(E)-3-{(R)-1-[Methoxycarbonyl-(triphenyl-λ5-phosphanylidene)-methyl]-4-oxo-azetidin-2-ylsulfanyl}-pent-2-enedioic acid dimethyl ester

(E)-3-{(R)-1-[Benzyloxycarbonyl-(triphenyl-λ5-phosphanylidene)-methyl]-4-oxo-azetidin-2-ylsulfanyl}-pent-2-enedioic acid dimethyl ester

(E)-3-[(R)-1-(Chloro-methoxycarbonyl-methyl)-4-oxo-azetidin-2-ylsulfanyl]-pent-2-enedioic acid dimethyl ester

(2S,5R)-3-[1-Methoxycarbonyl-meth-(Z)-ylidene]-7-oxo-4-thia-1-aza-bicyclo[3.2.0]heptane-2-carboxylic acid benzyl ester

(2S,5R)-3-[1-Methoxycarbonyl-meth-(Z)-ylidene]-7-oxo-4-thia-1-aza-bicyclo[3.2.0]heptane-2-carboxylic acid