(3S,4R)-3-benzyl-4-(4-carboxy)phenoxy-1-[(S)-1-(phenylethyl)aminocarbonyl]azetidin-2-one

(3S,4S)-4-acetoxy-3-benzylazetidin-2-one

(3S,4R)-3-benzyl-4-[4-(1,1-diphenylmethoxycarbonyl)phenoxy]azetidin-2-one

(3S,4R)-3-benzyl-4-(4-carboxy)phenoxy-1-[(R)-1-(phenylethyl)aminocarbonyl]azetidin-2-one

4-[(2S,3S)-3-Benzyl-4-oxo-1-((R)-1-phenyl-ethylcarbamoyl)-azetidin-2-yloxy]-benzoic acid