(2S,4R 5R)-2-Methoxy-4-methyl-3-(4-methylbenzenesulfonyl)-5-phenyl-1,3-oxazolidine (2c) adds trimethylsilyl 1-cycloalkenyl ethers under Lewis acid catalysis with complete induced and high simple diastereoselectivity to yield homochiral oxazolidine-masked 2-formylalkanones.