PS 5

(5R,6R)-3-[2-(Acetylamino)ethylthio]-6-ethyl-7-oxo-1-azabicyclo[3.2.0]hepta-2-ene-2-carboxylic acid

(5R,5β)-3-[[2-(Acetylamino)ethyl]thio]-6β-ethyl-7-oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid