3-Methyl-2-[(3S,4R)-3-(2-methyl-allyl)-2-oxo-4-phenylacetylsulfanyl-azetidin-1-yl]-but-2-enoic acid 2,2-dimethyl-propionyloxymethyl ester
(5R,6S)-3-Benzyl-7-oxo-6-(2-oxo-propyl)-4-thia-1-aza-bicyclo[3.2.0]hept-2-ene-2-carboxylic acid 2,2-dimethyl-propionyloxymethyl ester
2,2-Dimethyl-propionic acid 2-oxo-2-[(3S,4R)-2-oxo-3-(2-oxo-propyl)-4-phenylacetylsulfanyl-azetidin-1-yl]-acetoxymethyl ester
(pivaloyloxy)methyl 6,6-dihydropenicillanate S,S-dioxide