2-<3-benzyl-6-oxo-2-thia-4,7-diaza-(1R,5R)-bicyclo-<3.2.0>-hept-3-en-7-yl>-acetic acid-p-nitrobenzyl ester

2-oxo-bisnorpenicillin G-3-allyl carboxylate

2-oxo-bisnorpenicillin G-3-benzhydryl carboxylate

(6R)-phenylacetamido-2-benzoyloxy-(5R)-penem-3-t-butyl-carboxylate