(1S,3R,5R,6R)-8-aza-3-[1-(S)-benzyloxyethyl]-5-methyl-2,4-dioxa-bicyclo[4.2.0]octan-7-one

(3S,4R)-3-[(R)-1-(t-Butyldimethylsilyloxy)ethyl]-4-[(R)-1-(4,4-dimethyl-2-oxazolidone-3-carbonyl)ethyl]-2-azetidinone

(3R,4R)-4-acetoxy-3-[(R)-1-formyloxyethyl]-2-azetidinone

(3S,4R)-3-[(R)-1-(t-Butyldimethylsilyloxy)ethyl]-4-[(R)-1-(4,4-dibutyl-5,5-pentamethylene-2-oxazolidone-3-carbonyl)ethyl]-2-azetidinone

(3R,4S)-4-acetoxy-3-[(R)-1-formyloxyethyl]-2-azetidinone