(3R)-Methyl 4-((3S)-1b-(4-Chlorophenyl)-7-hydroxy-7-isopropyl-1a-phenyl-7a-(phenylcarbamoyl)hexahydro-1aH-oxireno[2',3':3,4]pyrrolo[2,1-b][1,3]oxazin-3-yl)-3-hydroxybutanoate