Trans-(+)-3-[[2-(acetylamino)ethyl]thio]-6-ethyl-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid, (4-nitrophenyl)methyl ester
4-tert-Butoxycarbonylamino-3-(diphenoxy-phosphoryloxy)-2,5-dihydro-thiophene-2-carboxylic acid ethyl ester
(R,S)-ethyl 4-<(tert-butoxycarbonyl)amino>-4,5-dihydro-3-<(diphenoxyphosphinyl)oxy>-2-thiophenecarboxylate