimethylsiloxy carbonyl compounds