(2S,5R,6S,7S)-7-[(R)-1-(tert-Butyl-dimethyl-silanyloxy)-ethyl]-5-methyl-4,8-dioxo-3-thia-1-aza-bicyclo[4.2.0]octane-2-carboxylic acid 2,2-dimethyl-propionyloxymethyl ester

pivaloyloxymethyl (4R)-{(2R,3S)-3-[(1R)-1-(tert-butyldimethylsilyloxy)ethyl]-4-oxoazetidin-2-y}-3-oxopentanoate

pivaloyloxymethyl (4R)-{(2R,3S)-3-[(1R)-1-(tert-butyldimethylsilyloxy)ethyl]-4-oxoazetidin-2-y}-2-diaza-3-oxopentanoate

2-((R)-2-{(2S,3S)-3-[(R)-1-(tert-Butyl-dimethyl-silanyloxy)-ethyl]-4-oxo-azetidin-2-yl}-propionylsulfanylmethyl)-malonic acid diethyl ester

2-{(R)-2-[(2S,3S)-3-[(R)-1-(tert-Butyl-dimethyl-silanyloxy)-ethyl]-1-(2,2-dimethyl-propionyloxymethoxyoxalyl)-4-oxo-azetidin-2-yl]-propionylsulfanylmethyl}-malonic acid diethyl ester