4-Thia-1-azabicyclo(3.2.0)heptane-2-carboxylic acid, 3,3-dimethyl-7-oxo-6-(2-oxopropylidene)-, (2S-cis)- 6-Acetylmethylenepenicillanic acid 83151-26-2 Ro 15-1903 Ro-15-1903