4-Thia-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid, 6-[(1R)-1-[[(1,1-dimethylethyl)dimethylsilyl]oxy]ethyl]-3-[[(4-methoxybenz oyl)oxy]methyl]-7-oxo-, 2-propenyl ester, (5R,6S)-