[1R-[1Alpha,5Alpha,6(R)]]-Alpha-(1-Methylethenyl)-3-(4-Methylphenyl)-7-Oxo-4-Oxa-2,6-Diazabicyclo[3.2.0]Hept-2-Ene-6-Acetic Acid Diphenylmethyl Ester [ 67978-05-6 ]