p-nitrobenzyl (5S,2'R)-2-2'-p-nitrobenzyloxycarbonyl-2'-p-nitrobenzyloxycarbonylaminoethylthio-1-carba-2-penem-3-carboxylate

(S)-4-carboxymethyl-2-azetidinone

(4R)-4-<3-(Benzyloxycarbonyl)-2-oxopropyl>azetidin-2-one

(R)-4-3'-p-nitrobenzyloxycarbonyl-2'-oxopropyl-2-azetidinone

p-nitrobenzyl (3R,5R)-1-carba-2-oxopenam-3-carboxylate