NEW MATERIAL:The compound of formula I [R1 is (protected) hydroxyl or (halogen-substituted) alkyl; R2 is H or alkyl; R3 is (substituted) aromatic group; X is O, S, sulfinyl, sulfonyl or NR4 (R4 is H, alkyl or phenyl); Y is O, S or NR5 (R5 is H, alkyl or phenyl)].

EXAMPLE: (R)-2-[(3S, 4R)-3-((R)-1-t-butyldimethylsilyloxyethyl)-2-oxoazetidin-4-yl] propionic acid 2-benzothianolinonamide.

USE: A synthetic intermediate for carbapenem derivative useful as an antibacterial substance.

PREPARATION: The compound of formula I can be produced by reacting the compound of formula II (Z is acetoxy, etc.) with the compound of formula III in the presence of a tertiary amine and boron triflate or tin triflate in an inert solvent (e.g. CH2Cl2).