methyl (2R,3R)-3-amino-4-oxoazetidine-2-carboxylate

cis-3-benzyloxycarbonylamino-1-(2,4-dimethoxybenzoyl)-4-methoxycarbonyl-2-azetidinone

(cis)-4-(methoxycarbonyl)-2-oxo-3-[[(phenylmethoxy)carbonyl]amino]-1-azetidine