Ketene-imine cycloaddition reactions (the Staudinger reaction) of ethoxycarbonyl(phenylthio)ketene with various imines and subsequent desulfurization reactions were employed to synthesize 3-ethoxycarbonyl β-lactam derivatives. The results indicate that the current approach provides a convenient, mild, and versatile method for synthesizing a variety of 3-alkoxycarbonyl trans-β-lactam derivatives with good to excellent yields and diastereoselectivities.