(3R, 4R)-4-Acetoxy-3-[(R)-1-(formyloxy) ethyl]-2-azetidinone 6 could be prepared highly stereoselectivity from (R)-3-hydroxybutyric acid by employing the [2+ 2]-cycloaddition reaction of chlorosulfonyl isocyanate with the 2H, 4H-1, 3-dioxin derivative and the Baeyer- Villiger reaction accompanying novel cleavage of the acetal moiety. The Reformatsky reaction of 6 with sterically crowded 3-(2-bromopropionyl)-2-oxazolidone derivatives ...