ethyl (2S)-2-[(2S,4aS,5aS,8aS)-4a-hydroxy-2-methyl-1,4-dioxo-5,5a,6,7,8,8a-hexahydro-2H-cyclopenta[3,4]pyrrolo[3,5-a]pyrazin-3-yl]-4-phenylbutanoate